The insulin and IGF signaling pathways are critical for development and maintenance of pancreatic β cell mass and function. The serine-threonine kinase Akt is one of several mediators regulated by these pathways. We have studied the role of Akt in pancreatic β cell physiology by generating transgenic mice expressing a kinase-dead mutant of this enzyme in β cells. Reduction of Akt activity in transgenic animals resulted in impaired glucose tolerance due to defective insulin secretion. The mechanisms involved in dysregulation of secretion in these mice lie at the level of insulin exocytosis and are not the result of abnormalities in glucose signaling or function of voltage-gated Ca2+ channels. Therefore, transgenic mice showed increased susceptibility to developing glucose intolerance and diabetes following fat feeding. These observations suggest that Akt plays a novel and important role in the regulation of distal components of the secretory pathway and that this enzyme represents a therapeutic target for improvement of β cell function in diabetes.
The insulin and IGF signaling pathways are critical for development and maintenance of pancreatic β cell mass and function. The serine-threonine kinase Akt is one of several mediators regulated by these pathways. We have studied the role of Akt in pancreatic β cell physiology by generating transgenic mice expressing a kinase-dead mutant of this enzyme in β cells. Reduction of Akt activity in transgenic animals resulted in impaired glucose tolerance due to defective insulin secretion. The mechanisms involved in dysregulation of secretion in these mice lie at the level of insulin exocytosis and are not the result of abnormalities in glucose signaling or function of voltage-gated Ca2+ channels. Therefore, transgenic mice showed increased susceptibility to developing glucose intolerance and diabetes following fat feeding. These observations suggest that Akt plays a novel and important role in the regulation of distal components of the secretory pathway and that this enzyme represents a therapeutic target for improvement of β cell function in diabetes.
Studies of the genetic basis of type 2 diabetes suggest that variation in the calpain-10 gene affects susceptibility to this common disorder, raising the possibility that calpain-sensitive pathways may play a role in regulating insulin secretion and/or action. Calpains are ubiquitously expressed cysteine proteases that are thought to regulate a variety of normal cellular functions. Here, we report that short-term (4-h) exposure to the cell-permeable calpain inhibitors calpain inhibitor II and E-64-d increases the insulin secretory response to glucose in mouse pancreatic islets. This dose-dependent effect is observed at glucose concentrations above 8 mmol/l. This effect was also seen with other calpain inhibitors with different mechanisms of action but not with cathepsin inhibitors or other protease inhibitors. Enhancement of insulin secretion with short-term exposure to calpain inhibitors is not mediated by increased responses in intracellular Ca2+ or increased glucose metabolism in islets but by accelerated exocytosis of insulin granules. In muscle strips and adipocytes, exposure to both calpain inhibitor II and E-64-d reduced insulin-mediated glucose transport. Incorporation of glucose into glycogen in muscle also was reduced. These results are consistent with a role for calpains in the regulation of insulin secretion and insulin action.
Calpains are a family of calcium-activated proteases involved in a number of cellular functions including cell death, proliferation and exocytosis. The finding that variation in the calpain-10 gene increases type 2 diabetes risk in some populations has increased interest in determining the potential role of calpains in pancreatic β-cell function. In the present study, transgenic mice (CastRIP) expressing an endogenous calpain inhibitor, calpastatin, in pancreatic β-cells were used to dissect the role of the calpain system in the regulation insulin secretion in vivo and in vitro. Glucose concentrations after the administration of intraperitoneal glucose were significantly increased in CastRIP mice compared with wildtype littermate controls. This was associated with a reduction in glucose-stimulated insulin secretion in vivo. Using pancreas perfusion, static islet incubation and islet perifusion, it was demonstrated that CastRIP islets hypersecreted insulin at low glucose, but exhibited significantly impaired insulin responses to high glucose. Examination of insulin release and calcium signals from isolated islets indicated that distal components of the insulin exocytotic pathway were abnormal in CastRIP mice. CastRIP islets had modestly reduced expression of Rab3a and other critical components in the late steps of insulin exocytosis. These studies provide the first evidence that blocking endogenous calpain activity partially impairs insulin release in vivo and in vitro by targeting distal components of the insulin exocytotic machinery.
The experiments in this study were undertaken to determine whether inhibition of calpain activity in skeletal muscle is associated with alterations in muscle metabolism. Transgenic mice that overexpress human calpastatin, an endogenous calpain inhibitor, in skeletal muscle were produced. Compared with wild type controls, muscle calpastatin mice demonstrated normal glucose tolerance. Levels of the glucose transporter GLUT4 were increased more than 3-fold in the transgenic mice by Western blotting while mRNA levels for GLUT4 and myocyte enhancer factors, MEF 2A and MEF 2D, protein levels were decreased. We found that GLUT4 can be degraded by calpain-2, suggesting that diminished degradation is responsible for the increase in muscle GLUT4 in the calpastatin transgenic mice. Despite the increase in GLUT4, glucose transport into isolated muscles from transgenic mice was not increased in response to insulin. The expression of protein kinase B was decreased by ∼60% in calpastatin transgenic muscle. This decrease could play a role in accounting for the insulin resistance relative to GLUT4 content of calpastatin transgenic muscle. The muscle weights of transgenic animals were substantially increased compared with controls. These results are consistent with the conclusion that calpain-mediated pathways play an important role in the regulation of GLUT4 degradation in muscle and in the regulation of muscle mass. Inhibition of calpain activity in muscle by overexpression of calpastatin is associated with an increase in GLUT4 protein without a proportional increase in insulin-stimulated glucose transport. These findings provide evidence for a physiological role for calpains in the regulation of muscle glucose metabolism and muscle mass. The experiments in this study were undertaken to determine whether inhibition of calpain activity in skeletal muscle is associated with alterations in muscle metabolism. Transgenic mice that overexpress human calpastatin, an endogenous calpain inhibitor, in skeletal muscle were produced. Compared with wild type controls, muscle calpastatin mice demonstrated normal glucose tolerance. Levels of the glucose transporter GLUT4 were increased more than 3-fold in the transgenic mice by Western blotting while mRNA levels for GLUT4 and myocyte enhancer factors, MEF 2A and MEF 2D, protein levels were decreased. We found that GLUT4 can be degraded by calpain-2, suggesting that diminished degradation is responsible for the increase in muscle GLUT4 in the calpastatin transgenic mice. Despite the increase in GLUT4, glucose transport into isolated muscles from transgenic mice was not increased in response to insulin. The expression of protein kinase B was decreased by ∼60% in calpastatin transgenic muscle. This decrease could play a role in accounting for the insulin resistance relative to GLUT4 content of calpastatin transgenic muscle. The muscle weights of transgenic animals were substantially increased compared with controls. These results are consistent with the conclusion that calpain-mediated pathways play an important role in the regulation of GLUT4 degradation in muscle and in the regulation of muscle mass. Inhibition of calpain activity in muscle by overexpression of calpastatin is associated with an increase in GLUT4 protein without a proportional increase in insulin-stimulated glucose transport. These findings provide evidence for a physiological role for calpains in the regulation of muscle glucose metabolism and muscle mass. The presence of calpains, calcium-activated proteases, in mammalian cells was first reported over 30 years ago (1Guroff G. J. Biol. Chem. 1964; 239: 149-155Abstract Full Text PDF PubMed Google Scholar). Since that time at least 14 members of the calpain family have been identified and their chemistry and biology have been extensively studied (2Haung Y. Wang K.K.W. Trends Mol. Med. 2001; 7: 355-362Abstract Full Text Full Text PDF PubMed Scopus (409) Google Scholar). It has been proposed that alterations in calpain activity result in a number of disease states, including stroke (2Haung Y. Wang K.K.W. Trends Mol. Med. 2001; 7: 355-362Abstract Full Text Full Text PDF PubMed Scopus (409) Google Scholar, 3Wang K.K.W. Yuen P.-W. Trends Pharmacol. Sci. 1997; 15: 412-419Abstract Full Text PDF Scopus (273) Google Scholar), traumatic brain injury (2Haung Y. Wang K.K.W. Trends Mol. Med. 2001; 7: 355-362Abstract Full Text Full Text PDF PubMed Scopus (409) Google Scholar, 3Wang K.K.W. Yuen P.-W. Trends Pharmacol. Sci. 1997; 15: 412-419Abstract Full Text PDF Scopus (273) Google Scholar), Alzheimer's disease (2Haung Y. Wang K.K.W. Trends Mol. Med. 2001; 7: 355-362Abstract Full Text Full Text PDF PubMed Scopus (409) Google Scholar, 3Wang K.K.W. Yuen P.-W. Trends Pharmacol. Sci. 1997; 15: 412-419Abstract Full Text PDF Scopus (273) Google Scholar), cataracts (3Wang K.K.W. Yuen P.-W. Trends Pharmacol. Sci. 1997; 15: 412-419Abstract Full Text PDF Scopus (273) Google Scholar), limb-girdle muscular dystrophy (2Haung Y. Wang K.K.W. Trends Mol. Med. 2001; 7: 355-362Abstract Full Text Full Text PDF PubMed Scopus (409) Google Scholar, 3Wang K.K.W. Yuen P.-W. Trends Pharmacol. Sci. 1997; 15: 412-419Abstract Full Text PDF Scopus (273) Google Scholar) and gastric cancer (2Haung Y. Wang K.K.W. Trends Mol. Med. 2001; 7: 355-362Abstract Full Text Full Text PDF PubMed Scopus (409) Google Scholar). Recent studies of the genetic basis of type 2 diabetes revealed that genetic variation in calpain 10 accounted for a significant component of the genetic risk for diabetes in a Mexican-American population (4Horikawa Y. Oda N. Cox N.J. Li X. Melander M.O. Hara M. Hinokio Y. Lindner T.H. Mashima H. Schwarz P.E.H. Plata L.B. Horikawa Y. Oda Y. Yoshiuchi I. Colilla S. Polonsky K.S. Wei H. Concannon P. Iwasaki N. Shulze J. Baier L.J. Bogardus C. Groop L. Boerwinkle E. Hanis C.L. Bell G.I. Nat. Genet. 2000; 26: 163-175Crossref PubMed Scopus (1263) Google Scholar). Subsequent studies have confirmed the role of calpain 10 as a diabetes susceptibility gene in some (5Hanis C.L. Boerwinkle E. Chakraborty R. Ellsworth D.L. Concannon P. Stirling B. Morrison V.A. Wapelhorst B. Spielman R.S. GogolinEwens K.J. Shepard J.M. Williams S.R. Risch N. Hinds D. Iwasaki N. Ogata M. Omori Y. Petzold C. Reitzch H. Schroder H.E. Schulze J. Cox N.J. Menzel S. Boriraj V.V. Chen X. Lim L.R. Linder T. Mereu L.E. Wang Y.Q. Xiang K. Yamagta K. Yang Y. Bell G.I. Nat. Genet. 1996; 13: 161-166Crossref PubMed Scopus (557) Google Scholar) but not all (6Duggirala R. Blangero J. Aomasy L. Dyer T.D. Williams K.L. Leach R.J. O'Connell P. 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The pathophysiological mechanism(s) whereby genetic variation in a calpain gene could lead to alterations in glucose tolerance are not known, and only a limited number of studies have examined the potential effects of alterations in calpain activity on insulin secretion or insulin action. These studies indicate that a reduction in calpain activity caused by calpain inhibitors induces a state of insulin resistance in isolated muscle strips (14Sreenan S.K. Zhou Y.P. Otani K. Hansen P.A. Currie K.P.M. Pan C.Y. Lee J.P. Ostrega D.M. Pugh W. Horikawa Y. Cox N.J. Hanis C.L. Burant C.F. Fox A.P. Bell G.I. Polonsky K.S. Diabetes. 2001; 50: 2013-2020Crossref PubMed Scopus (145) Google Scholar). Pancreatic islets exposed to calpain inhibitors for 4–6 h demonstrated increased glucose-induced insulin secretion, whereas longer periods of exposure induced significant defects in insulin secretory responses to glucose and other secretagogues (14Sreenan S.K. Zhou Y.P. Otani K. Hansen P.A. Currie K.P.M. Pan C.Y. Lee J.P. Ostrega D.M. Pugh W. Horikawa Y. Cox N.J. Hanis C.L. Burant C.F. Fox A.P. Bell G.I. Polonsky K.S. Diabetes. 2001; 50: 2013-2020Crossref PubMed Scopus (145) Google Scholar). These studies are limited by the fact that all experiments were conducted in vitro, and nonspecific effects of the calpain inhibitors, although unlikely, cannot be completely excluded. To examine the effects of inhibiting calpain activity using another in vivo experimental system, we produced transgenic mice that overexpress the endogenous calpain inhibitor, calpastatin. Generation of MCK-hCAST Transgenic Mice—Full-length human calpastatin (hCAST) 1The abbreviations used are: hCAST, human calpastatin; GLUT4, glucose transporter 4; MEF, myocyte enhancer factor; mMCK, mouse muscle-specific creatine kinase; EDL, extensor digitorum longus; WT, wild type; CsTg, calpastatin overexpression transgenic. corresponding to the 673-amino acid isoform described in GenBank™ accession number 1611327A was obtained using human pancreatic islet cDNA as template and amplified with forward and reverse primers 5′-TGG TGC AAC CAG CAA GTC TTC-3′ and 5′-GGA TGT TCA GAG ACT CAA C-3′, respectively. The PCR product was subcloned into pCR2.1-TOPO (Invitrogen) and the sequence of the insert confirmed. The insert was excised by digestion with EcoRI and cloned into the EcoRI site of the mouse muscle-specific creatine kinase (mMCK) promoter-bovine growth hormone (bGH) polyadenylation signal vector (15Zhu X. Hadhazy M. Groh M.E. Wheeler M.T. Wollmann R. McNally E.M. J. Biol. Chem. 2001; 276: 21785-21790Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar). The 4.28-kb mMCK-hCAST-bGH was excised from the vector by digestion with HhaI and purified by sucrose gradient centrifugation (Fig. 1). Transgenic mice were generated by microinjection of transgene DNA into the pronucleus of fertilized single-cell C57BL6 embryos at DNX Transgenic Sciences (Princeton, NJ). Animals were housed in a room maintained at 23 °C with a fixed 12-h light-dark cycle and given free access to Purina chow and water. After an overnight fast, mice were anesthetized by an intraperitoneal injection of pentobarbital sodium (5 mg/100 g body weight), and the soleus, epitrochlearis, extensor digitorum longus (EDL), and other muscles were excised. All experimental procedures were approved by the Washington University Animal Study Committee. DNA, RNA, and Protein Analysis—The mMCK-hCAST transgene was detected in transgenic mice by PCR analysis of tail DNA using forward and reverse primers 5′-GGC AAC GAG CTG AAA GCT CAT C-3′ and 5′-CAG TGA TAC CAG CAA CAC TCT CTC CAC C-3′. Total RNA was isolated from skeletal muscle using TRIzol reagent (Invitrogen) and suspended in Dnase- Rnase-free distilled water (Invitrogen). Mouse GLUT4 mRNA levels were determined by competitive RT-PCR using GLUT4 forward primer plus nested forward primer 5′-TCA ATG CCC CAC AGA AGG TGT CAA TGG TTG GGA AGG AAA AGG-3′ and reverse primer 5′-AAC CAG AAT GCC AAT GAC GAT G-3′ as described previously (34Garcia-Roves P.M. Han D.-H. Song Z. Jones T.E. Hucker K.A. Holloszy J.O. Am. J. Physiol. 2003; 285: E729-E736Crossref PubMed Scopus (74) Google Scholar). Total protein extracts were prepared by homogenizing muscle in 250 mm sucrose containing 20 mm HEPES and 1 mm EDTA, pH 7.4. Protein was resolved by SDS-PAGE and transferred to nitrocellulose membranes (Amersham Biosciences). The membranes were blocked overnight at 4 °C with 5% nonfat milk in phosphate-buffered saline containing 0.1% Tween 20. The membranes were probed with the following primary antibodies: polyclonal anti-CAST (Calbiochem, La Jolla, CA), anti-GLUT1 and -GLUT4 (a generous gift from Dr. Mike Mueckler, Washington University), anti-MEF 2A (Santa Cruz Biotechnology, Santa Cruz, CA), and anti-MEF 2D (Transduction Laboratories, Lexington, KY) as well as anti-insulin receptor β subunit, anti-insulin receptor substrate-1, anti-insulin receptor substrate-2, monoclonal anti-Akt/PKB, and anti-phosphatidylinositol 3-kinase, all from Upstate Cell Signaling Solution, Lake Placid, NY. Horseradish peroxidase-conjugated secondary antibodies were obtained from Jackson ImmunoResearch Laboratories (West Grove, PA), and reagents for enhanced chemiluminescence were obtained from Amersham Biosciences. Physiological Studies—Intraperitoneal glucose tolerance tests were performed following a 4-h fast. Blood was sampled from the tail vein prior to and 30, 60, and 120 min after injection of dextrose intraperitoneally (2 g/kg body weight). For glucose transport studies, epitrochlearis, soleus, and EDL muscles were removed and allowed to recover from dissection by incubation in 2 ml of Krebs-Henseleit buffer (KHB) containing 8 mm glucose, 32 mm mannitol, and 0.1% bovine serum albumin for 1 h at 35 °C in a shaking incubator. This was followed by 30 min of incubation in the same medium with or without a maximally effective insulin concentration (2 microunits/ml). The gas phase during the incubation was 95% O2, 5%CO2. The muscles were then washed for 10 min in KHB containing 40 mm mannitol, 0.1% bovine serum albumin, and insulin (if it was present in the previous incubation) to remove glucose from the extracellular space. Muscles were then transferred to 1.5 ml of KHB containing 3 mm 2-deoxy-d-[1,2-3H]glucose (2DG) (1.5 μCi/ml), 36 mm [14C]mannitol (0.2 μCi/ml), 0.1% bovine serum albumin, and insulin (if it was present in the previous incubation). Extracellular space and intracellular 2DG concentration were then determined as described previously (16Young D.A. Uhl J.J. Cartee G.D. Holloszy J.O. J. Biol. Chem. 1986; 261: 16049-16053Abstract Full Text PDF PubMed Google Scholar). Measurement of Muscle Weight—The soleus, EDL, tibialis anterior, gastrocnemius, and quadriceps muscles were dissected out using a consistent surgical approach. The muscles were weighed immediately following dissection after blood had been removed from the muscle surface. Measurement of Muscle Glycogen Content—Glycogen content was measured in perchloric acid extracts of muscle by the amyloglucosidase method (17Passonneau J.V. Lauderdale V.R. Anal. Biochem. 1974; 60: 405-412Crossref PubMed Scopus (635) Google Scholar). In Vitro Cleavage of Human GLUT4 by Calpain-2—Human GLUT4 (hGLUT4) protein was prepared using a coupled in vitro transcription and translation system (Promega, Madison, WI). One microgram of the hGLUT4-plasmid construct pGEM4Z-AMT7 (18Fukumoto H. Kayno T. Buse J.B. Edwards Y. Pilch P.F. Bell G.I. Seino S. J. Biol. Chem. 1989; 264: 7776-7779Abstract Full Text PDF PubMed Google Scholar) was incubated in the coupled transcription and translation reaction with 20 μCi of [35S]Met in a volume of 50 μl to generate [35S]-labeled hGLUT4. Unlabeled hGLUT4 was prepared by using 20 μm Met in the coupled transcription and translation reaction. Four microliters of the coupled transcription and translation reaction was incubated with 1.7 units of recombinant rat calpain-2 (Calbiochem) in 20 μl of calpain cleavage buffer (150 mm NaCl, 20 mm Tris-HCl (pH 7.4), 1 mm dithiothreitol, and 0.1% bovine serum albumin). Calpain was activated by addition of CaCl2 to 5 mm. Cleavage was terminated after 5 and 30 min by addition of 20 μl of SDS-sample buffer (0.125 m Tris-HCl (pH 6.8), 4% SDS, 20% glycerol, 0.2 m dithiothreitol, and 0.02% bromphenol blue). Samples were heated at 55 °C for 15 min and analyzed on a 4–20% SDS-PAGE followed by autoradiography or Western blotting. The Western blot was developed using a polyclonal anti-GLUT4 antibody generated using a cytoplasmic 12-amino acid C-terminal peptide from mouse GLUT4 (Alpha Diagnostic, San Antonio, TX). Generation of Transgenic Mice Overexpressing Calpastatin in Muscle—We obtained five founders (two females and three males). Human calpastatin was expressed in muscle (soleus and EDL) from all five founders. The founder used to generate the mice used in the present experiments showed a substantially greater level of calpastatin expression (∼20-fold) than the other founders, which all showed a similar level of overexpression. All transgenic mice were heterozygous. All were used around 4 months of age. Calpastatin Expression by Western Blot—Overexpression of human calpastatin in muscles was confirmed by Western blot (Fig. 2). Although human calpastatin was not expressed in muscle from WT mice, substantial expression was detected in transgenic (CsTg) mice. Mouse Body Weight and Glucose Tolerance—The body weights of male WT mice averaged 28.1 ± 0.6 g compared with 31.2 ± 0.5 g for the CsTg group (p <0.01, 8 mice per group, 23–25 weeks old). For the female mice, body weight averaged 21.8 ± 0.3 g for the WT group and 23.2 ± 0.4 g for the CsTg group (p <0.01, n = 16 per group, 23–25 weeks old). Differences in the intraperitoneal glucose tolerance test between the WT and the CsTg mice (WT versus CsTg: 182.3 ± 6.9 mg/dl versus 197.9 ± 9.3 mg/ dl at fasting, 189.3 ± 7.0 mg/ dl versus 198.3 ± 10.0 mg/ dl at 2 h after injection) were not statistically significant. Insulin Action in Muscle from CsTg Mice—The increase in 2-deoxy-d-[1,2-3H] glucose transport in response to a maximally effective insulin concentration (2 microunits/ml) was not significantly different in muscles from CsTg mice as compared with the WT controls in either EDL (Fig. 3), soleus (data not shown), or epitrochlearis (data not shown). Basal rates of glucose transport were also not different. GLUT4 Glucose Transporter Expression—As shown in Fig. 4, GLUT4 protein content of EDL, soleus, triceps, and tibialis anterior muscles was increased more than 3-fold in CsTg mice. The magnitude of insulin-stimulated muscle glucose transport is normally directly proportional to muscle GLUT4 content. This relationship is evident when muscle fiber types with different GLUT4 contents are compared (19Henriksen E.J. Bourey R.E. Rodnick K.J. Koranyi L. Permutt M.A. Holloszy J.O. Am. J. Physiol. 1990; 259: E593-E598Crossref PubMed Google Scholar), and in muscles that have undergone an adaptive increase in GLUT4 protein (20Weinstein S.P. Watts J. Haber R.S. Endocrinology. 1991; 129: 455-464Crossref PubMed Scopus (78) Google Scholar, 21Ren J.M. Semenkovich C.F. Holloszy J.O. Am. J. Physiol. 1993; 264: C146-C150Crossref PubMed Google Scholar, 22Ren J.-M. Semenkovich C.F. Gulve E.A. Gao J. Holloszy J.O. J. Biol. Chem. 1994; 269: 14396-14401Abstract Full Text PDF PubMed Google Scholar). Thus, the finding that stimulated glucose transport was the same in the CsTg as in the WT muscles despite 3-fold increases in the GLUT4 protein indicates that the increase in GLUT4 protein associated with calpastatin overexpression did not cause the expected increase in glucose transport in response to stimulation by insulin. GLUT4 mRNA Levels—Increases in GLUT4 expression induced by various adaptive stimuli may be mediated at the transcriptional level, as evidenced by an increased GLUT4 mRNA (23MacLean P.S. Zheng D. Jones J.P. Olson A.L. Dohm G.L. Biochem. Biophys. Res. Commun. 2002; 292: 409-414Crossref PubMed Scopus (68) Google Scholar). However, GLUT4 mRNA was significantly decreased in muscles from CsTg mice (Fig. 5), suggesting that the increase in GLUT4 protein was mediated by a post-transcriptional mechanism. Further support for this hypothesis is provided by the finding that MEF 2A and MEF 2D protein levels were decreased ∼50% in the CsTg muscles. Expression of GLUT4 in striated muscle is dependent on binding of a MEF 2A-MEF 2D heterodimer to the GLUT4 promoter (24Mora S. Pessin J.E. J. Biol. Chem. 2000; 275: 16323-16328Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar). We have found that stimuli that induce increased GLUT4 expression also result in increases in MEF 2A and MEF 2D (25Ojuka E.O. Jones T.E. Nolte L.A. Chen M. Wamhoff B.R. Sturek M. Holloszy J.O. Am. J. Physiol. 2002; 282: E1008-E1013Crossref PubMed Scopus (184) Google Scholar). As shown in Fig. 6, A and B, both MEF 2A and MEF 2D were significantly decreased in muscles from CsTg mice compared with WT controls.Fig. 6Measurement of immunoreactive MEF 2A and 2D. A, MEF 2A. B, MEF 2D. Myocyte enhancer factors MEF 2A and 2D were measured by Western blot. Expression of GLUT4 in striated muscle is dependent on binding of MEF 2A-2D heterodimer to the GLUT4 promoter. Values are means ± S.E. Triceps (n = 8), tibialis anterior (n = 8), and gastrocnemius (n = 6). *, p <0.05; **, p <0.01; ***, p <0.001.View Large Image Figure ViewerDownload (PPT) GLUT1 Glucose Transporter Expression—The finding that GLUT4 protein is increased in muscles of the CsTg mice, despite a decrease in GLUT4 mRNA, led us to hypothesize that GLUT4 is a calpain substrate and that inhibition of calpain by calpastatin is responsible for the increase in GLUT4. If this hypothesis is correct, one might also expect to see an increase in GLUT1 protein because its amino acid sequence is similar to that of GLUT4 (26Mueckler M. Eur. J. Biochem. 1994; 219: 713-725Crossref PubMed Scopus (967) Google Scholar). We, therefore, measured the GLUT1 protein level and found that it was also increased in skeletal muscle of the CsTg mice. Although highly significant (controls, 1.04 ± 0.14 versus CsTg, 1.66 ± 0.14 arbitrary units, mean ± S.E. for 16 muscles per group, p <0.004), the magnitude of the increase was smaller than that of GLUT4. Muscle Glycogen—Glucose transport is the primary ratelimiting step that determines the rate and extent of muscle glycogen accumulation (27Ren J.M. Marshall B.A. Gulve E.A. Gao J. Johnson D.W. Holloszy J.O. Mueckler M. J. Biol. Chem. 1993; 268: 16113-16115Abstract Full Text PDF PubMed Google Scholar). Increases in muscle GLUT4 are, therefore, generally associated with an increase in muscle glycogen content (20Weinstein S.P. Watts J. Haber R.S. Endocrinology. 1991; 129: 455-464Crossref PubMed Scopus (78) Google Scholar, 21Ren J.M. Semenkovich C.F. Holloszy J.O. Am. J. Physiol. 1993; 264: C146-C150Crossref PubMed Google Scholar, 22Ren J.-M. Semenkovich C.F. Gulve E.A. Gao J. Holloszy J.O. J. Biol. Chem. 1994; 269: 14396-14401Abstract Full Text PDF PubMed Google Scholar, 28Hansen P.A. Gulve E.A. Marshall B.A. Gao J. Pessin J.E. Holloszy J.O. Mueckler M. J. Biol. Chem. 1995; 270: 1679-1684Abstract Full Text Full Text PDF PubMed Google Scholar, 29Greiwe J.S. Hickner R.C. Hansen P.A. Racette S.B. Chen M.M. Holloszy J.O. J. Appl. Physiol. 1999; 87: 222-226Crossref PubMed Scopus (103) Google Scholar). Muscle glycogen concentrations were significantly increased in muscles of the CsTg mice, 20.4 ± 0.9 μmol/g EDL WT muscle compared with 46.0 ± 2.7 μmol/g EDL CsTg muscle. Effect of Muscle Glycogen Depletion on Glucose Transport— Accumulation of glycogen in muscle may induce insulin resistance. To determine whether the increase in glycogen in muscles from CsTg mice was masking an increase in muscle glucose transport in CsTg muscles, we exposed muscles to hypoxia for 80 min, a maneuver that reduces glycogen. The glucose concentration in the incubation medium for the CsTg muscles was also reduced compared with the incubations for the WT muscles (2 and 8 mm glucose, respectively). This approach markedly reduced glycogen concentrations to ∼7 μmol/g muscle wet weight. However, even after a reduction in muscle glycogen there were no significant differences in glucose transport induced by 2 microunits/ml insulin and hypoxia in the EDL and the soleus muscles of the CsTg compared with the WT mice (data not shown). Thus, the increase in GLUT4 resulting from calpain inhibition was not associated with a parallel increase in muscle glucose transport. Proteins of the Insulin Signaling Pathway—As a next step in our investigation of the mechanism responsible for the relative insulin resistance of the CsTg muscles, we measured the levels of some of the proteins of the insulin signaling pathway. Expression of the insulin receptor, insulin receptor substrates 1 and 2, and phosphatidylinositol 3-kinase proteins was similar in skeletal muscle of the transgenic and wild type mice (Fig. 7). However, there was a remarkable (∼60%) decrease in protein kinase B in skeletal muscle of the CsTg mice (Fig. 7). This finding could explain why insulin-stimulated glucose transport is not increased in CsTg muscle despite a ∼3-fold increase in GLUT4. Cleavage of Human GLUT4 in Vitro by Calpain-2—To determine whether human GLUT4 is a calpain cleavage substrate, we incubated 35S-labeled recombinant hGLUT4 with purified calpain-2. The recombinant hGLUT4 has a mass of 43.7 kDa, and incubation with activated calpain-2 generated a protein with an estimated mass of 37.2 kDa (Fig. 8A). This fragment was not identified by an antibody that binds to an epitope at the C terminus of GLUT4 (Fig. 8B), suggesting the calpain-2 cleavage site is located in the C-terminal portion of GLUT4. The mass of the cleavage product suggests that the calpain-sensitive site is about 60–70 residues from the C terminus. Effects on Muscle Mass—To determine whether overexpression of calpastatin affected muscle mass, muscles were systematically weighed in 23–25-week-old mice. Weight of EDL and soleus muscles was increased 12% (EDL, males), 33% (EDL, females), 17% (soleus, males), and 19% (soleus, females), respectively (Fig. 9A). Similar increases in mass were present in the other muscles, including the tibialis anterior (42% in males, 47% in females), gastrocnemius (13% in males, 14% in females), and quadriceps (21% in males, 14% in females) (Fig. 9B). Total muscle protein was increased in proportion to muscle weight in EDL (WT versus CsTg: 1.96 ± 0.10 mg versus 2.40 ± 0.10 mg, p <0.01) and soleus (WT versus CsTg: 1.58 ± 0.08 mg versus 1.85 ± 0.10 mg, p <0.05). The present study was designed to determine whether reducing calpain expression in muscle is associated with alterations in muscle glucose metabolism. A transgenic mouse with muscle-specific overexpression of calpastatin was used as the experimental model. The transgenic mice did not have overt changes in glucose tolerance and did not appear to be insulin-resistant in vivo. However, in vitro studies of muscle revealed a number of interesting effects of calpastatin overexpression. One important change observed was a greater than 3-fold increase in GLUT4 protein detected by Western blotting. The increase was found consistently in a number of different muscles, including EDL, soleus, triceps, and tibialis anterior. It is most likely that the increase in GLUT4 is due to a reduction in the rate of GLUT4 breakdown for the following reasons. GLUT4 mRNA was not increased and was actually decreased, as were protein levels for MEF 2A and 2D, two transcription factors that mediate induction of GLUT4 expression (24Mora S. Pessin J.E. J. Biol. Chem. 2000; 275: 16323-16328Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar). Our hypothesis is that GLUT4 serves as a calpain substrate and that the reduction in calpain activity resulting from increased expression of calpastatin leads to the increase in GLUT4 levels observed. The results of additional experiments that demonstrated that GLUT4 is a substrate for calpain-2 in vitro are consistent with this idea, as is the finding that GLUT1, which has an amino acid sequence similar to that of GLUT4 (26Mueckler M. Eur. J. Biochem. 1994; 219: 713-725Crossref PubMed Scopus (967) Google Scholar), was also increased. Previous experiments have shown that pharmacological inhibition of calpain causes insulin resistance of glucose transport in skeletal muscle (14Sreenan S.K. Zhou Y.P. Otani K. Hansen P.A. Currie K.P.M. Pan C.Y. Lee J.P. Ostrega D.M. Pugh W. Horikawa Y. Cox N.J. Hanis C.L. Burant C.F. Fox A.P. Bell G.I. Polonsky K.S. Diabetes. 2001; 50: 2013-2020Crossref PubMed Scopus (145) Google Scholar). In view of this observation, we expected that overexpression of calpastatin, an endogenous calpain inhibitor, would result in severe skeletal muscle insulin resistance. Our finding that insulin-stimulated glucose transport was normal in CsTg muscles provides evidence suggesting that the greater than 3-fold increase in GLUT4 protein induced by calpastatin overexpression compensated for insulin resistance caused by inhibition of calpain activity. Maximally insulin-stimulated glucose transport is normally directly proportional to the GLUT4 protein content of a muscle (19Henriksen E.J. Bourey R.E. Rodnick K.J. Koranyi L. Permutt M.A. Holloszy J.O. Am. J. Physiol. 1990; 259: E593-E598Crossref PubMed Google Scholar, 21Ren J.M. Semenkovich C.F. Holloszy J.O. Am. J. Physiol. 1993; 264: C146-C150Crossref PubMed Google Scholar, 22Ren J.-M. Semenkovich C.F. Gulve E.A. Gao J. Holloszy J.O. J. Biol. Chem. 1994; 269: 14396-14401Abstract Full Text PDF PubMed Google Scholar). Therefore, the finding that glucose transport rates were the same in muscles of the WT and CsTg mice, despite the higher GLUT4 content of the CsTg muscles, provides evidence for a severe resistance to stimulation of glucose transport of the CsTg muscles relative to their GLUT4 content. Our finding of an ∼60% reduction in protein kinase B expression in CsTg skeletal muscle raises the possibility that an attenuation of insulin signaling via this step may play a role in mediating the relative insulin resistance. The marked decrease in expression of MEF 2A, MEF 2D, and protein kinase B provides evidence suggesting that the large increase in GLUT4 directly or by means of a secondary effect, such as the increase in glycogen, results in suppression of the expression of these proteins. It used to be thought that calpains play a major role in muscle protein degradation only when cytosolic Ca2+ homeostasis is disturbed by trauma or ischemia and in muscular dystrophies (30Lecker S.H. Solomon V. Mitch W.E. Goldberg A.L. J. Nutr. 1999; 129: 227S-237SCrossref PubMed Google Scholar, 31Tidball J.G. Spencer J.M. Int. J. Biochem. Cell Biol. 2000; 32: 1-5Crossref PubMed Scopus (113) Google Scholar). The role of calpains in protein turnover in normal skeletal muscle is still unclear. However, evidence is accumulating that calpains mediate the initiating steps in the turnover of myofibrillar proteins. It appears that calpains are responsible for release of filaments from the myofibrils, making them available for degradation by the proteasome (32Goll D.E. Thompson V.F. Li H. Wei W. Cong J. Physiol. Rev. 2003; 83: 731-801Crossref PubMed Scopus (2423) Google Scholar). It was recently reported that overexpression of calpastatin in skeletal muscle protects mice against the muscle atrophy associated with hindlimb unloading (33Tidball J.G. Spencer M.J. J. Physiol. 2002; 545: 819-828Crossref PubMed Scopus (173) Google Scholar). The present finding that calpastatin overexpression results in skeletal muscle hypertrophy provides evidence that calpains are also involved in normal skeletal muscle protein turnover. The implications of the results of this study for our understanding of the pathophysiology of type 2 diabetes are unclear. Genetic variation in a specific calpain, calpain 10, has been associated with altered risk for type 2 diabetes (4Horikawa Y. Oda N. Cox N.J. Li X. Melander M.O. Hara M. Hinokio Y. Lindner T.H. Mashima H. Schwarz P.E.H. Plata L.B. Horikawa Y. Oda Y. Yoshiuchi I. Colilla S. Polonsky K.S. Wei H. Concannon P. Iwasaki N. Shulze J. Baier L.J. Bogardus C. Groop L. Boerwinkle E. Hanis C.L. Bell G.I. Nat. Genet. 2000; 26: 163-175Crossref PubMed Scopus (1263) Google Scholar, 5Hanis C.L. Boerwinkle E. Chakraborty R. Ellsworth D.L. Concannon P. Stirling B. Morrison V.A. Wapelhorst B. Spielman R.S. GogolinEwens K.J. Shepard J.M. Williams S.R. Risch N. Hinds D. Iwasaki N. Ogata M. Omori Y. Petzold C. Reitzch H. Schroder H.E. Schulze J. Cox N.J. Menzel S. Boriraj V.V. Chen X. Lim L.R. Linder T. Mereu L.E. Wang Y.Q. Xiang K. Yamagta K. Yang Y. Bell G.I. Nat. Genet. 1996; 13: 161-166Crossref PubMed Scopus (557) Google Scholar). Although calpastatin inhibits the activity of calpains I and II, it has not been determined whether it has similar effects on calpain 10. Nevertheless, our results provide further evidence that inhibition of calpain activity causes insulin resistance. This finding implies that calpain plays a role in the stimulation of glucose transport. Our results also provide additional new information indicating that inhibition of calpain activity in skeletal muscle results in an increase in GLUT4 protein and causes muscle hypertrophy. These findings strongly suggest that calpains play important roles in GLUT4 and skeletal muscle proteolysis. The support of the Kovler Foundation is gratefully acknowledged.
Exercise induces a rapid increase in expression of the GLUT4 isoform of the glucose transporter in skeletal muscle. One of the signals responsible for this adaptation appears to be an increase in cytosolic Ca2+. Myocyte enhancer factor 2A (MEF2A) is a transcription factor that is involved in the regulation of GLUT4 expression. It has been reported that the Ca2+-regulated phosphatase calcineurin mediates the activation of MEF2 by exercise. It has also been shown that the expression of activated calcineurin in mouse skeletal muscle results in an increase in GLUT4. These findings suggest that increases in cytosolic Ca2+ induce increased GLUT4 expression by activating calcineurin. However, we have obtained evidence that this response is mediated by a Ca2+-calmodulin−dependent protein kinase. The purpose of this study was to test the hypothesis that calcineurin is involved in mediating exercise-induced increases in GLUT4. Rats were exercised on 5 successive days using a swimming protocol. One group of swimmers was given 20 mg/kg body weight of cyclosporin, a calcineurin inhibitor, 2 h before exercise. A second group was given vehicle. GLUT4 protein was increased ∼80%, GLUT4 mRNA was increased ∼2.5-fold, MEF2A protein was increased twofold, and hexokinase II protein was increased ∼2.5-fold 18 h after the last exercise bout. The cyclosporin treatment completely inhibited calcineurin activity but did not affect the adaptive increases in GLUT4, MEF2A, or hexokinase expression. We conclude that calcineurin activation does not mediate the adaptive increase in GLUT4 expression induced in skeletal muscle by exercise.
It was previously found that transgenic mice that overexpress the calpain inhibitor calpastatin (CsTg) have an approximately 3-fold increase in GLUT4 protein in their skeletal muscles. Despite the increase in GLUT4, which appears to be due to inhibition of its proteolysis by calpain, insulin-stimulated glucose transport is not increased in CsTg muscles. PKB (Akt) protein level is reduced approximately 60% in CsTg muscles, suggesting a possible mechanism for the relative insulin resistance. Muscle contractions stimulate glucose transport by a mechanism that is independent of insulin signaling. The purpose of this study was to test the hypothesis that the threefold increase in GLUT4 in CsTg would result in a large increase in contraction-stimulated glucose transport. CAMKII and AMPK mediate steps in the contraction-stimulated pathway. The protein levels of AMPK and CAMKII were increased three- to fourfold in CsTg muscles, suggesting that these proteins are also calpain substrates. Despite the large increases in GLUT4, AMPK, and CAMKII, contraction-stimulated GLUT4 translocation and glucose transport were not increased above wild-type values. These findings suggest that inhibition of calpain results in impairment of a step in the GLUT4 translocation process downstream of the insulin- and contraction-signaling pathways. They also provide evidence that CAMKII and AMPK are calpain substrates.
Cells are programmed to die when critical signaling and metabolic pathways are disrupted. Inhibiting the type 2 ryanodine receptor (RyR2) in human and mouse pancreatic β-cells markedly increased apoptosis. This mode of programmed cell death was not associated with robust caspase-3 activation prompting a search for an alternative mechanism. Increased calpain activity and calpain gene expression suggested a role for a calpain-dependent death pathway. Using a combination of pharmacological and genetic approaches, we demonstrated that the calpain-10 isoform mediated ryanodine-induced apoptosis. Apoptosis induced by the fatty acid palmitate and by low glucose also required calpain-10. Ryanodine-induced calpain activation and apoptosis were reversed by glucagon-like peptide or short-term exposure to high glucose. Thus RyR2 activity seems to play an essential role in β-cell survival in vitro by suppressing a death pathway mediated by calpain-10, a type 2 diabetes susceptibility gene with previously unknown function. Cells are programmed to die when critical signaling and metabolic pathways are disrupted. Inhibiting the type 2 ryanodine receptor (RyR2) in human and mouse pancreatic β-cells markedly increased apoptosis. This mode of programmed cell death was not associated with robust caspase-3 activation prompting a search for an alternative mechanism. Increased calpain activity and calpain gene expression suggested a role for a calpain-dependent death pathway. Using a combination of pharmacological and genetic approaches, we demonstrated that the calpain-10 isoform mediated ryanodine-induced apoptosis. Apoptosis induced by the fatty acid palmitate and by low glucose also required calpain-10. Ryanodine-induced calpain activation and apoptosis were reversed by glucagon-like peptide or short-term exposure to high glucose. Thus RyR2 activity seems to play an essential role in β-cell survival in vitro by suppressing a death pathway mediated by calpain-10, a type 2 diabetes susceptibility gene with previously unknown function.
Pancreatic β-cell-restricted knockout of the insulin receptor results in hyperglycemia due to impaired insulin secretion, suggesting that this cell is an important target of insulin action. The present studies were undertaken in β-cell insulin receptor knockout (βIRKO) mice to define the mechanisms underlying the defect in insulin secretion. On the basis of responses to intraperitoneal glucose, ∼7-mo-old βIRKO mice were either diabetic (25%) or normally glucose tolerant (75%). Total insulin content was profoundly reduced in pancreata of mutant mice compared with controls. Both groups also exhibited reduced β-cell mass and islet number. However, insulin mRNA and protein were similar in islets of diabetic and normoglycemic βIRKO mice compared with controls. Insulin secretion in response to insulin secretagogues from the isolated perfused pancreas was markedly reduced in the diabetic βIRKOs and to a lesser degree in the nondiabetic βIRKO group. Pancreatic islets of nondiabetic βIRKO animals also exhibited defects in glyceraldehyde- and KCl-stimulated insulin release that were milder than in the diabetic animals. Gene expression analysis of islets revealed a modest reduction of GLUT2 and glucokinase gene expression in both the nondiabetic and diabetic mutants. Taken together, these data indicate that loss of functional receptors for insulin in β-cells leads primarily to profound defects in postnatal β-cell growth. In addition, altered glucose sensing may also contribute to defective insulin secretion in mutant animals that develop diabetes.
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