There are five major PGs (prostaglandins/prostanoids) produced from arachidonic acid via the COX (cyclo-oxygenase) pathway: PGE2, PGI2 (prostacyclin), PGD2, PGF2α and TXA2 (thromboxane A2). They exert many biological effects through specific G-protein-coupled membrane receptors, namely EP (PGE2 receptor), IP (PGI2 receptor), DP (PGD2 receptor), FP (PGF2α receptor) and TP (TXA2 receptor) respectively. PGs are implicated in physiological and pathological processes in all major organ systems, including cardiovascular function, gastrointestinal responses, reproductive processes, renal effects etc. This review highlights recent insights into the role of each prostanoid in regulating various aspects of renal function, including haemodynamics, renin secretion, growth responses, tubular transport processes and cell fate. A thorough review of the literature since Y2K (year 2000) is provided, with a general overview of PGs and their synthesis enzymes, and then specific considerations of each PG/prostanoid receptor system in the kidney.
Mesangial cells (MG) are an important source of renal PGE2 and PGI2. The purpose of this study was to examine the effects of cicaprost (CCP; PGI2 analog) on MG function and the expression of IP receptors in streptozotocin (STZ)-diabetic rats and glucose-treated MG cells. CCP increased cellular cAMP in immortalized rat MG cells. Both glucose and anisomycin attenuated CCP-cAMP, but not PMA, angiotensin II, or transforming growth factor-beta. Also, IP receptor protein was reduced in response to glucose. While CCP decreased the levels of the cell cycle inhibitor p27, it did not alter thymidine or leucine incorporation. However, CCP reduced fibronectin levels by 40% and increased matrix metalloproteinase-2 levels threefold, a key enzyme in matrix degradation. Finally, IP receptors were significantly reduced in the outer medulla of 4- and 12-wk STZ-diabetic rats and in the cortex, outer, and inner medullary regions in 6-mo uninephrectomized STZ-diabetic rats. The changes in the CCP/IP system observed in this study suggest that IP may serve as an alternate therapeutic target in diabetes.
The collecting duct (CD) expresses considerable amounts of PPAR δ . While its role is unknown in the CD, in other renal cells it has been shown to regulate both growth and apoptosis. We thus hypothesized that PPAR δ reduces apoptotic responses and stimulates cell growth in the mouse CD, and examined the effect of GW501516, a synthetic PPAR δ ligand, on these responses in mouse IMCD‐K2 cells. High doses of GW501516 decreased both DNA and protein synthesis in these cells by 80%, but had no overall effect on cell viability. Although anisomycin treatment resulted in an increase of caspase‐3 levels of about 2.59‐fold of control, GW501516 did not affect anisomycin‐induced changes in active caspase‐3 levels. These results show that a PPAR δ ligand inhibits growth but does not affect anisomycin‐apoptosis in a mouse IMCD cell line. This could have therapeutic implications for renal diseases associated with increased CD growth responses.
Peroxisome proliferator-activated receptor (PPAR)-γ is highly expressed in the collecting duct (CD), yet little is known about the effects of PPAR-γ ligands, thiazolidinediones (TZDs), on CD cell structure and function. M1 mouse cortical CD cells were treated with 5 μM troglitazone (TRO) and rosiglitazone (ROSI). First, growth was measured by [ 3 H]thymidine and [ 3 H]leucine incorporation, as well as analysis of cyclin D1 and the CDK inhibitor p27 by Western blot. [ 3 H]thymidine incorporation was reduced by 56 and 24% by TRO and ROSI at 6 h, and [ 3 H]leucine by 21 and 10%. A similar growth inhibition was also observed after 24 h for thymidine, but leucine was reduced by 48 and 24%, respectively. Likewise, cyclin D1 was diminished 60% by TRO, and p27 was elevated 1.6- and 1.7-fold in response to TRO and ROSI. Next, epithelial cell integrity was assessed by measuring different markers by Western blot analysis. While fibronectin and α-smooth muscle actin levels were unchanged, by 24 h E-cadherin was decreased by 50%, and β-catenin levels were reduced 2- and 1.5-fold in response to TRO and ROSI, respectively. GW9662, a PPAR-γ antagonist, did not reverse any of the TZD responses in M1 cells. Of interest, phosho-p38 levels were also elevated 2-fold in response to TRO and 2.3-fold to ROSI, but MAPK inhibition by PD98059 or SB203580 caused an additive inhibition of cell growth and did not alter E-cadherin or β-catenin in response to TZDs. Finally, apoptotic death was assessed by Western blot, but cleaved caspase-3 levels were unchanged from 15 min to 24 h in response to TZDs, and TRO did not affect cell viability or reactive oxygen species generation. Our data suggest that TZDs cause a disruption of M1 cell integrity that is preceded by an inhibition of cell growth. This response is independent of p38 or PPAR-γ activation.
This study examined the distribution of prostacyclin (PGI2 ) receptors (IP) along the rat nephron, the signaling responses coupled to IP activation, and any alterations in this system in high-glucose treated (HG)-mesangial cells (MG) and streptozotocin (STZ)-diabetic rat kidneys. In situ hybridization, RT-PCR, RealTime RT-PCR, and Northern blot were used to study mRNA, and immunohistochemistry and Western blot for protein. IP mRNA was detected in glomeruli, vasculature, and cortical and medullary tubules on rat tissue sections, and in different preparations: kidney (C: cortex, OM: outer medulla, and IM: inner medulla), cultured MG (primary and transformed), proximal tubule (PT; fresh and cultured), and inner medullary collecting duct (fresh: f-IMCD and cultured: c-IMCD). IP protein was noted in PT, MG, IMCD, OM, and IM, but not cortex.
Our work suggests that IP activation increases CAMP in: primary and immortalized MG, cultured PT, f-IMCD and c-IMCD. Inhibition of vasopressin-cAMP was obtained in f-IMCD. However IP was not linked to any calcium signaling in our studies. And we did not detect any IP subtypes or spliced variants.
The role of PGI2/IP in MG was examined following 24 hr stimulation with cicaprost (CCP). A decrease in fibronectin and p27 was observed in cell lysates, and an increase in matrix metalloproteinase-2; CCP had no effect on thymidine or leucine incorporation. In response to 24 hr and 4 day HG, CCP- and iloprost (ILP)-cAMP was attenuated in MG: immortalized and primary cultures, respectively. CCP-CAMP was also diminished by anisomycin, a MAPK activator.
PGI2 synthesis is dependent on cyclooxygenase (COX) and PGI 2 synthase (PGIS). We show increased COX-1 and -2 in the medulla of STZ rats, as well as HG-IMCD. Only COX-2 was elevated in HG-MG. While PGE 2 synthase was augmented in the OM of STZ rats, PGIS protein was reduced in HG-MG. HG did not affect IP mRNA in vitro, but reduced IP protein. However, IP mRNA was diminished in the OM of STZ rats, and all three-kidney regions in 6-mth uni-nephrectomized/STZ rats. The attenuation of PGI2/IP described in our work suggests that IP may serve as a target to prevent progressive injury to diabetic kidneys.
To clarify the role of the PGI 2 /PGI 2 receptor (IP) system in rabbit cortical collecting duct (RCCD), we characterized the expression of IP receptors in the rabbit kidney. We show by Northern and Western blotting that IP mRNA and protein was detectable in all three regions of the kidney. To determine how PGI 2 signals, we compared the effects of different PGI 2 analogs [iloprost (ILP), carba-prostacyclin (c-PGI 2 ), and cicaprost (CCP)] in the isolated perfused RCCD. PGI 2 analogs did not increase water flow ( L p ). Although PGI 2 analogs did not reduce an established L p response to 8-chlorophenylthio-cAMP, they equipotently inhibited AVP-stimulated L p by 45%. The inhibitory effect of ILP and c-PGI 2 on AVP-stimulated L p is partially reversed by the protein kinase C inhibitor staurosporine and abolished by pertussis toxin; no effect was obtained with CCP. In fura 2-loaded RCCD, CCP did not alter cytosolic Ca 2+ concentration ([Ca 2+ ] i ), but, in the presence of CCP, individual infusion of ILP and PGE 2 increased [Ca 2+ ] i , suggesting that CCP did not cause desensitization to either ILP or PGE 2 . We concluded that ILP and c-PGI 2 activate PKC and the liberation of [Ca 2+ ] i but not CCP. This suggested an important role for phosphatidylinositol hydrolysis in mediating ILP and c-PGI 2 effects but not CCP in RCCD.
