Regulation of Calreticulin Expression during Induction of Differentiation in Human Myeloid Cells
Robert A. ClarkSenlin LiDoran W. PearsonKevin G. LeidalJoshua R. ClarkGerene M. DenningRobert L. ReddickKarl-Heinz KrauseAnthony J. Valente
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Calreticulin is a ubiquitous endoplasmic reticulum Ca 2+ binding chaperone. The protein has been implicated in a variety of diverse functions. Calreticulin is a lectin-like chaperone and, together with calnexin, it plays an important role in quality control during protein synthesis, folding, and posttranslational modification. Calreticulin binds Ca 2+ and affects cellular Ca 2+ homeostasis. The protein increases the Ca 2+ storage capacity of the endoplasmic reticulum and modulates the function of endoplasmic reticulum Ca 2+ -ATPase. Calreticulin also plays a role in the control of cell adhesion and steroid-sensitive gene expression. Recently, the protein has been identified and characterized in higher plants but its precise role in plant cells awaits further investigation.Key words: calreticulin, endoplasmic reticulum, chaperone, Ca 2+ binding protein.
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Abstract The endoplasmic reticulum (ER) chaperones are highly conserved proteins that catalyze the posttranslational processing of all secretory and membrane proteins. Our studies suggest that chaperone declines are one of the two central defects in Alzheimer's disease. We propose that similar declines in other organ systems underlie the physiological deficits of aging. Rats were maintained in a colony from age 21 days to death. Animals were killed at regular intervals, and hepatic, ER chaperone contents were determined by immunoblotting. ERp55, ERp57, ERp72, BiP, and calnexin constitutive levels declined 30%–50% with age. Calreticulin was unaffected. BiP (also known as GRP78), ERp55, and ERp57 showed marked swings with peaks occurring in midwinter and midsummer. This cyclics declined 73% with age. Considering the role of the ER chaperones in membrane and secretory protein posttranslational processing, these data support the concept that their loss could lead to many of the physiological declines associated with aging.
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Calreticulin is a ubiquitous endoplasmic reticulum Ca2+ binding chaperone. The protein has been implicated in a variety of diverse functions. Calreticulin is a lectin-like chaperone and, together with calnexin, it plays an important role in quality control during protein synthesis, folding, and posttranslational modification. Calreticulin binds Ca2+ and affects cellular Ca2+ homeostasis. The protein increases the Ca2+ storage capacity of the endoplasmic reticulum and modulates the function of endoplasmic reticulum Ca2+-ATPase. Calreticulin also plays a role in the control of cell adhesion and steroid-sensitive gene expression. Recently, the protein has been identified and characterized in higher plants but its precise role in plant cells awaits further investigation.
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Quality control of the endoplasmic reticulum plays a critical role in protein folding, modification and modification of a secretory pathway. As endoplasmic reticulum chaperones, calreticulin and calnexin have similar substrate specificity and share several common features. Yet, surprisingly, mice bearing a disruption in the calreticulin gene die from a lesion in cardiac development and develop significant metabolic problems whereas calnexin-deficient mice are born alive with, yet not understood, neurological problems. Studies with calreticulin and calnexin gene knockout mice and calreticulin- and calnexin-deficient cell lines indicate that calnexin is unable to compensate for the loss of calreticulin and conversely, calreticulin cannot compensate for the loss of calnexin. Calreticulin or calnexin deficiency or reduction in the level of ERp57 protein (ERp57 heterozygote mice) leads to development of metabolic disorders as documented by sever changes serum lipids and carbohydrates composition in these animals. These observations indicate that calreticulin, calnexin and ERp57, in addition of being involved in maturation of glycoproteins in the endoplasmic reticulum, perform other distinct functions including affecting energy metabolism.
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The present study was addressed on the effect of 3,3',4,4',5-pentachlorobiphenyl (PenCB) to the expression of molecular chaperon proteins, glucose regulated protein (GRP) 78, GRP94, calreticulin and calnexin in liver endoplasmic reticulum of rat by treatment with acute exposure. Male Wistar rats received PenCB in corn oil at once a dose of 10 mg/kg i.p., then at 5 days after treatment the microsomes were prepared. Free-fed and pair-fed control groups were given the vehicle. The microsomal proteins were separated on SDS-PAGE, transferred to membrane and blotted using antibody towards respective chaperone proteins. The protein levels of GRP78, GRP94, calreticulin and calnexin were significantly decreased with the acute exposure. In addition, albumin level in the microsomes was also significantly reduced by the PenCB treatment. The transferrin level was significantly higher than pair-fed but not from free-fed group. These chaperone proteins have important physiological functions against synthesized and/or denatured proteins, which include assembling, folding of proteins. PenCB-treatment may alter the extent of biosynthesis of secretory protein such as albumin through the decreasing levels of chaperone proteins in endoplasmic reticulum. Interestingly, reduced GRP78 protein level by PenCB was not due to decreased mRNA level. Our results suggested that a part of the toxicity of PenCB is associated to significant decrease of the chaperone proteins in the endoplasmic reticulum.
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