Reduction of rat liver endoplasmic reticulum Ca(2+)-ATPase activity and mobilization of hepatic intracellular calcium by ciprofibrate, a peroxisome proliferator.
Ciprofibrate, a peroxisome proliferating agent, induces cell proliferation in rodent liver during the early periods of exposure. Since Ca2+ plays an important role in mitogenesis, we have investigated the effects of ciprofibrate on hepatic endoplasmic reticulum (ER) Ca(2+)-ATPase, which in part regulates Ca2+ homeostasis. A single oral dose of 200 mg/kg ciprofibrate to male F344 rats produced a transient decrease in liver microsomal Ca(2+)-ATPase activity to 48% of control levels at 24 hr post-exposure. Activity had returned to control levels by 48 and 72 hr after exposure. The decrease in Ca(2+)-ATPase activity was not a function of non-specific enzymatic inhibition, since activity of another microsomal enzyme, glucose-6-phosphatase, was not altered in ciprofibrate-exposed rats. Using an ATP-driven 45Ca2+ accumulation assay, rats exposed to 25, 100 and 200 mg/kg ciprofibrate exhibited a dose-dependent inhibition of liver microsomal Ca2+ accumulation at 24 hr post-exposure. Analysis of Western immunoblots using a polyclonal antibody to the liver ER Ca(2+)-ATPase revealed a marginal increase in Ca(2+)-ATPase protein content in microsomes prepared from ciprofibrate-exposed rats compared to controls 24 hr post-exposure. These data indicate that the reduction of Ca(2+)-ATPase activity is not attributable to diminished Ca(2+)-ATPase protein content in vivo and, therefore, is due to a functional inhibition of the enzyme. Ciprofibrate also produced a concentration-dependent inhibition of rat liver ER Ca(2+)-ATPase activity in vitro (IC50 approximately 170 microM). In freshly isolated rat hepatocytes, ciprofibrate elevated the free intracellular calcium concentration ([Ca2+]i) in the presence and absence of extracellular calcium. Collectively, these results suggest that ciprofibrate mobilizes hepatic [Ca2+]i via inhibition of the ER Ca(2+)-ATPase. These events may lead to an environment of elevated [Ca2+]i during the early stages of ciprofibrate exposure and may serve to augment Ca(2+)-dependent processes, thus playing a pivotal role in the acute mitogenic response.
Bennett
A M
AM
American Health Foundation, Valhalla, NY 10595.
Williams
G M
GM
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000096:biosynthesis*; D002995:Clofibric Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000201:enzymology; D058607:Fibric Acids; D008099:Liver / Q000187:drug effects* / Q000201:enzymology; D008297:Male; D008830:Microbodies / Q000187:drug effects*; D008862:Microsomes, Liver / Q000187:drug effects; D009929:Organ Size; D051381:Rats; D011916:Rats, Inbred F344; D013997:Time Factors
D058607:Fibric Acids; D002995:Clofibric Acid; D000252:Calcium-Transporting ATPases; C019304:ciprofibrate; D002118:Calcium