REDUCTION OF RAT-LIVER ENDOPLASMIC-RETICULUM CA2+-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) Ca2+-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 Ca2+-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 Ca2+-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 Ca-45(2+) 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 Ca2+-ATPase revealed a marginal increase in Ca2+-ATPase protein content in microsomes prepared from ciprofibrate-exposed rats compared to controls 24 hr post-exposure. These data indicate that the reduction of Ca2+-ATPase activity is not attributable to diminished Ca2+-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 Ca2+-ATPase activity in vitro (IC50 almost-equal-to 170-mu-M). 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 Ca2+-ATPase. These events may lead to an environment of elevated [Ca2+]i during the early stages of ciprofibrate exposure and may serve to augment Ca2+-dependent processes, thus playing a pivotal role in the acute mitogenic response.