Bennett A M, Williams G M
American Health Foundation, Valhalla, NY 10595.
Biochem Pharmacol. 1992 Feb 4;43(3):595-605. doi: 10.1016/0006-2952(92)90583-5.
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.
环丙贝特是一种过氧化物酶体增殖剂，在接触初期可诱导啮齿动物肝脏细胞增殖。由于Ca2+在有丝分裂中起重要作用，我们研究了环丙贝特对肝脏内质网（ER）Ca(2+)-ATP酶的影响，该酶部分调节Ca2+的稳态。给雄性F344大鼠单次口服200mg/kg环丙贝特后，暴露24小时时肝脏微粒体Ca(2+)-ATP酶活性短暂下降至对照水平的48%。暴露后48小时和72小时活性恢复到对照水平。Ca(2+)-ATP酶活性的下降不是非特异性酶抑制的结果，因为在环丙贝特处理的大鼠中，另一种微粒体酶葡萄糖-6-磷酸酶的活性没有改变。使用ATP驱动的45Ca2+积累试验，暴露于25、100和200mg/kg环丙贝特的大鼠在暴露24小时时表现出肝脏微粒体Ca2+积累的剂量依赖性抑制。使用针对肝脏ER Ca(2+)-ATP酶的多克隆抗体进行的Western免疫印迹分析显示，与暴露24小时后的对照相比，从环丙贝特处理的大鼠制备的微粒体中Ca(2+)-ATP酶蛋白含量略有增加。这些数据表明，Ca(2+)-ATP酶活性的降低不是由于体内Ca(2+)-ATP酶蛋白含量减少，因此是由于该酶的功能抑制。环丙贝特在体外也对大鼠肝脏ER Ca(2+)-ATP酶活性产生浓度依赖性抑制（IC50约为170 microM）。在新鲜分离的大鼠肝细胞中，无论有无细胞外钙，环丙贝特都会升高细胞内游离钙浓度（[Ca2+]i）。总体而言，这些结果表明环丙贝特通过抑制ER Ca(2+)-ATP酶来动员肝脏[Ca2+]i。这些事件可能导致环丙贝特暴露早期[Ca2+]i升高的环境，并可能有助于增强Ca(2+)依赖性过程，从而在急性有丝分裂反应中起关键作用。
