Regulation of sterol carrier protein 2 (SCP 2) levels in the soluble fraction of rat Leydig cells. Kinetics and the possible role of calcium influx

Abstract

The rate-determining step in steroidogenesis is the conversion of cholesterol to pregnenolone by the cholesterol side-chain cleavage enzyme. The transport of substrate for this reaction may be facilitated by sterol carrier protein 2(SCP 2). In rat testis tissue SCP 2 is specifically localized in the Leydig cells and tissue levels of SCP 2 are regulated by luteinizing hormone (LH). The present study concerns short-term regulation of SCP 2 in isolated rat Leydig cells. Levels of SCP 2 in the membrane-free supernatant are increased 2-fold already after 2 min incubation with LH and remain elevated for 24 h. The same response occurs with cells preincubated in the presence of cycloheximide for 4 h. SCP 2 levels are also 2-fold increased after incubation with dibutyryl cAMP or 4β-phorbol 12-myristate 13-acetate (PMA) whereas these compounds stimulate steroid production 5.5- and 2-fold respectively. Luteinizing hormone releasing hormone (LHRH), which can stimulate steroid production more than 3-fold, does not influence SCP 2 levels, neither are SCP 2 levels altered when LH is added in the presence of the Ca 2+-channel blocker diltiazem or in the absence of extracellular Ca 2+. A restoration of the LH effect on SCP 2 levels was already obtained in the presence of 1 μM extracellular Ca 2+. These results suggest that Ca 2+ influx through the plasma membrane may play an important role in the control of SCP 2 levels. In most of the experiments no correlation between steroid production and SCP 2 levels could be observed. The soluble amount of SCP 2 is probably not the rate-limiting factor in the control of cholesterol side-chain cleavage activity but SCP 2 in the membrane-free supernatant may play a permissive role.