Regulation of sterol carrier protein2 (SCP2) levels in the soluble fraction of rat Leydig cells. Kinetics and the possible role of calcium influx.

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 protein2 (SCP2). In rat testis tissue SCP2 is specifically localized in the Leydig cells and tissue levels of SCP2 are regulated by luteinizing hormone (LH). The present study concerns short-term regulation of SCP2 in isolated rat Leydig cells. Levels of SCP2 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. SCP2 levels are also 2-fold increased after incubation with dibutyryl cAMP or 4 beta-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 SCP2 levels, neither are SCP2 levels altered when LH is added in the presence of the Ca2+-channel blocker diltiazem or in the absence of extracellular Ca2+. A restoration of the LH effect on SCP2 levels was already obtained in the presence of 1 microM extracellular Ca2+. These results suggest that Ca2+ influx through the plasma membrane may play an important role in the control of SCP2 levels. In most of the experiments no correlation between steroid production and SCP2 levels could be observed.(ABSTRACT TRUNCATED AT 250 WORDS)