The Calcium Signaling Pathway Regulates Leydig Cell Steroidogenesis through a Transcriptional Cascade Involving the Nuclear Receptor NR4A1 and the Steroidogenic Acute Regulatory Protein

In the gonads and adrenal glands, the transient increase in steroidogenesis after hormonal stimulation requires modulation of steroidogenic acute regulatory protein (Star) expression and activity in a tightly regulated process involving cAMP and Ca2+. In Leydig cells, the cAMP and Ca2+ pathways account for most if not all of LH-induced steroidogenesis. Although the cAMP-activated molecular network has been well characterized in Leydig cells, little is known about the molecular cascade triggered by the Ca2+ signaling pathway and the transcription factors responsible for mediating the genomic response. It is established that LH induces an increase in cytoplasmic Ca2+ from the endoplasmic reticulum primarily through the ryanodine receptors. Previous reports also suggested a role of the Ca2+ signaling pathway in Star expression based on the fact that inhibition of the Ca2+/calmodulin (CaM) protein kinase pathway greatly impaired Star expression in Leydig and adrenal cells. In this study, we used ryanodine receptors and CaM antagonists to show that the increase in intracellular Ca2+ level is an essential modulator of progesterone synthesis through the regulation of Star gene expression in MA-10 Leydig cells. Furthermore, we mapped a Ca2+/CaM-sensitive element in the Star promoter, which led to the identification of the nuclear receptor 4A1 (NR4A1) as a key mediator of the Ca2+/CaM signaling pathway in these cells. These data provide new insights into the Ca2+ molecular pathway essential for steroidogenesis in Leydig cells.