Trafficking of cholesterol from lipid droplets to mitochondria in bovine luteal cells: Acute control of progesterone synthesis

The corpus luteum is a transient endocrine gland that synthesizes and secretes the steroid hormone, progesterone, which is vital for establishment and maintenance of pregnancy. Luteinizing hormone (LH) via activation of protein kinase A (PKA) acutely stimulates luteal progesterone synthesis via a complex process, converting cholesterol via a series of enzymatic reactions, into progesterone. Lipid droplets in steroidogenic luteal cells store cholesterol in the form of cholesterol esters, which are postulated to provide substrate for steroidogenesis. Early enzymatic studies showed that hormone sensitive lipase (HSL) hydrolyzes luteal cholesterol esters. In this study, we tested whether HSL is a critical mediator of the acute actions of LH on luteal progesterone production. Using LH-responsive bovine small luteal cells our results reveal that LH, forskolin, and 8-Br cAMP-induced PKA-dependent phosphorylation of HSL at Ser563 and Ser660, events known to promote HSL activity. Small molecule inhibition of HSL activity and siRNA-mediated knock down of HSL abrogated LH-induced progesterone production. Moreover, western blotting and confocal microscopy revealed that LH stimulates phosphorylation and translocation of HSL to lipid droplets. Furthermore, LH increased trafficking of cholesterol from the lipid droplets to the mitochondria, which was dependent on both PKA and HSL activation. Taken together, these findings identify a PKA/HSL signaling pathway in luteal cells in response to LH and demonstrate the dynamic relationship between PKA, HSL, and lipid droplets in luteal progesterone synthesis.