Constitutive Steroidogenesis in Ovine Large Luteal Cells May Be Mediated by Tonically Active Protein Kinase A1

Abstract

The mechanisms responsible for the increased basal rates of progesterone secretion from large steroidogenic luteal cells (LLC) relative to small steroidogenic luteal cells (SLC) have not been clearly defined. To determine if protein kinase A (PKA) is tonically active in LLC, the adenylate cyclase activator forskolin and a specific PKA inhibitor (PKI) were utilized in a 2 x 2 factorial treatment with each steroidogenic cell type. Progesterone and cAMP production were quantified after the different treatments. In addition, the effects of the treatments on the concentrations and relative phosphorylation status of the steroidogenic acute regulatory (STAR) protein in the two cell types were determined as a measure of PKA activity. Treatment with PKI blocked forskolin-induced increases in progesterone secretion by SLC without affecting the production of cAMP. The treatment of LLC with PKI significantly decreased basal progesterone secretion in the presence or absence of forskolin, indicating that the high level of steroidogenesis in this cell type requires PKA activity. There were no differences in the steady-state concentrations of STAR protein in either cell type after treatment. However, the percentage of relative STAR phosphorylation was higher in the LLC than in SLC, and PKI treatment significantly decreased the phosphorylation of STAR in the LLC. The relative phosphorylation status of STAR and the concentrations of progesterone in the media were significantly correlated with the treatments in both cell types. The amount of progesterone secreted per picogram of cAMP was higher in the LLC than in the SLC, and this was accompanied by a significant increase in the ratio of relative STAR phosphorylation to the steady-state concentration of STAR protein. These data are compatible with the theory that LLC are constitutively steroidogenic, partly because they have tonically active PKA. In addition, the phosphorylation of STAR appears to be a primary activity of PKA in both types of ovine steroidogenic luteal cells.

Intracellular mechanisms underlying the nicotinic enhancement of LTP in the rat dentate gyrus

“…, 2007a, b). It is also known that both PKA and adenylate cyclase demonstrate basal activity in the absence of agonist activation (Salazar & González, 2002; Bogan & Niswender, 2007). Under control conditions inhibition of this basal activity does not impact on LTP induction.…”

Physiological and practical effects of progesterone on reproduction in dairy cattle

“…Thus, the acute or even chronic effects of LH pulses on CL function in ruminants remain to be unequivocally defined in the scientific literature. In general, P4 production by the ruminant CL appears to be very high, due to high constitutive P4 production by the large luteal cells (Wiltbank, 1994; Diaz et al, 2002;Bogan and Niswender, 2007;Wiltbank et al, 2012a).…”

Regulation of Sterol Regulatory Element-Binding Transcription Factor 1a by Human Chorionic Gonadotropin and Insulin in Cultured Rat Theca-Interstitial Cells1

“…Because activation of PRKA has been shown to be involved in LH/hCG-stimulated steroidogenesis [43][44][45], the role of PRKA in stimulating SREBF1a in response to hCG was examined. Our results show that PRKA inhibitor completely blocked the stimulatory effect of hCG on SREBF1a.…”