Submitted December 19, 2006
Returned for revision January 7, 2007
Accepted April 3, 2007
Mechanisms of Hormone Action
Constitutive Steroidogenesis in Ovine Large Luteal Cells
May Be Mediated by Tonically Active Protein Kinase A
R. L. Bogan
and
G. D. Niswender *
* To whom correspondence should be addressed. E-mail: gordon.niswender{at}colostate.edu.
Abstract
The mechanisms responsible for the increased basal rates
of progesterone secretion from large steroidogenic luteal
cells compared to small steroidogenic luteal cells have
not been clearly defined. To determine if protein kinase A
(PKA) is tonically active in large cells, the adenylate
cyclase activator forskolin and a specific PKA inhibitor
(PKI) were utilized in a 2x2 factorial treatment with each
steroidogenic cell type. Progesterone and cAMP production
were quantified following treatments. In addition, the
effect of treatments on concentrations and the relative
phosphorylation state of steroidogenic acute regulatory
(STAR) protein in the two cell types was determined as a
marker for PKA activity. Treatment with PKI blocked
forskolin-induced increases in progesterone secretion by
small luteal cells without affecting cAMP production.
Treating large cells with PKI significantly decreased
basal progesterone secretion in the presence or absence of
forskolin, indicating that the high level of
steroidogenesis by this cell type required PKA activity.
There were no differences in steady-state concentrations
of STAR protein due to treatment in either cell type.
However, the percentage of relative STAR phosphorylation
was higher in large compared to small cells and PKI
treatment significantly decreased the phosphorylation
state of STAR in large cells. Relative phosphorylation
state of STAR and concentrations of progesterone in media
were significantly correlated over treatments in both cell
types. The ratio of progesterone secreted per picogram of
cAMP was higher in large than small cells, and this was
accompanied by a significant increase in the ratio of
relative STAR phosphorylation to steady-state
concentrations of STAR protein. These data are compatible
with the theory that large luteal cells are constitutively
steroidogenic at least in part because they have tonically
active PKA. In addition, phosphorylation of STAR appears
to be a primary effect of PKA on the activity of STAR
protein in both types of ovine steroidogenic luteal cells.
Key words:
Mechanisms of Hormone Action •
Corpus luteum •
Kinases •
Progesterone
