Roles of type I and type II isoenzymes of cyclic AMP-dependent protein kinase in steroidogenesis in rat adrenal cells

Effect of swimming on the production of aldosterone in rats

It has been demonstrated that exercise is one of the stresses known to increase the aldosterone secretion. Both potassium and angiotensin II (Ang II) levels are shown to be correlated with aldosterone production during exercise, but the mechanism is still unclear. In an in vivo study, male rats were catheterized via right jugular vein (RJV), and divided into four groups namely water immersion, swimming, lactate infusion (13 mg/kg/min) and pyruvate infusion (13 mg/kg/min) groups. Each group was treated for 10 min. Blood samples were collected at 0, 10, 15, 30, 60 and 120 min from RJV after administration. In an in vitro study, rat zona glomerulosa (ZG) cells were challenged by lactate (1–10 mM) in the presence or absence of Ang II (10⁻⁸ M) for 60 min. The levels of aldosterone in plasma and medium were measured by radioimmunoassay. Cell lysates were analyzed by immunoblotting assay. After exercise and lactate infusion, plasma levels of aldosterone and lactate were significantly higher than those in the control group. Swimming for 10 min significantly increased the plasma Ang II levels in male rats. Administration of lactate plus Ang II significantly increased aldosterone production and enhanced protein expression of steroidogenic acute regulatory protein (StAR) in ZG cells. These results demonstrated that acute exercise led to the increase of both aldosterone and Ang II secretion, which is associated with lactate action on ZG cells and might be dependent on the activity of renin-angiotensin system.

The differential regulation of steroidogenic acute regulatory protein-mediated steroidogenesis by type I and type II PKA in MA-10 cells

Following tropic hormone challenge, steroidogenic tissues utilize PKA to phosphorylate unique subsets of proteins necessary to facilitate steroidogenesis. This includes the PKA-dependent expression and activation of the steroidogenic acute regulatory protein (STAR), which mediates the rate-limiting step of steroidogenesis by inducing the transfer of cholesterol from the outer to the inner mitochondrial membrane. Since both type I and type II PKA are present in steroidogenic tissues, we have utilized cAMP analog pairs that preferentially activate each PKA subtype in order to examine their impact on STAR synthesis and activity. In MA-10 mouse Leydig tumor cells Star gene expression is more dependent upon type I PKA, while the post-transcriptional regulation of STAR appears subject to type II PKA. These experiments delineate the discrete effects that type I and type II PKA exert on STAR-mediated steroidogenesis, and suggest complimentary roles for each subtype in coordinating steroidogenesis.

Cloning and mitochondrial localization of full-length D-AKAP2, a protein kinase A anchoring protein

Differential compartmentalization of signaling molecules in cells and tissues is being recognized as an important mechanism for regulating the specificity of signal transduction pathways. A kinase anchoring proteins (AKAPs) direct the subcellular localization of protein kinase A (PKA) by binding to its regulatory (R) subunits. Dual specific AKAPs (D-AKAPs) interact with both RI and RII. A 372-residue fragment of mouse D-AKAP2 with a 40-residue C-terminal PKA binding region and a putative regulator of G protein signaling (RGS) domain was previously identified by means of a yeast two-hybrid screen. Here, we report the cloning of full-length human D-AKAP2 (662 residues) with an additional putative RGS domain, and the corresponding mouse protein less the first two exons (617 residues). Expression of D-AKAP2 was characterized by using mouse tissue extracts. Full-length D-AKAP2 from various tissues shows different molecular weights, possibly because of alternative splicing or posttranslational modifications. The cloned human gene product has a molecular weight similar to one of the prominent mouse proteins. In vivo association of D-AKAP2 with PKA in mouse brain was demonstrated by using cAMP agarose pull-down assay. Subcellular localization for endogenous mouse, rat, and human D-AKAP2 was determined by immunocytochemistry, immunohistochemistry, and tissue fractionation. D-AKAP2 from all three species is highly enriched in mitochondria. The mitochondrial localization and the presence of RGS domains in D-AKAP2 may have important implications for its function in PKA and G protein signal transduction.

Type II cAMP-dependent protein kinase regulates electrogenic ion transport in rabbit collecting duct

cAMP mediates many of the effects of vasopressin, prostaglandin E2, and beta-adrenergic agents upon salt and water transport in the renal collecting duct. The present studies examined the role of cAMP-dependent protein kinase (PKA) in mediating these effects. PKA is a heterotetramer comprised of two regulatory (R) subunits and two catalytic (C) subunits. The four PKA isoforms may be distinguished by their R subunits that have been designated RIalpha, RIbeta, RIIalpha, and RIIbeta. Three regulatory subunits, RIalpha, RIIalpha, and RIIbeta, were detected by immunoblot and ribonuclease protection in both primary cultures and fresh isolates of rabbit cortical collecting ducts (CCDs). Monolayers of cultured CCDs grown on semipermeable supports were mounted in an Ussing chamber, and combinations of cAMP analogs that selectively activate PKA type I vs. PKA type II were tested for their effect on electrogenic ion transport. Short-circuit current (Isc) was significantly increased by the PKA type II-selective analog pairs N6-monobutyryl-cAMP plus 8-(4-chlorophenylthio)-cAMP or N6-monobutyryl-cAMP plus 8-chloro-cAMP. In contrast the PKA type I-selective cAMP analog pair [N6-monobutyryl-cAMP plus 8-(6-aminohexyl)-amino-cAMP] had no effect on Isc. These results suggest PKA type II is the major isozyme regulating electrogenic ion transport in the rabbit collecting duct.

Effects of estradiol on aldosterone secretion in ovariectomized rats

The effects and action mechanisms of estradiol on aldosterone secretion in female rats were studied. Replacement of estradiol benzoate (EB) increased the levels of plasma estradiol and aldosterone in ovariectomized (Ovx) rats. The aldosterone release from zona glomerulosa (ZG) cells was higher in EB-treated rats than in oil-treated animals. EB treatment potentiated the responses of aldosterone release to adrenocorticotropic hormone (ACTH), forskolin (FSK), and 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP). Administration of EB in vivo did not alter cAMP production in response to ACTH or FSK. Although angiotensin II (Ang II) increased aldosterone secretion by rat ZG cells, the stimulatory effect of Ang II on the release of aldosterone was not altered by EB treatment. The conversions of [3H]-deoxycorticosterone to [3H]-corticosterone and [3H]-corticosterone to [3H]-aldosterone in EB-treated groups were greater than those in the oil-treated group. These results suggest that estradiol increases aldosterone secretion in part through the mechanisms involving the activation of the post-cAMP pathway, 11beta-hydroxylase and aldosterone synthase activity.

Effects of prolactin on aldosterone secretion in rat zona glomerulosa cells

Acute effects and action mechanisms of prolactin (PRL) on aldosterone secretion in zona glomerulosa (ZG) cells were investigated in ovariectomized rats. Administration of ovine PRL (oPRL) increased aldosterone secretion in a dose-dependent manner. Incubation of [3H]-pregnenolone combined with oPRL increased the production of [3H]-aldosterone and [3H]-deoxycorticosterone but decreased the accumulation of [3H]-corticosterone. Administration of oPRL produced a marked increase of adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in ZG cells. The stimulatory effect of oPRL on aldosterone secretion was attenuated by the administration of angiotensin II (Ang II) and high potassium. The Ca2+ chelator, ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA, 10(-2) M), inhibited the basal release of aldosterone and completely suppressed the stimulatory effects of oPRL on aldosterone secretion. The stimulatory effects of oPRL on aldosterone secretion were attenuated by the administration of nifedipine (L-type Ca2+ channel blocker) and tetrandrine (T-type Ca2+ channel blocker). These data suggest that the increase of aldosterone secretion by oPRL is in part due to (1) the increase of cAMP production, (2) the activation of both L- and T-type Ca2+ channels, and (3) the activation of 21-hydroxylase and aldosterone synthase in rat ZG cells.

Type II protein kinase A regulates CFTR in airway, pancreatic, and intestinal cells

The type of protein kinase A (PKA) responsible for cystic fibrosis transmembrane conductance regulator (CFTR) activation was determined with adenosine 3', 5'-cyclic monophosphate analogs capable of selectively activating type I or type II PKA. The type II-selective pair stimulated chloride efflux in airway, pancreatic, and colonic epithelial cells; the type I-selective pair only stimulated a calcium-dependent efflux in airway cells. The type II-selective analogs activated larger increases in CFTR-mediated current than did the type I-selective analogs. Measurement of soluble PKA activity demonstrated similar levels stimulated by type I- and type II-selective analogs, creating an apparent paradox regarding PKA activity and current generated. Also, addition of forskolin after the type I-selective analogs resulted in an increase in current; little increase was seen after the type II-selective analogs. Measurement of insoluble PKA activity stimulated by the analogs resolved this paradox. Type II-selective analogs stimulated three times as much insoluble PKA activity as the type I-selective pair, indicating that differential activation of PKA in cellular compartments is important in CFTR regulation.