Cyclic nucleotide-dependent phosphorylation of endogenous proteins in bovine adrenocortical cell membranes

Effect of protein kinase inhibitors on ACTH-stimulated aldosterone production in rat zona glomerulosa cells

In order to obtain further evidence for the involvement of protein kinases in the short-term ACTH-stimulated aldosterone synthesis in rat zona glomerulosa cells, the effects of three different compounds with protein kinase inhibitory properties were investigated. Staurosporine, H-7 and trifluoperazine inhibited ACTH-stimulated aldosterone release in a dose-dependent manner. While the inhibitory effect of H-7 was reversible upon washing of the cells with inhibitor-free medium, the inhibition was maintained in cells treated with staurosporine or trifluoperazine. In contrast to the stimulated production, basal release of aldosterone even at the highest drug concentrations tested was not completely inhibited. We thus conclude that protein kinases may play a crucial role in short-term ACTH-stimulated aldosterone production in rat glomerulosa cells.

Transduction of ACTH signal from plasma membrane to mitochondria in adrenocortical steroidogenesis. Effects of peptide, phospholipid, and calcium

The conversion of cholesterol to pregnenolone by adrenocortical mitochondria is the rate-limiting step in steroidogenesis. This process is stimulated dramatically by the action of ACTH through the sequential reactions, in which adenyl cyclase, cAMP-dependent protein kinase, cholesterol esterase and ribosomal protein synthesis are all involved. The de novo synthesized protein, the so-called labile protein with a half-life of approx 10 min, is believed to stimulate the cholesterol side chain cleavage reaction by an unknown mechanism. Available evidence indicates that the electron on transfer reaction from NADPH to P-450scc is mediated rapidly by adrenodoxin reductase and p-450 scc. In addition, these redox components are inactivated slowly with a half-life of 3.5 days after hypophysectomy. It is known that the corticoid output from adrenocortical cells starts within 5 min and reaches the maximum after 10-15 min of ACTH administration to animals. One can assume that under normal physiological conditions, both O2 and NADPH are not limiting. Additionally, mitochondrial inner membranes are poor in cholesterol. In this context, the availability of substrate cholesterol to P450scc is the most likely candidate for the regulatory mechanism.

Cyclic AMP-dependent protein kinase activity and protein phosphorylation in zones of the adrenal cortex

Steroidogenesis is not stimulated by ACTH in the inner zone of the guinea pig adrenal cortex; adenylate cyclase is normally stimulated. To further explore the lack of a steroidogenic response to ACTH in the inner zone, cAMP-dependent protein kinase activity and protein phosphorylation were examined in the outer and inner adrenocortical zones. To summarize: total cAMP-dependent protein kinase activity was 40% higher in the outer zone than in the inner zone; of the total cAMP-dependent protein kinase activity, cytosol contained 80% for the outer and 70% for the inner zone. In both zones only the type II isozyme was present. Qualitative and quantitative differences in protein phosphorylation were noted for the two zones.

Protein synthesis requirement for acute ACTH stimulation of adrenal corticosteroidogenesis

The rapid, cAMP mediated increase produced by ACTH in adrenal corticosteroidogenesis depends also on the rapid synthesis of protein. This obligatory involvement of protein synthesis has been established in several laboratories mainly on the basis of studies demonstrating a parallelism, under a variety of experimental conditions, of the capacity of adrenal cells to synthesize protein with their ability to increase steroid production in response to ACTH or cAMP. More recent correlative studies have disclosed the existence of two proteins, denoted as p and i. Protein i appears only in ACTH or cAMP stimulated cells and with the same time course and ACTH dose response as the increase in corticosteroid synthesis. Protein p, which closely resembles i in molecular weight and primary structure but differs in pI, is synthesized only in unstimulated cells. Neither the function of these two proteins nor the exact nature of the interplay of their synthesis on regulation has been established, although it seems reasonably certain that i is stimulatory rather than p inhibitory. The protein glycosylation inhibitor tunicamycin was found to inhibit the ACTH produced increase in steroidogenesis and also to inhibit the synthesis of protein i under conditions which did not inhibit overall protein synthesis. Therefore it seems probable that i is an N-glycosylated form of protein p and that adrenal cells are unresponsive to acute ACTH action if they are incapable of synthesizing glycosylated proteins specifically.

Polypeptide activators of cholesterol side-chain cleavage

A rate-determining step in the cAMP-dependent action of ACTH on adrenal steroid biosynthesis is the interaction of cholesterol substrate with the cholesterol side-chain cleavage cytochrome P-450 in the mitochondrion. This interaction is rapidly and reversibly sensitive to inhibitors of protein synthesis. For this reason a hormone-dependent, labile protein activator of cholesterol side-chain cleavage has long been postulated as an obligatory intermediate in the tropic regulation of this reaction. Applying recent advances in liquid chromatography, two-dimensional gel electrophoresis, and enzyme reconstitution into liposomes, several laboratories have now reported the isolation and partial characterization of polypeptide candidates for the status of "labile protein."

Intrinsic protein phosphorylation in synaptosomal plasma membrane fragments: a comparison of cerebral cortex tissue from several species, including human biopsy specimens

Intrinsic protein phosphorylation was studied in synaptosomal membrane fragments made from cerebral cortex tissue taken from the following species: human (biopsy specimens), ox, rat, rabbit, guinea pig and mouse. Membrane fragments from all species exhibited a qualitatively similar range of protein acceptors phosphorylated by cyclic AMP-dependent protein kinase activity; contrary to a previous report, no evidence for cyclic GMP-dependent protein kinase activity was found in the human material. With the exception of membrane fragments prepared from ox brain, all the preparations exhibited the same range of Ca2+-dependent protein kinase activity. Ox brain obtained from a slaughterhouse yielded membranes containing no Ca2+-dependent protein kinase activity, but this may have been due to unavoidable postmortem losses.