Phorbol ester mimics ACTH action in corticoadrenal cells stimulating steroidogenesis, blocking cell cycle, changing cell shape, and inducing c-fos proto-oncogene expression

Studying mechanisms of cAMP and cyclic nucleotide phosphodiesterase signaling in Leydig cell function with phosphoproteomics

Many cellular processes are modulated by cyclic AMP and nucleotide phosphodiesterases (PDEs) regulate this second messenger by catalyzing its breakdown. The major unique function of testicular Leydig cells is to produce testosterone in response to luteinizing hormone (LH). Treatment of Leydig cells with PDE inhibitors increases cAMP levels and the activity of its downstream effector, cAMP-dependent protein kinase (PKA), leading to a series of kinase-dependent signaling and transcription events that ultimately increase testosterone release. We have recently shown that PDE4B and PDE4C as well as PDE8A and PDE8B are expressed in rodent Leydig cells and that combined inhibition of PDE4 and PDE8 leads to dramatically increased steroid biosynthesis. Here we investigated the effect of PDE4 and PDE8 inhibition on the molecular mechanisms of cAMP actions in a mouse MA10 Leydig cell line model with SILAC mass spectrometry-based phosphoproteomics. We treated MA10 cells either with PDE4 family specific inhibitor (Rolipram) and PDE8 family specific inhibitor (PF-04957325) alone or in combination and quantified the resulting phosphorylation changes at five different time points between 0 and 180min. We identified 28,336 phosphosites from 4837 proteins and observed significant regulation of 749 sites in response to PDE4 and PDE8 inhibitor treatment. Of these, 132 phosphosites were consensus PKA sites. Our data strongly suggest that PDE4 and PDE8 inhibitors synergistically regulate phosphorylation of proteins required for many different cellular processes, including cell cycle progression, lipid and glucose metabolism, transcription, endocytosis and vesicle transport. Our data suggests that cAMP, PDE4 and PDE8 coordinate steroidogenesis by acting on not one rate-limiting step but rather multiple pathways. Moreover, the pools of cAMP controlled by these PDEs also coordinate many other metabolic processes that may be regulated to assure timely and sufficient testosterone secretion in response to LH.

Deconstructing the molecular mechanisms of cell cycle control in a mouse adrenocortical cell line: roles of ACTH

This is a progress report of an attempt to deconstruct the signaling network underlying cell cycle control in the mouse Y1 adrenocortical cell line, aiming to uncover ACTH growth regulatory pathways. Y1 adrenocortical tumor cells possess amplified and overexpressed c-Ki-ras proto-oncogene. Despite this oncogenic lesion, Y1 cells retain tight regulatory mechanisms of cell cycle control typified by the sequential events comprising the mitogenic response triggered by FGF2 in G0/G1-arrested Y1 cells: 1) activation of ERK1/2 and PI3K, by 5 minutes; 2) induction of c-Fos and c-Myc proteins by 2 hours; 3) induction of cyclin D1 protein by 5 hours; 4) phosphorylation of Rb protein between 6 and 8 hours; 5) onset of DNA synthesis by 8-9 hours. In this cell line, ACTH-receptor (ACTH-R) activates contradictory pathways of growth regulation. First, ACTH coordinately induces fos and jun gene families via activation of both ERK1/2 and cAMP/PKA pathways, resembling a mitogen. Second, ACTH-R triggers cAMP/PKA-mediated antimitogenic mechanisms comprised of Akt/PKB dephosphorylation/deactivation, c-Myc protein degradation, and p27(Kip1) protein induction. Induction of cyclin D1 depends on activation of both ERK1/2 and PI3K, but is not affected by ACTH action. As a consequence, ACTH antagonizes FGF2 mitogenic activity but ectopic expression of the c-Myc protein (via MycER fusion protein) is sufficient to abrogate this ACTH antagonistic effect over FGF2 mitogenic activity. Ectopic expression of both c-Myc and cyclin D1 is not sufficient to drive G0/G1-arrested Y1 cells into S phase, but when the sustained expression of these two proteins is complemented by ACTH treatment it promotes G1 phase progression and DNA synthesis initiation. In conclusion, ACTH-receptor lacks signaling potential sufficient to initiate a mitogenic response in Y1 adrenocortical cells and, therefore, cannot substitute for bona fide mitogens like FGF2.

Salt-inducible kinase is involved in the ACTH/cAMP-dependent protein kinase signaling in Y1 mouse adrenocortical tumor cells

The involvement of salt-inducible kinase, a recently cloned protein serine/threonine kinase, in adrenal steroidogenesis was investigated. When Y1 mouse adrenocortical tumor cells were stimulated by ACTH, the cellular content of salt-inducible kinase mRNA, protein, and enzyme activity changed rapidly. Its level reached the highest point in 1-2 h and returned to the initial level after 8 h. The mRNA levels of cholesterol side-chain cleavage cytochrome P450 and steroidogenic acute regulatory protein, on the other hand, began to rise after a few hours, reaching the highest levels after 8 h. The salt-inducible kinase mRNA level in ACTH-, forskolin-, or 8-bromo-cAMP-treated Kin-7 cells, mutant Y1 with less cAMP-dependent PKA activity, remained low. However, Kin-7 cells, when transfected with a PKA expression vector, expressed salt-inducible kinase mRNA. Y1 cells that overexpressed salt-inducible kinase were isolated, and the mRNA levels of steroidogenic genes in these cells were compared with those in the parent Y1. The level of cholesterol side-chain cleavage cytochrome P450 mRNA in the salt-inducible kinase-overexpressing cells was markedly low compared with that in the parent, while the levels of Ad4BP/steroidogenic factor-1-, ACTH receptor-, and steroidogenic acute regulatory protein-mRNAs in the former were similar to those in the latter. The ACTH-dependent expression of cholesterol side-chain cleavage cytochrome P450- and steroidogenic acute regulatory protein-mRNAs in the salt-inducible kinase-overexpressing cells was significantly repressed. The promoter activity of the cholesterol side-chain cleavage cytochrome P450 gene was assayed by using Y1 cells transfected with a human cholesterol side-chain cleavage cytochrome P450 promoter-linked reporter gene. Addition of forskolin to the culture medium enhanced the cholesterol side-chain cleavage cytochrome P450 promoter activity, but the forskolin-dependently activated promoter activity was inhibited when the cells were transfected with a salt-inducible kinase expression vector. This inhibition did not occur when the cells were transfected with a salt-inducible kinase (K56M) vector that encoded an inactive kinase. The salt-inducible kinase's inhibitory effect was also observed when nonsteroidogenic, nonAd4BP/steroidogenic factor-1 -expressing, NIH3T3 cells were used for the promoter assays. These results suggested that salt-inducible kinase might play an important role(s) in the cAMP-dependent, but Ad4BP/steroidogenic factor-1-independent, gene expression of cholesterol side-chain cleavage cytochrome P450 in adrenocortical cells.

Signal transduction in G0/G1-arrested mouse Y1 adrenocortical cells stimulated by ACTH and FGF2

In G0/G1 cell cycle arrested mouse Y1 adrenocortical tumor cells ACTH39, a weak mitogen and strong anti-mitogenic agent, blocks FGF2 mitogenic activity at G1 phase, keeping untouched ERK-MAPK activation and c-Fos protein induction. Here we report two anti-mitogenic mechanisms initiated in ACTH receptors and mediated by cAMP/PKA: a) post-transcriptional down regulation of c-Myc protein; b) dephosphorylation of AKT/PKB. In Y-1 cells the activity of the Mad/Max/Myc network of transcription factors seems to be regulated by c-Myc levels. FGF2 induces c-myc gene and stabilizes c-Myc protein by a process dependent on ERK-MAPK (PD98059 sensitive), but not on PI3K (Wortmannin resistant). ACTH39, on the other hand, causes rapid decrease in c-Myc levels induced by FGF2 in wild type Y1 cells, but not in PKA-deficient Y1 clones. The ACTH inhibition of DNA synthesis stimulated by FGF2 is reversed by transient transfection and induction of the MycER chimera (fusion of c-Myc and estrogen-receptor), suggesting that c-Myc down regulation is an efficient anti-mitogenic mechanism activated by ACTH. Y1 cells display high constitutive levels of AKT/PKB, that is dependent on elevated Ras x GTP. FGF2 up regulates Ras x GTP, PI3K and AKT/PKB. ACTH antagonizes this mitogenic effect of FGF2, promoting rapid dephosphorylation of AKT/PKB.

Proliferative signaling initiated in ACTH receptors

This article reviews recent results of studies aiming to elucidate modes of integrating signals initiated in ACTH receptors and FGF2 receptors, within the network system of signal transduction found in Y1 adrenocortical cells. These modes of signal integration should be central to the mechanisms underlying the regulation of the G0-->G1-->S transition in the adrenal cell cycle. FGF2 elicits a strong mitogenic response in G0/G1-arrested Y1 adrenocortical cells, that includes a) rapid and transient activation of extracellular signal-regulated kinases-mitogen-activated protein kinases (ERK-MAPK) (2 to 10 min), b) transcription activation of c-fos, c-jun and c-myc genes (10 to 30 min), c) induction of c-Fos and c-Myc proteins by 1 h and cyclin D1 protein by 5 h, and d) onset of DNA synthesis stimulation within 8 h. ACTH, itself a weak mitogen, interacts with FGF2 in a complex manner, blocking the FGF2 mitogenic response during the early and middle G1 phase, keeping ERK-MAPK activation and c-Fos and cyclin D1 induction at maximal levels, but post-transcriptionally inhibiting c-Myc expression. c-Fos and c-Jun proteins are mediators in both the strong and the weak mitogenic responses respectively triggered by FGF2 and ACTH. Induction of c-Fos and stimulation of DNA synthesis by ACTH are independent of PKA and are inhibited by the PKC inhibitor GF109203X. In addition, ACTH is a poor activator of ERK-MAPK, but c-Fos induction and DNA synthesis stimulation by ACTH are strongly inhibited by the inhibitor of MEK1 PD98059.

Control of the adrenocortical cell cycle: interaction between FGF2 and ACTH

FGF2 elicits a strong mitogenic response in the mouse Y-1 adrenocortical tumor cell line, that includes a rapid and transient activation of the ERK-MAPK cascade and induction of the c-Fos protein. ACTH, itself a very weak mitogen, blocks the mitogenic response effect of FGF2 in the early and middle G1 phase, keeping both ERK-MAPK activation and c-Fos induction at maximal levels. Probing the mitogenic response of Y-1 cells to FGF2 with ACTH is likely to uncover reactions underlying the effects of this hormone on adrenocortical cell growth.

Protein kinase C-alpha levels are inversely associated with growth rate in cultured human dermal fibroblasts

Human dermal fibroblasts are known to express the alpha, delta, epsilon, and zeta isoforms of protein kinase C (PKC). We asked whether the growth of human dermal fibroblasts correlates with expression of a particular PKC isoform. Of total PKC activity measured in the presence of calcium, a condition permissive for activation of all PKC isoforms, 75%) was contributed by PKC-alpha, suggesting that PKC-alpha is the dominant isoform in human dermal fibroblasts. We then further studied PKC-alpha under different culture conditions and in cultures derived from different aged donors. In both subconfluent and confluent cultures, total PKC activity and the level of PKC-alpha protein were consistently higher in slowly proliferating adult cells than in more rapidly proliferating newborn cells. Moreover, in newborn fibroblasts density strongly influenced these parameters. At subconfluent density, when cells were dividing exponentially, total PKC activity was 345+/-63 cpm/,ug protein; whereas at confluent density, when cells were growth arrested, it was 6-7 fold higher, 2334+/-50 cpm/ug protein. Immunoblot analysis using a specific monoclonal antibody against PKC-alpha exhibited a similar 6-7 fold increase in the level of PKC-alpha protein at confluent density. However, in adult cells, density had no influence on the already high total activity or level of PKC-alpha. To further determine whether the increases in the levels of total PKC activity and the alpha isoform correlate with the decreased growth rate, a characteristic of both adult donor-derived and confluent cells, total PKC activity and the level of PKC-alpha in subconfluent quiescent cells was compared to that in paired exponentially growing cells at the same density. Total PKC activity was 8836+/-71 cpm/microg protein in subconfluent quiescent cells versus 4415+/-175 cpm/microg protein in dividing cells. The level of PKC-alpha protein was also 2-3 fold higher in quiescent than in growing cultures. However, the amount of PKC-alpha mRNA in these two conditions was identical as determined by northern blot analysis. Taken together, these results suggest an inverse relationship between the levels of total PKC activity and PKC-alpha protein and fibroblast growth rate that is regulated at the post-transcriptional level.

c-Fos protein is a mediator in mitogenic response to ACTH

Pulses (up to 2 h) of the adrenocorticotropic hormone (ACTH) rapidly activate p42 and p44 MAPK (5 min), induce the c-Fos protein (1 h, 80% of cells) and stimulate entry of mouse Y-1 adrenocortical cells into the S phase of the cell cycle. This set of sequential events is also triggered in Y-1 cells by bFGF, and reflects a mitogenic response to ACTH. We report here that 90% inhibition of c-fos mRNA translation with a c-fos antisense oligodeoxynucleotide completely blocks the entry of Y1 cells into S phase stimulated by pulses of ACTH. These results indicate that c-Fos protein is an intracellular mediator of the mitogenic response to ACTH.

Unmasking a growth-promoting effect of the adrenocorticotropic hormone in Y1 mouse adrenocortical tumor cells

The adrenocorticotropic hormone (ACTH) inhibits the growth of Y1 mouse adrenocortical tumor cells as well as normal adrenocortical cells in culture but stimulates adrenocortical cell growth in vivo. In this study, we investigated this paradoxical effect of ACTH on cell proliferation in Y1 adrenal cells and have unmasked a growth-promoting effect of the hormone. Y1 cells were arrested in the G1 phase of the cell cycle by serum starvation and monitored for progression through S phase by measuring [3H]thymidine incorporation into DNA and by measuring the number of nuclei labeled with bromodeoxyuridine. Y1 cells were stimulated to progress through S phase and to divide after a brief pulse of ACTH (up to 2 h). This effect of ACTH appeared to be cAMP independent, since ACTH also induced cell cycle progression in Kin-8, a Y1 mutant with defective cAMP-dependent protein kinase activity. The growth-promoting effect of ACTH in Y1 was preceded by the rapid activation of p44 and p42 mitogen-activated protein kinases and by the accumulation of c-FOS protein. In contrast, continuous treatment with ACTH (14 h) inhibited cell cycle progression in Y1 cells by a cAMP-dependent pathway. The inhibitory effect of ACTH mapped to the midpoint of G1. Together, the results demonstrate a dual effect of ACTH on cell cycle progress, a cAMP-independent growth-promoting effect early in G1 possibly mediated by mitogen-activated protein kinase and c-FOS, and a cAMP-dependent inhibitory effect at mid-G1. It is suggested that the growth-inhibitory effect of ACTH at mid-G1 represents an ACTH-regulated check point that limits cell cycle progression.

Adrenocorticotropin induction of stress-activated protein kinase in the adrenal cortex in vivo

A broad array of stressors induce ACTH release from the anterior pituitary, with consequent stimulation of the adrenal cortex and release of glucocorticoids critical for survival of the animal. ACTH stimulates adrenocortical gene expression in vivo and inhibits adrenocortical cell proliferation. Binding of ACTH to its G-protein-coupled receptor stimulates the production of cAMP and activation of the protein kinase A pathway. The stress-activated protein kinases (SAPKs) (or c-Jun N-terminal kinases) and the extracellular signal-regulated kinases (ERKs) are members of the mitogen-activated protein kinase family of serine/threonine kinases, which have recently been implicated in G-protein-coupled receptor intracellular signaling. The SAPKs are preferentially induced by osmotic stress and UV light, whereas the ERKs are preferentially induced by growth factors and proliferative signals in cultured cells. In these studies, ACTH stimulated SAPK activity 3-4-fold both in the adrenal cortex in vivo and in the Y1 adrenocortical cell line. 12-O-Tetradecanoylphorbol-13-acetate but not cAMP induced SAPK activity in Y1 cells. The isoquinolinesulfonamide inhibitors H-8 and H-89 blocked ACTH induction of SAPK activity at protein kinase C inhibitory doses but not at protein kinase A inhibitory doses. The calcium chelating agent EGTA inhibited ACTH-induced SAPK activity and the calcium ionophore A23187 induced SAPK activity 3-fold. In contrast with the induction of SAPK by ACTH, ERK activity was inhibited in the adrenal cortex in vivo and in Y1 adrenal cells. Together these findings suggest that ACTH induces SAPK activity through a PKC and Ca+2-dependent pathway. The induction of SAPK and inhibition of ERK by ACTH in vivo may preferentially regulate target genes involved in the adrenocortical stress responses in the whole animal.

Regulation of growth by ACTH in the Y-1 line of mouse adrenocortical cells

Y-1 adrenal cells were cell cycle arrested by serum starvation to characterize a G0-->G1-->S transition in these cells. Cycle arrested Y-1 cells start to enter S phase 8h after serum feeding, reaching more than 90% cells synthesizing DNA by 24h. ACTH displays a dual effect in the G0-->G1-->S transition: 2h ACTH treatment stimulates DNA synthesis initiation, but longer treatments inhibit S phase entry. This dual effect of ACTH is similar to the antagonistic actions of PMA (phorbol-12-miristate-13-acetate) on the G0-->G1-->S transition. However ACTH and PMA are likely to have different mechanisms of action. ACTH inhibitory effect requires PKA, whereas PMA inhibitory effect is not dependent on PKA. ACTH induces the proto-oncogenes c-fos and c-jun, but inhibits the expression of the c-myc proto-oncogene. PMA, on the other hand, induces equally well c-fos, c-jun and c-myc. We hypothesize that ACTH promotes G0-->G1 transition by induction of c-fos and c-jun and blocks G1-->S transition by c-myc inhibition.

Regulation of primary response and specific genes in adrenal cells by peptide hormones and growth factors

Using cultured bovine adrenal fasciculata cells (BAC), we investigated the effects of two hormones, corticotropin (ACTH) and angiotensin II (Ang-II) and two growth factors, insulin-like growth factors I (IGF-I) and transforming growth factor beta 1 (TGF beta 1), on the mRNA levels of nuclear proto-oncogenes of the Fos and Jun families and on the mRNA levels of genes expressed in BAC coding for ACTH and AT1 receptors, cytochrome P450scc and P450 17 alpha and 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD). ACTH and IGF-1 increased c-fos and jun-B mRNA levels early with later increases in the levels of mRNA for the ACTH receptor and the three steroidogenic enzymes, and enhanced steroidogenic responses to both ACTH and Ang-II. In contrast, Ang-II increased mRNA coding for the three proto-oncogenes (cfos, c-jun, and jun-B), decreased those for P450 17 alpha and 3 beta-HSD, and caused marked homologous and heterologous steroidogenic desensitization. TGF beta 1 increased only jun-B mRNA and markedly reduced BAC-differentiated functions and steroidogenic responsiveness to both ACTH and Ang-II. The long-term effects of ACTH on human adrenal fasciculata cells were comparable with those observed in BAC, whereas the long term effects of Ang-II and TGF beta 1 were different from those observed in BAC. Whether these species-specific differences are related to a different effect of these factors on proto-oncogene expression is not yet known.

The ACTH receptor

The ACTH-R is a receptor that has a prominent role in mammalian physiology, but which has been notoriously difficulty to study. The cloning of the gene encoding this receptor in 1992 should permit significant advances in the understanding of the physiology, pharmacology and pathophysiology of ACTH. Areas of particular interest that should be clarified in the next few years are the control of the tissue specific expression of the gene, and an understanding of the molecular determinants of the ligand-receptor interaction. This latter area raises the prospect of clinically useful, orally active ACTH agonists and antagonists.

Peptide hormone and growth factor regulation of nuclear proto-oncogenes and specific functions in adrenal cells

Among the large number of immediate early genes, nuclear proto-oncogenes of the Fos and Jun families, have been postulated to be involved in the long-term effects of several growth factors on cell differentiation and/or multiplication. Since adrenal cell differentiated functions appear to be regulated by specific hormones and growth factors, the effects of these factors on proto-oncogene mRNA levels were analysed in bovine adrenal fasciculata cells (BAC) in culture. Corticotropin (ACTH) and insulin-like growth factor I increased c-fos and jun-B mRNA, but had no effect on c-jun mRNA and these early changes were associated with a later increase in BAC specific function [ACTH receptors, cytochrome P450 17 alpha) and 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD)] and an enhanced steroidogenic responsiveness to both ACTH and angiotensin-II (A-II). On the other hand, A-II increased the three proto-oncogene (c-fos, c-jun and jun-B) mRNAs, induced a decrease of P450 17 alpha and 3 beta-HSD and caused a marked homologous and heterologous (ACTH) densitization. Transforming growth factor beta 1 which only increased jun-B mRNA, markedly reduced BAC differentiated functions and the steroidogenic responsiveness to both ACTH and A-II. Thus, it is postulated that the proto-oncoproteins encoded by the immediate early genes may play a role in the long-term effects of peptide hormones and growth factors on BAC differentiated functions.

A model for studying regulation of aldosterone secretion: freshly isolated cells or cultured cells?

Practically all studies relating to zona glomerulosa function have been performed either with freshly isolated cells or with cells used after 2 or 3 days in culture. This study compares the step-by-step response (binding, second messenger production and aldosterone response) of isolated glomerulosa cells vs cells maintained in primary culture to the main stimuli of aldosterone secretion. One day in culture induces a decrease of 77 and 65% in the basal level of corticosterone and aldosterone secretions, compared to that observed in freshly isolated cells. In these conditions, the cells become more sensitive to most of their stimuli, but not all: e.g. important differences are noted in the dose-response of aldosterone secretion to adrenocorticotropin (ACTH), which is often shifted to a lower concentration sensitivity in cultured cells. For example, 0.1 nM ACTH stimulates steroid secretion by three-fold in isolated cells while 1 pM ACTH already induces a 25 and nine-fold increase, respectively, in corticosterone and aldosterone output in cultured cells. Moreover, some stimuli such as isoproterenol do not have any effect in isolated cells but do stimulate steroid secretion in cultured cells. In contrast, other stimuli, such as serotonin or DA (via DA2 receptors) act preferentially in freshly isolated cells. The main observation derived from this study is that glomerulosa cells, under appropriate conditions, are able to respond to their main secretagogues even after 4 days in culture. At this time, glomerulosa cells maintain their ultrastructural characteristics and functional properties and, aside from a few exceptions, demonstrate higher sensitivity to their known stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

Relevance of c-fos proto-oncogene induction for the steroidogenic response to ACTH, dcAMP and phorbol ester in adrenocortical cells

We previously reported that ACTH, but not dibutyryl cAMP, rapidly induces the c-fos proto-oncogene in Y-1 adrenocortical cells. Here we show that PMA induces c-fos with similar kinetics when compared with ACTH (0.5-1 h peak) but reaches only 60% of the maximal ACTH induction and dcAMP is a weak c-fos inducer (15% of ACTH). However, combination of PMA and dcAMP has a synergistic effect leading to maximal c-fos induction. c-fos expression may play a role in the RNA synthesis-dependent corticosteroidogenesis response and/or growth regulation by ACTH. We also show that, in contrast to dcAMP, PMA is a poor steroidogenesis stimulator (15 to 17% of maximum ACTH-stimulated level), its activity being completely dependent on RNA synthesis. Combination of dcAMP and PMA yields an additive steroidogenesis stimulation, an effect that is also dependent on RNA synthesis. Although no strict correlation was found between c-fos induction and early steroidogenesis stimulation, particularly with respect to cAMP derivatives, the results suggest that a PKC pathway is likely to cooperate with the classical cAMP-PKA pathway in adrenal cells' RNA-dependent steroidogenesis.

Transcriptional regulation of the adrenal steroidogenic enzymes

Corticosteroid biosynthesis requires the concerted action of a related group of cytochrome P450 steroid hydroxylases. The genes encoding these steroid hydroxylases exhibit two distinct levels of transcriptional regulation: selective expression in steroidogenic cells and induction in response to trophic hormones. With respect to cell-selective expression, recent studies have identified a nuclear receptor protein expressed only in steroidogenic cells that is postulated to regulate the expression of all cytochrome P450 steroid hydroxylases through common promoter elements. In contrast, the coordinate responses of these genes to trophic hormones are not readily explained by a unifying mechanism, and their hormone responsive expression probably involves multiple promoter elements.

The protein kinase C content is increased in the nuclear fraction of rat adrenal zona glomerulosa following long-term ACTH administration

It is well known that the adrenal zona glomerulosa is transformed to zona fasciculata-reticularis in rats exposed chronically to ACTH. This model was used to study the intracellular distribution of protein kinase C, which is known to be involved in differentiation processes. Under basal conditions, in zona glomerulosa, 70, 23, and 7% of the protein kinase C was located in the cytosol, membrane and nuclear fractions, respectively. At 30 min after ACTH administration to rats, the protein kinase C content remained unchanged in the nuclear fraction, whereas that of the cytosolic fraction was decreased to 43% while in the membranes it was increased to 48%. After 2 days of ACTH treatment, we observed a significant increase, up to 25%, of protein kinase C in the nuclear fraction, a decrease to 47% in the cytosol, whereas the membrane fraction content had returned to its basal value. The intracellular distribution of inner zones was 17% in nuclear fraction, 47% in cytosol and 36% in membranes. ACTH treatments did not change these proportions. The total protein kinase C content of ACTH-treated groups was not different than that of their respective controls, in zona glomerulosa and in inner zones respectively. The cytosolic protein kinase C formed complexes with detergent-treated nuclei; this association was saturable, and could be measured by the ability of the kinase to bind [3H]PDBu ([20(n)-3H]phorbol-12,13-dibutyrate). The number of nuclear 'acceptor sites' thus measured was calculated to be 5245 fmol/mg DNA in the zona glomerulosa; this did not change significantly following a 3-day administration of ACTH. Protein kinase C prepared from the adrenal inner zones also bound zona glomerulosa detergent-treated nuclei but occupied fewer sites than the protein kinase C from the zona glomerulosa. In conclusion, the effects of chronic ACTH treatment on rat adrenal zona glomerulosa could be mediated by an increased level of protein kinase C in the nuclear fraction and possibly through its binding to specific 'acceptor sites'.