Protein kinase A and C activities and endogenous substrates in ovine small and large luteal cells

Effects of selective protein kinase c isozymes in prostaglandin2alpha-induced Ca2+ signaling and luteinizing hormone-induced progesterone accumulation in the mid-phase bovine corpus luteum

A single-cell approach for measuring the concentration of cytoplasmic calcium ions ([Ca(2+)](i)) and a protein kinase C-epsilon (PKCepsilon)-specific inhibitor were used to investigate the developmental role of PKCepsilon in the prostaglandin F(2alpha)(PGF(2alpha))-induced rise in [Ca(2+)](i) and the induced decline in progesterone accumulation in cultures of cells isolated from the bovine corpus luteum. PGF(2alpha) increased [Ca(2+)](i) in Day 4 large luteal cells (LLCs), but the response was significantly lower than in Day 10 LLCs (4.3 +/- 0.6, n = 116 vs. 21.3 +/- 2.3, n = 110). Similarly, the fold increase in the PGF(2alpha)-induced rise in [Ca(2+)](i) in Day 4 small luteal cells (SLCs) was lower than in Day 10 SLCs (1.6 +/- 0.2, n = 198 vs. 2.7 +/- 0.1, n = 95). A PKCepsilon inhibitor reduced the PGF(2alpha)-elicited calcium responses in both Day 10 LLCs and SLCs to 3.5 +/- 0.3 (n = 217) and 1.3 +/- 0.1 (n = 205), respectively. PGF(2alpha) inhibited LH-stimulated progesterone (P(4)) accumulation only in the incubation medium of Day 10 luteal cells. Both conventional and PKCepsilon-specific inhibitors reversed the ability of PGF(2alpha) to decrease LH-stimulated P(4) accumulation, and the PKCepsilon inhibitor was more effective at this than the conventional PKC inhibitor. In conclusion, the evidence indicates that PKCepsilon, an isozyme expressed in corpora lutea with acquired PGF(2alpha) luteolytic capacity, has a regulatory role in the PGF(2alpha)-induced Ca(2+) signaling in luteal steroidogenic cells, and that this in turn may have consequences (at least in part) on the ability of PGF(2alpha) to inhibit LH-stimulated P(4) synthesis at this developmental stage.

Effects of second messengers on gap junctional intercellular communication of ovine luteal cells throughout the estrous cycle

Corpora lutea (CL) from Days 5, 10, and 15 after superovulation were enzymatically dispersed, and a portion of the cells were elutriated to obtain fractions enriched with small or large luteal cells. Mixed, small, and large luteal cell fractions were incubated with no treatment or with agonists or antagonists of cAMP (dbcAMP or Rp-cAMPS), protein kinase C (PKC; TPA or H-7), or calcium (A23187, EGTA, or A23187 + EGTA). The rate of contact-dependent gap junctional intercellular communication (GJIC) was evaluated by laser cytometry. Media were collected for progesterone (P(4)) radioimmunoassay, and luteal cells cultured with no treatment were fixed for immunocytochemistry or frozen for Western blot analysis. Luteal cells from each stage of the estrous cycle exhibited GJIC. The dbcAMP increased (P < 0.05) GJIC for all cell types across the estrous cycle. The Rp-cAMPS decreased (P < 0.05) GJIC for small luteal cells on Day 5 and for all cell types on Days 10 and 15. The TPA inhibited (P < 0.01), but H-7 did not affect, GJIC for all cell types across the estrous cycle. The A23187 decreased (P < 0.05) GJIC for large luteal cells touching only small or only large luteal cells, whereas A23187 + EGTA decreased (P < 0.05) GJIC for all cell types across the estrous cycle. For the mixed and large luteal cell fractions, dbcAMP increased (P < 0.05), but TPA and A23187 + EGTA decreased (P < 0.05), P(4) secretion. The A23187 alone decreased (P < 0.05) P(4) secretion by large, but not by mixed, luteal cells. For all days and cell types, the rate of GJIC and P(4) secretion were correlated (r = 0.113-0.249; P < 0.01). Connexin 43 was detected in cultured luteal cells by immunofluorescence and Western immunoblotting. Thus, intracellular regulators like cAMP, PKC, or calcium appear to regulate GJIC, which probably is an important mechanism for coordinating function of the ovine CL.

Effect of follicle-stimulating hormone on sertoli cell division in cultures of fetal rat testes

The present study was performed to determine when follicle-stimulating hormone (FSH) begins to promote Sertoli cell division in fetal rats, and to determine whether the effect of FSH is mediated by cAMP-dependent protein kinase (PKA). When testes from 15- to 17-day fetuses were cultured with or without FSH for 48 h, FSH did not promote Sertoli cell division in 15-day testes, but did in 16- and 17-day testes. Anti-rat FSH was injected into 16-day fetuses in utero. Twenty-four hours later, the testes of the injected fetuses and those of their intact littermates were cultured with or without FSH for 48 h. Without FSH, the Sertoli cell division index was significantly lower in anti-FSH-treated fetuses than in intact fetuses. With FSH, however, the index increased. When PKA inhibitor was added to cultures of 16-day testes with FSH, the promotion of Sertoli cell division by FSH was inhibited. We conclude that between 16 and 17 days of gestation, fetal pituitary FSH stimulates the division of Sertoli cells by activating the PKA activity.

Electrical permeabilization of rat luteal cells: in situ phosphorylation of endogenous protein

Progesterone synthesis in the corpus luteum is regulated primarily by luteinizing hormone which acts via the adenylate cyclase/cyclic AMP/protein kinase A signalling cascade. Protein phosphorylation therefore plays a key role in the regulation of steroidogenesis, but there are relatively few studies of the in situ phosphorylation of luteal cell substrates. This may in part reflect the difficulties inherent in measuring changes in protein phosphorylation in intact cells preloaded with 32P and difficulties in interpreting data obtained using broken cell preparations. We have now applied a method of stable permeabilization of luteal cell plasma membranes by exposure of cell populations to a high intensity electric field. Under optimum conditions (5 kV/cm, six discharges) electrical permeabilization reproducibly produced populations of luteal cells in which 70-80% of the cells were permeabilized, as assessed by Trypan blue exclusion and [14C] sucrose space measurements. Pores were stable for at least 1 h, and there were no ultrastructural changes to the cells that could be detected by transmission electron microscopy. Permeabilized cells showed rapid cyclic AMP-induced changes in phosphorylation of endogenous proteins when provided with [gamma - 32 P] ATP. Our results demonstrate that the electricity permeabilized luteal cell offers a useful model for studying intracellular events in steroidogenic stimulus-response coupling cascades.

Steroidogenic enzyme activity after acute activation of protein kinase (PK) A and PKC in ovine small and large luteal cells

Intracellular effector systems which utilize PKA and PKC can be pharmacologically activated by forskolin and phorbol 12-myristate 13-acetate (PMA) and appear to be important for regulation of steroidogenesis by cells of the corpus luteum. In this study the effect of pharmacologic activation of PKA (forskolin) or PKC (PMA) on the activity of adenylate cyclase, cholesterol esterase, P450 cholesterol side chain cleavage (P450scc) and 3 beta-hydroxysteroid dehydrogenase/delta 5, delta 4 isomerase (3 beta HSD) was determined. Basal adenylate cyclase activity (as measured by intracellular and secreted cAMP) was extremely low in both large and small luteal cells. Forskolin stimulated adenylate cyclase activity in both large and small luteal cells but progesterone production was increased only in small cells. PMA inhibited progesterone production by large and forskolin-stimulated small cells without altering adenylate cyclase activity. Basal cholesterol esterase activity was greater in small than in large cells and was stimulated by forskolin only in small cells. PMA did not significantly alter cholesterol esterase activity in either cell type. Activity of P450scc or 3 beta HSD was measured by conversion of hydroxylated cholesterol derivatives (P450scc) or pregnenolone (3 beta HSD) to progesterone. Although basal progesterone production was 47 times greater in large than small cells, there was only 5.1 (P450scc) and 6.4 (3 beta HSD) times greater enzyme activity in large than in small luteal cells. Activation of PKA and/or PKC did not alter the activity of P450scc or 3 beta HSD in either cell type.(ABSTRACT TRUNCATED AT 250 WORDS)

Steroidogenic enzymes: structure, function, and role in regulation of steroid hormone biosynthesis

In the pathways of steroid hormone biosynthesis there are two major types of enzymes: cytochromes P450 and other steroid oxidoreductases. This review presents an overview of the function and expression of both types of enzymes with emphasis on steroidogenic P450s. The final part of the review on regulation of steroidogenesis includes a description of the normal physiological fluctuations in the steroid output of adrenal cortex and gonads, and provides an analysis of the relative role of enzyme levels in the determination of these fluctuations. The repertoire of enzymes expressed in a steroidogenic cell matches the cell's capacity for the biosynthesis of specific steroids. Thus, steroidogenic capacity is regulated mainly by tissue and cell specific expression of enzymes, and not by selective activation or inhibition of enzymes from a larger repertoire. The quantitative capacity of steroidogenic cells for the biosynthesis of specific steroids is determined by the levels of steroidogenic enzymes. The major physiological variations in enzyme levels, are generally associated with parallel changes in gene expression. The level of expression of each steroidogenic enzyme varies in three characteristics: (a) tissue- and cell-specific expression, determined during tissue and cell differentiation; (b) basal expression, in the absence of trophic hormonal stimulation; and (c) hormonal signal regulated expression. Each of these three types of expression probably represent the functioning of distinct gene regulatory elements. In adult steroidogenic tissues, the levels of most of the cell- and tissue-specific steroidogenic enzymes depend mainly on trophic hormonal stimulation mediated by a complex network of signal transduction systems.

Hormonal regulation of the type III isoform of C-kinase in porcine ovarian tissues

C-kinase activity is notably increased in corpora lutea (CL) compared to preovulatory follicles of porcine ovaries. Our purpose was to identify the C-kinase isoform(s) involved in this increase and to examine the expression of C-kinase in ovarian tissues at different stages. The major component of C-kinase activity in the CL was isoform III, with a molecular weight (M(r)) of 80,000. Minor activities were attributed to the type II isoform (M(r) = 80,000) and an unidentified C-kinase activity (M(r) = 77,000). C-kinase was not partitioned differently in preovulatory follicles and CL as demonstrated by tissue homogenization in the presence of a detergent and increased chelators. A 3-fold increase in immunoreactive C-kinase was detected in postovulatory follicles relative to preovulatory. A second, nearly 3-fold increase in C-kinase was detected in mature CL relative to postovulatory follicles. These increases are examined in the context of the complex hormonal regulation of the porcine CL.

Calcium-independent phospholipid/diolein-dependent phosphorylation of a soluble ovarian Mr 80,000 substrate protein: biochemical characteristics

Soluble ovarian extracts were incubated with protein kinase effectors in the presence of [gamma 32P]ATP and proteins were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Autoradiograms revealed phosphorylation of an ovarian Mr = 80,000 substrate in the presence of EGTA ([ethylenebis(oxyethylenenitrilo)]tetraacetic acid), phosphatidylserine and 1,2-diolein. In contrast to a classical response pattern to C-kinase effectors, the ovarian Mr = 80,000 phosphorylation was inhibited by 2 x 10(-7) M or greater free Ca2+. The ovarian Mr = 80,000 substrate was distinguished from the myristoylated acidic Mr = 80,000 C-kinase substrate of brain tissue on the basis of heat stability and phosphorylative response to effectors. Phosphorylation of the exogenous substrate myelin basic protein by DEAE-resolved ovarian kinase showed the variant effector dependence, maximal in the presence of EGTA, phosphatidylserine and 1,2-diolein. Finally, the effect of Ca2+ on ovarian Mr = 80,000 [32P]phosphate content could not be accounted for by post-phosphorylation activities, or by DEAE-resolvable or hydroxylapatite-resolvable inhibitory activities.

Phorbol ester receptors in bovine luteal cells: relationship to protein kinase C

We investigated the binding kinetics of the tumor-promoting phorbol ester, phorbol-12,13-dibutyrate (PBt2) to dispersed total bovine luteal cells, purified small luteal cells, and purified luteal protein kinase C (PKC). Saturation analysis and competitive displacement techniques were used. Binding of [3H]PBt2 to total luteal cell preparations resulted in two distinct affinities. The high affinity component was characterized by a Kd of 4.5 +/- 1.5 nM. Analysis of [3H]PBt2 binding to total cells using competitive displacement demonstrated that the low affinity binding was specific and displaceable but dependent on concentrations of [3H]PBt2 far above the Kd for the high affinity binding. In contrast to the total cell preparations, only high affinity binding was observed in intact purified small luteal cells (Kd = 0.96 +/- 0.04 nM). Partial purification of luteal cytosolic PKC by DEAE-Sephadex chromatography resulted in co-elution of PKC enzyme activity and the [3H]PBt2 binding activity. Under conditions of saturating calcium (0.1 mM) and phosphatidylserine (PS) (100 micrograms/tube) concentrations, binding to the partially purified PKC preparation was found to be of a single high affinity and exhibited a Kd (1.3 +/- 0.2 nM) similar to the high affinity binding observed in intact cells. These results suggest that the primary phorbol ester receptor in luteal cells is PKC. However, a low affinity, high capacity [3H]PBt2 binding site also exists within the corpus luteum, either in the large cells or in the accessory cell fraction which consists mainly of endothelial cells.

Phorbol ester- and luteinizing hormone-induced phosphorylation of membrane proteins in bovine luteal cells

In this study we have investigated the protein phosphorylation pattern in the membrane fraction prepared from bovine luteal cells. The phosphorylation reaction was carried out in vitro, under defined conditions, using either [gamma-32P]ATP or [gamma-35S]ATP as the phosphate donor. The results obtained show that [gamma-35S]ATP was a suitable phosphate donor for performing in vitro phosphorylation studies, and that thiophosphorylation of at least eight protein bands (120 kDa to 18 kDa) was observed. The extent of phosphorylation was dependent upon the duration of incubation and the amount of membrane protein used. The presence of Ca2+ was obligatory for phosphorylation and an enhanced phosphorylation was observed in the presence of Ca2+, phosphatidyl serine and phorbol 12-myristate 13-acetate (PMA), agents known to activate protein kinase C. Interestingly, when phosphorylation was carried out in the presence of luteinizing hormone (LH), a phosphorylation pattern was obtained which was similar to that obtained in the presence of calcium and phospholipid. Furthermore, in the case of two protein bands corresponding to 80-82 and 44-46 kDa, an additive phosphorylation was observed when the phosphorylation reaction was carried out for 5 min in the presence of both LH and Ca2+, phosphatidyl serine and PMA. To conclude, we have demonstrated a calcium- and phospholipid-dependent endogenous protein phosphorylation in the membrane fraction prepared from bovine luteal cells and the data obtained suggest that LH is able to stimulate this endogenous protein phosphorylation via a protein kinase C-mediated mechanism.