Pertussis toxin can distinguish the augmentary effect elicited by epidermal growth factor from that of phorbol ester on luteal adenylate cyclase activity

Hormonal regulation of messenger ribonucleic acid expression for steroidogenic factor-1, steroidogenic acute regulatory protein, and cytochrome P450 side-chain cleavage in bovine luteal cells

To examine hormonal regulation of genes pertinent to luteal steroidogenesis, bovine theca and granulosa cells derived from preovulatory follicles were cultured with various combinations of forskolin and insulin. On Day 8 of culture, progesterone production was measured, and mRNA levels of steroidogenic factor-1 (SF-1), cytochrome P450 side-chain cleavage enzyme (P450scc), and steroidogenic acute regulatory protein (StAR) were determined by means of semiquantitative reverse transcription-polymerase chain reaction. Notably, the combination of forskolin plus insulin stimulated progesterone production in luteinized theca cells. This was probably a result of a synergistic interaction between forskolin and insulin, observed on both StAR and P450scc mRNA levels. However, in luteinized granulosa cells (LGC), forskolin and insulin each independently were able to up-regulate the levels of P450scc and StAR mRNA levels, respectively. Moreover, insulin alone was sufficient to maintain the high steady-state levels of StAR mRNA in LGC. Both insulin and insulin-like growth factor I enhanced StAR gene expression in LGC. SF-1 was constitutively expressed in bovine luteal cells; its amounts did not vary between the two luteal cell types or with hormonal treatments. In summary, this study demonstrates a distinct, cell-type specific regulation of StAR and P450scc mRNA in the two bovine luteal cell types.

Functional lysophosphatidic acid receptor in bovine luteal cells

The aim of this study was to determine whether luteal cells possess functional receptors for lysophosphatidic acid (LPA). We present evidence that [3H]LPA binds to a 38-40 kDa protein in a membrane fraction prepared from luteal cells and that this phospholipid is able to induce tyrosine phosphorylation of several proteins (65-125 kDa). Furthermore, LPA upregulates forskolin- and LH/GTP-stimulated adenylyl cyclase activity by changing its Vmax. Although a pertussis toxin-sensitive G-protein has been reported to transmit the inhibitory signals between the LPA receptor and adenylyl cyclase, the observed upregulation of the enzyme activity in luteal cells is not abolished after pre-treating the cells with the toxin, suggesting that a different mechanism is operative in these cells. According to the pharmacological regulatory pattern it is suggested that the modulated adenylyl cyclase isoform is the enzyme subtype V expressed in luteal cells.

Epidermal growth factor (EGF) receptor localization in cultured human granulosa lutein cells and the stimulation of progesterone production by EGF and transforming growth factor-alpha (TGF-alpha)

PURPOSE

The purpose of this study was to investigate the expression of EGF receptor (EGF-R) in human granulosa cells undergoing luteinization and progesterone production by these cells in response to EGF an TGF-alpha alone or in combination with luteinizing hormone (LH). Granulosa cells were obtained from IF patients following oocyte retrieval 34 to 36 hr post-hCG injection. EGF receptor was localized in cells by means of immunoperoxidase staining using a polyclonal primary antibody directed against the human EGF-R. To assess progesterone production, cells were seeded overnight, washed, and cultured with the growth factors +/- LH. Medium and treatments were changed every 24 hr for 3 days.

RESULTS

Specific EGF-R staining was observed in the cultured cells compared to those incubated with antibody that was preabsorbed with a 10-fold excess of EGF. Basal progesterone accumulation per 24-hr period was stimulated dose dependently on each day of culture, by both EGF (up to 3.5-fold at 5 or 50 ng/ml) and TGF-alpha (up to 4-fold at 50 ng/ml). The addition of LH alone also stimulated progesterone accumulation daily, and this effect was further enhanced dose dependently by cotreatment with EGF or TGF-alpha. Furthermore, tyrphostin, an EGF-R-specific tyrosine kinase inhibitor, inhibited both basal and growth factor-stimulated progesterone production.

CONCLUSION

These data suggest an EGF receptor-mediated physiological role for EGF and TGF-alpha in human luteal function involving an autocrine and/or a paracrine stimulation of progesterone production.

LH/hCG-receptor is coupled to both adenylate cyclase and protein kinase C signaling pathways in isolated mouse Leydig cells

The aim of this study was to examine whether or not a protein kinase C-dependent pathway is involved in the desensitization process of the LH/hCG-receptor-linked adenylate cyclase system in isolated mouse Leydig cells. Treatment of these cells with the phorbol ester, 4-β-phorbol 12-myristate 13-acetate (PMA) leads to a translocation (and a putative activation) of protein kinase C from the cytosol to the plasma membrane, as evidenced by the Western blotting procedure using particulate and cytosolic fractions of Percoll-purified mouse Leydig cells. A similar translocation is also observed following the treatment of mouse Leydig cells with hCG. Data obtained show that this effect is time-dependent and is mediated specifically through the LH/hCG-receptor. Furthermore, we show that the treatment of Leydig cells with either PMA or hCG leads to a desensitization of the adenylate cyclase stimulated with hCG, hCG plus GppNHp or AIF (4) (-) . This desensitization was not accompanied by a change in the [(125)I]-hCG binding to membrane receptors. Thus we provide here direct evidence that hCG is capable of activating protein kinase C. In addition, we postulate that PMA as well as hCG-treatment leads to a lesion located at a site distal to the receptor/G-protein interaction but proximal to the adenylate cyclase activation and that the translocation (and activation) of protein kinase C may be a common mechanism involved in this desensitizing effect caused by both PMA and hCG on Leydig cells.

Stimulation of neurite outgrowth in PC12 cells by EGF and KCl depolarization: a Ca(2+)-independent phenomenon

MAP kinase activity is necessary for growth factor induction of neurite outgrowth in PC12 cells. Although NGF and EGF both stimulate MAP kinase activity, EGF does not stimulate neurite extension. We report that EGF, in combination with KCl, stimulates neurite outgrowth in PC12 cells. This phenomenon was independent of intracellular Ca2+ increases and not due to enhancement of MAP kinase activity over that seen with EGF alone. However, EGF plus KCl increased intracellular cAMP, and other cAMP elevating agents acted synergistically with EGF to promote neurite outgrowth. Stimulation of neurite outgrowth by cAMP and EGF was blocked by inhibitors of transcription suggesting that synergistic regulation of transcription by the cAMP and MAP kinase pathways may stimulate neurite growth.

The early stimulation of glycolysis by epidermal growth factor in isolated rat hepatocytes is secondary to the glycogenolytic effect

We have studied the relationship between the effect of epidermal growth factor (EGF) on glycogen metabolism and its effect on glycolysis, in rat hepatocyte suspensions. Although 10 nM glucagon or 10 microM adrenaline increased glycogen degradation by more than 120%, 10 nM EGF increased glycogenolysis by less than 20% in hepatocytes incubated in glucose-free medium. Both glucagon and adrenaline increased phosphorylase a activity by more than 130%; EGF increased this activity by about 30%. Under basal conditions, 65% of the glucosyl residues were released as free glucose and about 30% ended up as C3 molecules (lactate and pyruvate). Both glucagon and adrenaline decreased the proportion of glucosyl units that rendered glycolysis end-products (to 2% for glucagon and 6% for adrenaline) and increased the proportion that ended up as free glucose (to 94% and 88% of the glucosyl residues for glucagon and adrenaline respectively). EGF increased the production of both free glucose and lactate+pyruvate, but the proportion of glucosyl residues that ended up as free glucose or glycolysis end-products was unchanged. In glycogen-depleted hepatocytes incubated in the presence of 25 mM glucose, EGF affected neither glycogen deposition nor glycolysis. EGF increased cytosolic free Ca2+, and neomycin decreased both the Ca2+ signal and the glycogenolytic effect. In conclusion, our results indicate that the effect of EGF on glycolysis is secondary to the Ca(2+)-mediated stimulation of glycogenolysis in rat hepatocyte suspensions.

Role of heterotrimeric G-proteins in epidermal growth factor signalling

Since in 1986 it was reported that a pertussis toxin-sensitive substrate was involved in the Ca2+ signal induced by epidermal growth factor (EGF) in rat hepatocytes, much evidence accumulated to implicate heterotrimeric G-proteins in EGF action. EGF can also induce a cyclic AMP signal, but while the generation of a Ca2+ signal appears to be quite general in EGF action, the increase in cyclic AMP occurs only in few cell types. In non-transformed cell types these effects appear to involve G-proteins. EGF not only induces cell proliferation but also interacts with hormones in the short-term control of cell function in quiescent cells. Most of the known interactions are on cyclic AMP mediated hormone effects, and in many cases, the interaction between EGF and hormones involves G-proteins. Here we review the evidence accumulated in recent years that implicate G-proteins in EGF action. An understanding of the mechanisms involved may reveal new mechanisms of G-protein regulation and will contribute to our knowledge of EGF function and signal transduction.