Human chorionic gonadotropin-alpha gene is transcriptionally activated by epidermal growth factor through cAMP response element in trophoblast cells

Stimulus-Induced Activation of the Glycoprotein Hormone α-Subunit Promoter in Human Placental Choriocarcinoma Cells: Major Role of a tandem cAMP Response Element

The glycoprotein hormones LH, FSH, TSH and chorionic gonadotropin consist of a common α-subunit and a hormone-specific β-subunit. The α-subunit is expressed in the pituitary and the placental cells, and its expression is regulated by extracellular signal molecules. Much is known about the regulation of the α-subunit gene in the pituitary, but few studies have addressed the regulation of this gene in trophoblasts. The aim of this study was to characterize the molecular mechanism of stimulus-induced α-subunit gene transcription in JEG-3 cells, a cellular model for human trophoblasts, using chromatin-embedded reporter genes under the control of the α-subunit promoter. The results show that increasing the concentration of the second messengers cAMP or Ca2+, or expressing the catalytic subunit of cAMP-dependent protein kinase in the nucleus activated the α-subunit promoter. Similarly, the stimulation of p38 protein kinase activated the α-subunit promoter, linking α-subunit expression to stress response. The stimulation of a Gαq-coupled designer receptor activated the α-subunit promoter, involving the transcription factor CREB, linking α-subunit expression to hormonal stimulation and an increase in intracellular Ca2+. Deletion mutagenesis underscores the importance of a tandem cAMP response element within the glycoprotein hormone α-subunit promoter, which acts as a point of convergence for a multiple signaling pathway.

Physiological profile of undifferentiated bovine blastocyst-derived trophoblasts

Trophectoderm of blastocysts mediate early events in fetal-maternal communication, enabling implantation and establishment of a functional placenta. Inadequate or impaired developmental events linked to trophoblasts directly impact early embryo survival and successful implantation during a crucial period that corresponds with high incidence of pregnancy losses in dairy cows. As yet, the molecular basis of bovine trophectoderm development and signaling towards initiation of implantation remains poorly understood. In this study, we developed methods for culturing undifferentiated bovine blastocyst-derived trophoblasts and used both transcriptomics and proteomics in early colonies to categorize and elucidate their functional characteristics. A total of 9270 transcripts and 1418 proteins were identified and analyzed based on absolute abundance. We profiled an extensive list of growth factors, cytokines and other relevant factors that can effectively influence paracrine communication in the uterine microenvironment. Functional categorization and analysis revealed novel information on structural organization, extracellular matrix composition, cell junction and adhesion components, transcription networks, and metabolic preferences. Our data showcase the fundamental physiology of bovine trophectoderm and indicate hallmarks of the self-renewing undifferentiated state akin to trophoblast stem cells described in other species. Functional features uncovered are essential for understanding early events in bovine pregnancy towards initiation of implantation.

Chemosensitizing effect of podophyllotoxin acetate on topoisomerase inhibitors leads to synergistic enhancement of lung cancer cell apoptosis

Podophyllotoxin acetate (PA) acts as a radiosensitizer against non-small cell lung cancer (NSCLC) in vitro and in vivo. In this study, we examined its potential role as a chemosensitizer in conjunction with the topoisomerase inhibitors etoposide (Eto) and camptothecin (Cpt). The effects of combinations of PA and Eto/Cpt were examined with CompuSyn software in two NSCLC cell lines, A549 and NCI-H1299. Combination index (CI) values indicated synergistic effects of PA and the topoisomerase inhibitors. The intracellular mechanism underlying synergism was further determined using propidium iodide uptake, immunoblotting and electrophoretic mobility shift assay (EMSA). Combination of PA with Eto/Cpt promoted disruption of the dynamics of actin filaments, leading to subsequent enhancement of apoptotic cell death via induction of caspase-3, -8, and -9, accompanied by increased phosphorylation of p38. Conversely, suppression of p38 phosphorylation blocked the apoptotic effect of the drug combinations. Notably, CREB-1, a transcription factor, was constitutively activated in both cell types, and synergistically inhibited upon combination treatment. Our results collectively indicate that PA functions as a chemosensitizer by enhancing apoptosis through activation of the p38/caspase axis and suppression of CREB-1.

Γ-Ionizing radiation-induced activation of the EGFR-p38/ERK-STAT3/CREB-1-EMT pathway promotes the migration/invasion of non-small cell lung cancer cells and is inhibited by podophyllotoxin acetate

Here, we report a new intracellular signaling pathway involved in γ-ionizing radiation (IR)-induced migration/invasion and show that podophyllotoxin acetate (PA) inhibits the IR-induced invasion and migration of A549 cells (a non-small cell lung cancer (NSCLC) cell line). Our results revealed that IR increased the invasion/migration of A549 cells, and this effect was decreased by 10 nM PA treatment. PA also inhibited the expressions/activities of matrix metalloprotase (MMP) -2, MMP-9, and vimentin, suggesting that PA could block the IR-induced epithelial-mesenchymal transition (EMT). The IR-induced increases in invasion/migration were associated with the activation of EGFR-AKT, and PA inhibited this effect. P38 and p44/42 ERK were also involved in IR-induced invasion/migration, and combined treatments with PA plus inhibitors of each MAPK synergistically blocked this invasion/migration. In terms of transcription factors (TFs), IR-induced increases in cyclic AMP response element-binding protein-1 (CREB-1) and signal transducer and activator of transcription 3 (STAT3) increased invasion/migration and EMT. PA also inhibited these transcription factors and then blocked IR-induced invasion/migration. Collectively, these results indicate that IR induces cancer cell invasion/migration by activating the EGFR-p38/ERK-CREB-1/STAT3-EMT pathway and that PA blocks this pathway to inhibit IR-induced invasion/migration.

Involvement of RelA-associated inhibitor in regulation of trophoblast differentiation via interaction with transcriptional factor specificity protein-1

Glucose transporter-1 (GLUT1), one of the key functional indicators of placental differentiation, has an important role in placental glucose transport. We previously showed that the protein levels of GLUT1 and nuclear transcription factor specificity protein-1 (Sp1) in rat choriocarcinoma cells (Rcho-1 cells) decreased during the differentiation of these cells to giant cells. We also showed that Sp1 was involved in the regulation of GLUT1 gene expression during this process. RelA-associated inhibitor (RAI) is an inhibitor of nuclear factor-kappaB that was identified by a yeast two-hybrid screen and is preferably expressed in human placenta and heart. RAI was also found to interact with Sp1 and exert an inhibitory effect against the DNA-binding activity of Sp1. We first show here that RAI mRNA expression increased as gestation proceeded and that RAI was localized mainly in the syncytiotrophoblast throughout pregnancy. The chloramphenicol acetyltransferase activity assay in Rcho-1 cells revealed that cotransfection of RAI expression vector resulted in decreased activity of the rat GLUT1 promoter but not in that of a mutated rat GLUT1 promoter lacking the Sp1 binding site. Furthermore, the protein level of RAI increased during differentiation. In addition, transfection of RAI expression vector promoted the morphological differentiation of Rcho-1 cells, and RAI knockdown using RAI-specific small interfering RNA reveals inhibitory effects on the morphological differentiation, as assessed by photomicroscopy. Taken together, these findings suggest that RAI may be involved in the regulation of trophoblast differentiation via interaction with Sp1.

Hypoxia Represses the Differentiation of Rcho-1 Rat Trophoblast Giant Cells

BACKGROUND

A hypoxic environment is known to be essential for early placentation. A low oxygen tension induces hypoxia-inducible factor-1 (HIF-1alpha) which may play an important role as a transcription factor in maintaining the proliferative and undifferentiated phenotype in human trophoblasts.

METHODS

We analyzed the effect of a low oxygen tension on the rat trophoblast giant cell differentiation pathway using Rcho-1 cells which were derived from rat choriocarcinomas and consist of trophoblast stem cells.

RESULTS

We found that a low oxygen tension suppressed the morphological changes and steroidogenesis during differentiation. The anticipated downregulation of the Id-1 transcription factor, a negative regulator of trophoblast giant cell differentiation, was not observed in the hypoxic environment. On the other hand, deferoxamine, which mimics hypoxia and induces HIF-1alpha, caused downregulation of Id-1 transcription factor and trophoblast giant cell differentiation.

CONCLUSION

These results indicate that hypoxia represses rat trophoblast giant cell differentiation via an HIF-1alpha-independent pathway.

Endocrine cell lines from the placenta

Cell-lines derived from human placenta and chorion have been used extensively to model the endocrine functions of human trophoblast. In general terms, the endocrine functions of the primary cells and tissues are at least partially replicated within the cell-lines, suggesting that they may be used as appropriate models. There are, however, two major provisos that compromise this generalisation. Firstly, the endocrine function of placenta represents a complex interaction between cytotrophoblast, syncytiotrophoblast and multiple regulators, so a single cell population digested from the normal environment is unlikely to represent this. Secondly, the characterisation of primary trophoblast populations and of cell-lines is incomplete, complicating the assignment of functions to trophoblast populations. Despite these difficulties, useful information has been obtained from the available cell-lines, regardless of whether they have arisen spontaneously, been transformed in vitro, or derived from cancers in vivo.

What turns CREB on?

The transactivation domain of the cAMP response element-binding protein (CREB) consists of two major domains. The glutamine-rich Q2 domain, which interacts with the general transcription factor TAFII130/135, is sufficient for the recruitment of a functional RNA polymerase II complex and allows basal transcriptional activity. The kinase-inducible domain, however, mediates signal-induced activation of CREB-mediated transcription. It is generally believed that recruitment of the coactivators CREB-binding protein (CBP) and p300 after signal-induced phosphorylation of this domain at serine-133 strongly enhances CREB-dependent transcription. Transcriptional activity of CREB can also be potentiated by phosphoserine-133-independent mechanisms, and not all stimuli that provoke phosphorylation of serine-133 stimulate CREB-dependent transcription. This review presents an overview of the diversity of stimuli that induce CREB phosphorylation at Ser-133, focuses on phosphoserine-133-dependent and -independent mechanisms that affect CREB-mediated transcription, and discusses different models that may explain the discrepancy between CREB Ser-133 phosphorylation and activation of CREB-mediated transcription.

An activator protein-1 complex mediates epidermal growth factor regulation of equine glycoprotein alpha subunit expression in trophoblast cells

Equids and primates are the only species known to express the placental hormone chorionic gonadotropin (CG). CG is a member of the heterodimeric glycoprotein family and is composed of an alpha subunit linked to a hormone-specific beta subunit. Previously, we have reported that epidermal growth factor (EGF) regulates the equine glycoprotein hormone alpha subunit promoter through a protein kinase C (PKC)/mitogen-activated protein kinase (MAPK) signal transduction pathway in trophoblasts. The current study investigates the regulatory element/factors involved in the induction of equine glycoprotein alpha subunit gene expression by EGF. Using 5' deletion mutagenesis, we have delineated the primary EGF/PKC responsive region of the equine alpha subunit gene to be located between -2039 to -2032 base pairs upstream of the transcriptional start site. The sequence within this region contains an activator protein 1 (AP-1)-like response element (TGAATCA) and is similar to a consensus AP-1 (TGAC/GTCA) response element. This element appeared to preferentially interact with a c-fos/JunD heterodimer. Stimulation by EGF induced the binding of c-fos and JunD to this element and subsequently elevated promoter activity. In conclusion, an EGF/PKC/MAPK signal transduction pathway regulates equine glycoprotein alpha subunit gene expression through a distinct regulatory element(s) that lies between -2039 to -2032 of the equine glycoprotein alpha subunit promoter in trophoblasts and involves an AP-1 complex.

Induction of transcriptional activity of the cyclic adenosine monophosphate response element binding protein by parathyroid hormone and epidermal growth factor in osteoblastic cells

Previously, we have shown that parathyroid hormone (PTH) transactivation of cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) requires both serine 129 (S129) and serine 133 (S133) in rat osteosarcoma cells UMR 106-01 (UMR) cells. Furthermore, although protein kinase A (PKA) is responsible for phosphorylation at S133, glycogen synthase kinase 3beta (GSK-3beta) activity is required and may be responsible for phosphorylation of CREB at S129. Here, we show, using the GAL4-CREB reporter system, that epidermal growth factor (EGF) can transactivate CREB in UMR cells in addition to PTH. Additionally, treatment of UMR cells with both PTH and EGF results in greater than additive transactivation of CREB. Furthermore, using mutational analysis we show that S129 and S133 are required for EGF-induced transcriptional activity. EGF activates members of the MAPK family including p38 and extracellular signal-activated kinases (ERKs), and treatment of UMR cells with either the p38 inhibitor (SB203580) or the MEK inhibitor (PD98059) prevents phosphorylation of CREB at S133 by EGF but not by PTH. Treatment of cells with either SB203580 or PD98059 alone or together significantly inhibits transactivation of CREB by EGF but not by PTH, indicating that EGF regulates CREB phosphorylation and transactivation through p38 and ERKs and PTH does not. Finally, the greater than additive transactivation of CREB by PTH and EGF is significantly inhibited by the PKA inhibitor H-89 or by cotreatment with SB203580 and PD98059. Thus, several different signaling pathways in osteoblastic cells can converge on and regulate CREB activity. This suggests, in vivo, that circulating agents such as PTH and EGF are acting in concert to exert their effects.

Molecular mechanisms involved in gastrin-mediated regulation of cAMP-responsive promoter elements

In the present study, we explore the role of cAMP-responsive (CRE) promoter elements in gastrin-mediated gene activation. By using the minimal CRE promoter reporter plasmid, pCRELuc, we show that gastrin can activate CRE. This activation is blocked by H-89 and GF 109203x, which inhibit protein kinases A and C, respectively. Moreover, Ca(2+)-activated pathways seem to be involved, because the calmodulin inhibitor W-7 reduced gastrin-mediated activation of pCRELuc. Deletion of CRE from the c-fos promoter rendered this promoter completely unresponsive to gastrin, indicating that CRE plays a central role in c-fos transactivation. Interestingly, gastrin-induced expression of the inducible cAMP early repressor (ICER), a gene that is known to be regulated by CRE promoter elements, was not reduced by H-89, W-7, or GF 109203x. Furthermore, bandshift analyses indicated that the region of the ICER promoter containing the CRE-like elements CARE 3-4 binds transcription factors that are not members of the CRE-binding protein-CRE modulator protein-activating transcription factor, or CREB/CREM/ATF-1, family. Our results underline the significance of the CRE promoter element in gastrin-mediated gene regulation and indicate that a variety of signaling mechanisms are involved, depending on the CRE promoter context.

Progesterone inhibits transcriptional activation of human chorionic gonadotropin-alpha gene through protein kinase A pathway in trophoblast cells

The purpose of this study was to analyze the mechanism of transcriptional inhibition of human chorionic gonadotropin-alpha (hCGalpha) gene by progesterone in trophoblast cells. We stably transfected -290 bp hCGalpha promoter-CAT constructs (-290halphaCAT) into Rcho-1 cells and monitored the promoter activities. Differentiation-dependent activation of -290 bp hCGalpha promoter containing a tandem repeat of cAMP response element (CRE) was inhibited by progesterone in a dose-dependent manner. To further analyze the mechanism of the progesterone action, Rcho-1 cells stably transfected with -290halphaCAT were treated with forskolin in the presence of progesterone. Progesterone inhibited forskolin-induced transcriptional activation of hCGalpha gene. Moreover, progesterone inhibited forskolin-induced transcriptional activation of CRE-CRE-tk-CAT. These results suggest that progesterone may inhibit cAMP-induced transcriptional activation of hCGalpha gene through CRE. Although progesterone did not alter the amount of CRE-binding protein (CREB), which is a main transcriptional factor bound to CRE(s) on hCGalpha promoter, progesterone abolished forskolin-induced CREB phosphorylation. In addition, pretreatment with progesterone abolished forskolin-induced activation of nuclear protein kinase A (PKA). In conclusion, progesterone inhibits hCGalpha gene transcription, at least in part, via the CRE region by inhibiting CREB phosphorylation through PKA pathway in trophoblast cells.

Epidermal growth factor regulation of equine glycoprotein hormone alpha subunit expression in trophoblast cells

Primates and equids are the only species known to express the placental glycoprotein hormone, chorionic gonadotropin (CG), a heterodimeric glycoprotein composed of an alpha subunit linked to a hormone-specific beta subunit. The regulatory mechanisms involved in the induction of equine glycoprotein alpha subunit gene expression have not been identified. Epidermal growth factor (EGF) receptor is known to transduce signals that alter a number of different cellular functions (cell proliferation, differentiation, hormone secretion, and gene regulation). In the present study, we investigated the regulation of the equine alpha subunit gene by EGF in trophoblasts. We found that 2800 base pairs of 5' flanking sequence from the equine alpha subunit promoter is sufficient for basal expression in human choriocarcinoma cells. Epidermal growth factor and phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), increased transcriptional activity of the equine alpha subunit promoter (-2800/+21). These responses were blocked by pretreatment with bisindolylmaleimide-I, an inhibitor of PKC, suggesting an involvement of this pathway downstream of EGF. In addition, PD98059, an inhibitor of the extracellular signal-regulated kinase (ERK) pathway, completely blocked activation of the equine alpha promoter by PMA, suggesting that mitogen-activated protein kinase (MAPK) cascade was involved downstream of the PKC pathway. In conclusion, the EGF/PKC/MAPK pathway regulates equine glycoprotein alpha subunit gene expression through a distinct regulatory region (-2300 to -1900) in trophoblasts, while essential elements for basal expression appear to exist within the -2800 to -1900 region of the promoter.

The Egr-1 gene is induced by epidermal growth factor in ECV304 cells and primary endothelial cells

The early growth response (Egr)-1 transcription factor serves to couple changes in the extracellular environment to alterations in gene expression. An understanding of the mechanisms that underlie Egr-1 gene regulation should provide important insights into how environmental signals are transduced by endothelial cells. The aim of the present study was to determine whether epidermal growth factor (EGF) induces Egr-1 expression in endothelial cells. In ECV304 cells, Egr-1 mRNA and protein levels were increased in response to EGF. In stable transfection assays, the 1,200-bp promoter of the mouse Egr-1 gene contained information for EGF response via a protein kinase C-independent, mitogen-activated protein kinase-dependent pathway. The endogenous Egr-1 gene was similarly responsive to EGF in primary human umbilical vein endothelial cells, human coronary artery endothelial cells, and rat fat pad endothelial cells, but not in bovine aortic endothelial cells, calf pulmonary artery endothelial cells, or PY-4-1 endothelial cells. Together, these results suggest that the Egr-1 gene is responsive to EGF in a subset of endothelial cells.

Role of the cyclic AMP response element binding complex and activation of mitogen-activated protein kinases in synergistic activation of the glycoprotein hormone alpha subunit gene by epidermal growth factor and forskolin

The aim of these studies was to elucidate a role for epidermal growth factor (EGF) signaling in the transcriptional regulation of the glycoprotein hormone alpha subunit gene, a subunit of chorionic gonadotropin. Studies examined the effects of EGF and the adenylate cyclase activator forskolin on the expression of a transfected alpha subunit reporter gene in a human choriocarcinoma cell line (JEG3). At maximal doses, administration of EGF resulted in a 50% increase in a subunit reporter activity; forskolin administration induced a fivefold activation; the combined actions of EGF and forskolin resulted in synergistic activation (greater than eightfold) of the alpha subunit reporter. Mutagenesis studies revealed that the cyclic AMP response elements (CRE) were required and sufficient to mediate EGF-forskolin-induced synergistic activation. The combined actions of EGF and forskolin resulted in potentiated activation of extracellular signal-regulated kinase (ERK) enzyme activity compared with EGF alone. Specific blockade of ERK activation was sufficient to block EGF-forskolin-induced synergistic activation of the alpha subunit reporter. Pretreatment of JEG3 cells with a p38 mitogen-activated protein kinase inhibitor did not influence activation of the alpha reporter. However, overexpression of c-Jun N-terminal kinase (JNK)-interacting protein 1 as a dominant interfering molecule abolished the synergistic effects of EGF and forskolin on the alpha subunit reporter. CRE binding studies suggested that the CRE complex consisted of CRE binding protein and EGF-ERK-dependent recruitment of c-Jun-c-Fos (AP-1) to the CRE. A dominant negative form of c-Fos (A-Fos) that specifically disrupts c-Jun-c-Fos DNA binding inhibited synergistic activation of the alpha subunit. Thus, synergistic activation of the alpha subunit gene induced by EGF-forskolin requires the ERK and JNK cascades and the recruitment of AP-1 to the CRE binding complex.

Acceleration of trophoblast differentiation by heparin-binding EGF-like growth factor is dependent on the stage-specific activation of calcium influx by ErbB receptors in developing mouse blastocysts

Heparin-binding EGF-like growth factor (HB-EGF) is expressed in the mouse endometrial epithelium during implantation exclusively at sites apposed to embryos and accelerates the development of cultured blastocysts, suggesting that it may regulate peri-implantation development in utero. We have examined the influence of HB-EGF on mouse trophoblast differentiation in vitro and the associated intracellular signaling pathways. HB-EGF both induced intracellular Ca2+ signaling and accelerated trophoblast development to an adhesion-competent stage, but only late on gestation day 4 after ErbB4, a receptor for HB-EGF, translocated from the cytoplasm to the apical surface of trophoblast cells. The acceleration of blastocyst differentiation by HB-EGF was attenuated after inhibition of protein tyrosine kinase activity or removal of surface heparan sulfate, as expected. Chelation of intracellular Ca2+ blocked the ability of HB-EGF to accelerate development, as did inhibitors of protein kinase C or calmodulin. The absence of any effect by a phospholipase C inhibitor and the requirement for extracellular Ca2+ suggested that the accrued free cytoplasmic Ca2+ did not originate from inositol phosphate-sensitive intracellular stores, but through Ca2+ influx. Indeed, N-type Ca2+ channel blockers specifically inhibited the ability of HB-EGF to both induce Ca2+ signaling and accelerate trophoblast development. We conclude that HB-EGF accelerates the differentiation of trophoblast cells to an adhesion-competent stage by inducing Ca2+ influx, which activates calmodulin and protein kinase C. An upstream role for ErbB4 in this pathway is implicated by the timing of its translocation to the trophoblast surface.

Insulin produces a growth hormone-like increase in intracellular free calcium concentration in okadaic acid-treated adipocytes

In vivo, GH and insulin usually produce opposing effects on carbohydrate and lipid metabolism in adipocytes, even though their signal transduction pathways overlap. However, when added to rat adipocytes that have been made GH deficient, GH briefly produces responses that are qualitatively like those of insulin. Subsequently, GH induces refractoriness to this acute insulin-like response, in a sense restricting its effects to a unique subset of possible physiological actions. Okadaic acid is an inhibitor of type I and IIa phosphoprotein phosphatases and affects glucose metabolism in fat cells in a manner that is reminiscent of GH. Okadaic acid initially mimics the actions of insulin, and subsequently, even after it has been removed by thorough washing, blunts the ability of adipocytes to accelerate glucose metabolism in response to insulin or GH. Because refractoriness to the insulin-like effect of GH is associated with GH-induced increases in intracellular free calcium concentrations ([Ca2+]i), we examined the effects of insulin on [Ca2+]i in okadaic acid-treated adipocytes. Adipocytes were incubated with 0.25 microM okadaic acid for 1 h, washed, and reincubated without okadaic acid for 2 h before measurement of [Ca2+]i using fura-2 as a calcium indicator. Neither GH (500 ng/ml) nor insulin (100 microU/ml) affected [Ca2+]i in cells in which glucose metabolism was stimulated, but both hormones rapidly increased [Ca2+]i in adipocytes that were refractory to insulin-like stimulation. The characteristics of the increase in [Ca2+]i produced by insulin were identical to those previously reported for GH. The effect of insulin was mimicked by the dihydropyridine calcium channel activator BayK 5552 or depolarization of the cell membrane with 30 mM KCl and was blocked by the dihydropyridine calcium channel blocker, nimodipine (100 nM), implicating activation of voltage-sensitive L-type Ca2+ channels. The increase in [Ca2+]i was also mimicked by sn-1,2-dioctanoylglycerol and blocked by inhibitors of protein kinase C (staurosporine, chelerythrine chloride, and calphostin), and D609, an inhibitor of phospholipase C, as reported for GH. Acquisition of the ability to increase [Ca2+]i in response to insulin required a lag period of at least 2 h after removal of okadaic acid and was prevented by inhibitors of RNA and protein synthesis. Adipocytes that were incubated with inhibitors of protein kinase A (KT-5720), or protein kinase C (staurosporine) along with okadaic acid also failed to increase [Ca2+]i in response to insulin. Conversely, adipocytes that were incubated with dibutyryl cAMP, methylisobutyl xanthine, or phorbol ester instead of okadaic acid increased [Ca2+]i when treated with insulin 2 h later. These results suggest that phosphorylated substrates of protein kinases A and C may mediate the transcriptional event(s) that enable adipocytes to activate L-type Ca2+ channels in response to insulin. Blockade of protein kinases A or C or removal of calcium from the incubation medium did not restore the ability of okadaic acid-treated adipocytes to increase glucose metabolism in response to insulin, nor did pretreatment of adipocytes with dibutyryl cAMP or phorbol ester decrease insulin-induced stimulation of glucose metabolism. The failure of insulin to increase glucose metabolism in okadaic acid-treated adipocytes thus cannot be ascribed to the increase in [Ca2+]i. These findings indicate that just as GH can produce an insulin-like response, so too can insulin produce a GH-like response, and highlight the need to understand how specificity of hormone action is achieved in cells that respond to different hormones that share elements of their transduction pathways.