Activation and glucagon regulation of mitogen-activated protein kinases (MAPK) by insulin and epidermal growth factor in cultured rat and human hepatocytes

Regulation of human hepatocytes by P2Y receptors: control of glycogen phosphorylase, Ca2+, and mitogen-activated protein kinases

In the rat both short-term liver function, such as glycogen metabolism, and long-term events such as proliferation after partial hepatectomy, are in part controlled by release of nucleotides such as ATP acting on hepatocyte P2Y(1) and P2Y(2) receptors (members of a family of P2Y receptors for extracellular nucleotides such as ATP and UTP). Here, we have studied P2Y receptor regulation of signaling pathways involved in glycogen phosphorylase activation and proliferation of primary human hepatocytes. Stimulation of cultured hepatocytes with either ATP and UTP, but not UDP or 2-methylthio ADP, led to concentration-dependent increases in cytosolic free Ca(2+) concentration ([Ca(2+)](c); EC(50) for ATP = 3.3 microM, for UTP = 2.3 microM) and [(3)H]inositol (poly)phosphates (EC(50) for ATP = 9.4 microM, for UTP = 15.4 microM). ATP and UTP also stimulated glycogen phosphorylase in human hepatocytes, each with a threshold for activation of less than 1 microM. Application of 2-methylthio ADP up to 100 microM was ineffective. Phosphorylation of both extracellular signal-related kinase and c-Jun N-terminal kinase was stimulated by ATP and UTP, but not by 2-methylthio ADP or UDP, either alone or when costimulated with epidermal growth factor. In conclusion, in human hepatocytes P2Y receptors control both glycogen metabolism and proliferation-associated responses such as increased [Ca(2+)](c) and mitogen-activated protein kinase cascades. Regulation seems to be primarily through P2Y(2) receptors. In contrast with previous studies on rat hepatocytes, there is an absence of responses mediated by P2Y(1) receptors.

Possible mechanisms underlying the mitogenic actionof heptachlor in rat hepatocytes

The worldwide use of the organochlorine pesticide heptachlor has led to widespread contamination in the environment. Like many other organochlorine pesticides, heptachlor is considered to pose a threat to human health. It has been shown that heptachlor is a tumor-promoting agent, but the mechanisms involved still remain unclear. The negative response of heptachlor in in vitro genotoxicity test suggests that this pesticide displays its carcinogenicity through epigenetic pathways. With the growing evidence that proliferation accounts for the tumor-promoting effects of many agents, the purpose of this work was to investigate the mechanisms involved in the mitogenic activity of heptachlor in quiescent rat hepatocytes and to understand the properties of this compound as a tumor promoter in the liver. Heptachlor triggered significant proliferation in quiescent rat hepatocytes. Two mechanisms were delineated to support the mitogenic effect in the hepatocyte: activation of key kinases in signaling pathways and inhibition of apoptosis. Exposure to heptachlor led to activation of protein kinase C mitogenactivated protein kinases. Moreover, these results indicate that like many tumor promoters, heptachlor strongly inhibited TGFbeta-induced apoptosis and cytochrome c release into the cytosol. The levels of the anti-apoptotic protein Bcl-2 were also increased in the presence of heptachlor. In conclusion, these results indicate that heptachlor alters basic cell function by interfering with key cellular signaling pathways.

In vitro models to study hepatotoxicity

Drug discovery and development consists of a series of processes starting with the demonstration of pharmacological effects in experimental cell and animal models and ending with drug safety and efficacy studies in patients. A main limitation is often the unacceptable level of toxicity with the liver as the primary target organ. Therefore, approaches to study hepatic toxicity in the early phase of drug discovery represent an important step towards rational drug development. A variety of in vitro liver models have been developed in the past years. Next to their use in drug development, they can also be applied to study environmental toxins and their hepatotoxicity. The 3 main approaches are ex vivo isolated and perfused organ models, precision-cut liver slices and cell culture models. Although the advantage of whole organ perfusions is based on the assessment of physiologic parameters such as bile production and morphologic parameters such as tissue histology, cell culture models can be efficiently used to assess cellular metabolism, cytotoxicity and genotoxicity. The advantage of precision-cut liver slices is based on the juxtaposition of cellular assays and tissue morphology. None of these models can be compared as they all focus on different fields of hepatoxicology. For the future, the ideal setup for testing the hepatic toxicity of a new compound could of primary studies in cell or slice cultures to assess cellular effects and secondary studies using ex vivo perfused organs to examine gross organ function parameters and histology.

cAMP potentiates H(2)O(2)-induced ERK1/2 phosphorylation without the requirement for MEK1/2 phosphorylation

In Jurkat T lymphocytes, hydrogen peroxide (H(2)O(2)) potentiates the phosphorylation level of extracellular signal-regulated kinase 1 and 2 (ERK1/2) caused by T cell receptor (TCR) stimulation with anti-CD3 and anti-CD28 or anti-CD3 alone. Submillimolar concentrations of H(2)O(2)-induced phosphorylation of ERK1/2 and MAP/ERK kinase 1 and 2 (MEK1/2) without antigenic stimulation. H(2)O(2) also induced the electrophoretic mobility shift of Lck from 56 to 60 kDa. The MEK inhibitor, PD98059 attenuated ERK1/2 and MEK1/2 phosphorylation, as well as the migration shift of Lck induced by H(2)O(2). The phospholipase C (PLC) inhibitor, U73122, and EGTA reduced the phosphorylation of both ERK1/2 and MEK1/2 induced by H(2)O(2). Interestingly, an increase of intracellular cAMP level with forskolin or 8-(4-chlorophenylthio)-cAMP augmented ERK1/2 phosphorylation by H(2)O(2), while inhibiting MEK1/2 phosphorylation by H(2)O(2). These results demonstrate an alternative pathway that results in augmentation of ERK1/2 phosphorylation without concomitant MEK1/2 phosphorylation in T cells.

Link of nonhemodynamic factors to hemodynamic determinants of left ventricular hypertrophy

Despite current evidence suggesting that hemodynamic load is the fundamental stimulus to begin the sequence of biological events leading to the development of left ventricular hypertrophy, genotype, gender, body size, and less easily recognizable environmental factors may contribute to generate the cascade of molecular changes that eventually yield the increase in protein synthesis needed to increase left ventricular mass. However, even nonhemodynamic factors such as gender and body size eventually regulate the growth of left ventricular mass by at least in part influencing loading conditions. Consideration of measurable factors, such as gender, body size, and hemodynamic load, allows evaluation of individual echocardiographic left ventricular mass as the deviation from the level that would be required to face a gender-specific hemodynamic load at a given body size. Values of left ventricular mass that are inappropriately high for individual gender, body size, and hemodynamic load are associated with a high cardiovascular risk phenotype, even independent of the presence of arterial hypertension. Thus, the condition of inappropriately high left ventricular mass may be recognized as a more advanced stage of pathological structural changes initially induced by overload, going beyond the compensatory needs. The biological process that yields inappropriate left ventricular mass is probably linked to the protracted activity over time of biological mediators of left ventricular hypertrophy, such as proto-oncogenes and other growth factors, neurohormones, and cytokines, inducing structural modifications that initially compensate imposed overload but eventually change the structure of myocardial tissue and the composition of motor units.

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.