Promotion-resistant JB6 mouse epidermal cells exhibit defects in phosphatidylethanolamine synthesis and phorbol ester-induced phosphatidylcholine hydrolysis

Metabolic and molecular aspects of ethanolamine phospholipid biosynthesis: the role of CTP:phosphoethanolamine cytidylyltransferase (Pcyt2)

The CDP-ethanolamine branch of the Kennedy pathway is the major route for the formation of ethanolamine-derived phospholipids, including diacyl phosphatidylethanolamine and alkenylacyl phosphatidylethanolamine derivatives, known as plasmalogens. Ethanolamine phospholipids are essential structural components of the cell membranes and play regulatory roles in cell division, cell signaling, activation, autophagy, and phagocytosis. The physiological importance of plasmalogens has not been not fully elucidated, although they are known for their antioxidant properties and deficiencies in a number of inherited peroxisomal disorders. This review highlights important aspects of ethanolamine phospholipid metabolism and reports current molecular information on 1 of the regulatory enzymes in their synthesis, CTP:phosphoethanolamine cytidylyltransferase (Pcyt2). Pcyt2 is encoded by a single, nonredundant gene in animal species that could be alternatively spliced into 2 potential protein products. We describe properties of the mouse and human Pcyt2 genes and their regulatory promoters and provide molecular evidence for the existence of 2 distinct Pcyt2 proteins. The goal is to obtain more insight into Pcyt2 catalytic function and regulation to facilitate a better understanding of the production of ethanolamine phospholipids via the CDP-ethanolamine branch of the Kennedy pathway.

Novel protein kinase C isoforms and mitogen-activated kinase kinase mediate phorbol ester-induced osteopontin expression

BACKGROUND AND AIMS

The expression of osteopontin (OPN), a protein postulated to play a role in tumorigenesis, is induced by the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) in vivo and in the in vitro initiation-promotion skin carcinogenesis model (JB6 cells). Although TPA-induced OPN expression in JB6 cells has been suggested to involve protein kinase C (PKC), the PKC isoforms and the downstream pathway mediating OPN expression have not been extensively studied.

METHODS

Using the JB6 cell model, we determined the involvement of PKC isoforms, mitogen-activated protein kinase kinase (MAPK kinase/MEK) and MAPK in TPA-induced OPN expression using inhibitors specific to PKC isoforms and MEK and performing Northern blot analyses. Western blot analyses of cells treated with specific inhibitors were also performed to determine whether PKC isoforms or MEK were involved in activation of MAPK.

KEY RESULTS

TPA increased the steady-state level of OPN mRNA as early as 2-4h and this expression persisted for at least 4 days. TPA induction of OPN expression in JB6 cells is mediated through PKC epsilon and PKC delta, which also mediated the phosphorylation of MAPK. Additionally, inhibition of MEK activity, which activates MAPK, attenuated TPA-induced OPN expression. These findings suggest that activation of MAPK is important in mediating OPN expression.

CONCLUSION

TPA-induced steady-state OPN mRNA expression in mouse JB6 cells involves the activation of MAPK mediated through PKC epsilon and/or PKC delta.

Activator protein 1 (AP-1)- and nuclear factor kappaB (NF-kappaB)-dependent transcriptional events in carcinogenesis

Generation of reactive oxygen species (ROS) during metabolic conversion of molecular oxygen imposes a constant threat to aerobic organisms. Other than the cytotoxic effects, many ROS and oxidants are also potent tumor promoters linking oxidative stress to carcinogenesis. Clonal variants of mouse epidermal JB6 cells originally identified for their differential susceptibility to tumor promoters also show differential reduction-oxidation (redox) responses providing a unique model to study oxidative events in tumor promotion. AP-1 and NF-kappaB, inducible by tumor promoters or oxidative stimuli, show differential protein levels or activation in response to tumor promoters in JB6 cells. We further demonstrated that AP-1 and NF-kappaB are both required for maintaining the transformed phenotypes where inhibition of either activity suppresses transformation response in JB6 cells as well as human keratinocytes and transgenic mouse. NF-kappaB proteins or extracellular signal-regulated kinase (ERK) but not AP-1 proteins are shown to be sufficient for conversion from transformation-resistant to transformation-susceptible phenotype. Insofar as oxidative events regulate AP-1 and NF-kappaB transactivation, these oxidative events can be important molecular targets for cancer prevention.

Ethanol potentiates the stimulatory effects of insulin and phosphocholine on mitogenesis by a zinc-dependent and rapamycin-sensitive mechanism in fibroblasts and JB6 cells

In most cellular systems ethanol inhibits growth factor-induced cell growth. Here we examined the effects of ethanol on DNA synthesis and cell proliferation induced by insulin and phosphocholine (PCho) in NIH3T3 fibroblasts, Swiss 3T3 fibroblasts and mouse epidermal JB6 cells. In serum-starved low (12-18) passage NIH3T3 fibroblasts, 60 mM ethanol enhanced the mitogenic effect of insulin in the absence or presence of 25 microM zinc about 2- or 12-fold, respectively. In contrast, in serum-starved high (30-47) passage NIH3T3 cells 60 mM ethanol had large (20-40-fold) potentiating effects on insulin-induced DNA synthesis even in the absence of zinc. Furthermore, ethanol also enhanced the effects of PCho on DNA synthesis in both the absence and presence of insulin. The potentiating effects of ethanol on insulin- and PCho-induced DNA synthesis were associated with 1.2-1.3-fold stimulation of cell proliferation. Rapamycin, an inhibitor of p70 S6 kinase action, strongly inhibited the potentiating effects of ethanol on insulin- and PCho-induced mitogenesis. Unexpectedly, ethanol inhibited synergistic activation of p42/p44 mitogen-activated protein kinases by insulin and PCho. In both Swiss 3T3 and JB6 cells, ethanol potentiated insulin-induced DNA synthesis only in the presence of zinc. In these cells, ethanol also increased the effects of PCho on both DNA synthesis and cell proliferation in the co-presence of either insulin or ATP. The results indicate that in various cell lines physiologically relevant concentrations of ethanol can increase the ability of insulin and PCho to induce DNA synthesis and, to smaller extents, cell proliferation. In low passage NIH3T3 cells as well as in Swiss 3T3 and JB6 cells potentiation of insulin-induced DNA synthesis by ethanol requires the presence of zinc.