Protein kinase C and phospholipase D activation in rat parotid glands

Protein kinase Calpha determines HER2 fate in breast carcinoma cells with HER2 protein overexpression without gene amplification

In some HER2-positive breast tumors, cell surface overexpression of HER2 is not associated with gene amplification but may instead rest in altered gene transcription, half-life, or recycling of the oncoprotein. Here, we show that HER2 overexpression in HER2 2+ carcinomas is associated with neither an increase in gene transcription nor a deregulation in the ubiquitin-dependent pathways, but instead seems to be regulated by protein kinase Calpha (PKCalpha) activity. The stimulation of PKCalpha up-regulated HER2 expression, whereas PKCalpha inhibition by pharmacologic treatments and PKCalpha-specific small interfering RNA led to a dramatic down-regulation of HER2 levels only in breast cancer cells HER2 2+. Consistent with the in vitro data, our biochemical analysis of HER2 2+ human primary breast specimens revealed significantly higher levels of phosphorylated PKCalpha compared with HER2-negative tumors. Inhibition of HER2 activation by the tyrosine kinase inhibitor lapatinib led to decreased levels of PKCalpha phosphorylation, clearly indicating a cross-talk between PKCalpha and HER2 molecules. These data suggest that HER2 overexpression in HER2 2+ carcinomas is due to an accumulation of the recycled oncoprotein to the cell surface induced by activated PKCalpha.

Modulation of protein kinase C (PKC)-mediated contraction and the possible role of PKC epsilon in rat mesenteric arteries

he involvement of protein kinase C (PKC) in isometric tension development of rat mesenteric arteries was investigated. Non-selective inhibition of PKC and selective inhibition of the epsilon isoform were performed using the PKC inhibitor, chelerythrine, and non-viral gene-transfer of a kinase inactive mutant of PKCepsilon (PKCepsilon-KN), respectively. Chelerythrine (2.5 or 5.0 microM) significantly and equally attenuated phenylephrine-induced but not potassium-induced contractions. Higher concentrations of chelerythrine (10 microM) caused the vessels to lose responsiveness to both phenylephrine and potassium chloride. Transfection of blood vessels with epsilon-KN also resulted in significant attenuation of contractile responses to phenylephrine. Potassium chloride-induced responses were not altered in transfected arteries. In a separate group of vessels, the relationship between [Ca2+]i and isometric tension was evaluated. These studies suggested that calcium sensitivity of the contractile apparatus was decreased in vessels when PKC-epsilon activity was compromised. The results of the study suggest that PKC-epsilon can modulate phenylephrine-induced contraction in mesenteric arteries via calcium-independent pathways.

Chelerythrine and bisindolylmaleimide I prolong cardiac action potentials by protein kinase C-independent mechanism

Effects of chelerythrine and bisindolylmaleimide I on action potential duration and on voltage-activated K(+) and Ca(2+) currents in rat ventricular myocytes were studied using perforated patch-clamp technique. The action potentials were markedly prolonged after application of 20 microM chelerythrine or 100 nM bisindolylmaleimide I. Chelerythrine and bisindolylmaleimide I reduced the amplitude of sustained current (I(K,sus)) significantly. Transient K(+) current (I(to)) was inhibited only by chelerythrine. Ca(2+) current was reduced only with highest chelerythrine concentration (50 microM). Application of chelerythrine and bisindolylmaleimide I inhibited outward K(+) currents significantly also in ruptured patch-clamp configuration. Bisindolylmaleimide V, an inactive analogue of bisindolylmaleimide I, decreased I(K,sus) substantially. However, I(to) and I(K,sus) were not affected by calphostin C. Direct protein kinase C activators resulted in decrease of outward K(+) currents. Chelerythrine blocked I(to) in a use-dependent manner and the block did not recover during a 4-min washout. I(K,sus) was not blocked by this mechanism by either inhibitor. We conclude that chelerythrine and bisindolylmaleimide I inhibit outward K(+) currents independently of protein kinase C inhibition.

Relationship between phosphatidic acid level and regulation of protein transit in colonic epithelial cell line HT29-cl19A

Colonic epithelial HT29-cl19A cells are polarized and secrete proteins among which alpha(1)-antitrypsin represents about 95%. Secretion occurs via a constitutive pathway, so that the rates of secretion directly reflect the rates of protein transit. In this paper we have demonstrated that: 1) in resting cells phospholipase D (PLD) is implicated in the control of apical protein transit; 2) phorbol esters stimulate apical protein transit (stimulation factor 2.2), which is correlated with a PLD-catalyzed production of phosphatidic acid (PA) (2.45-fold increase); 3) the stimulation of cholinergic receptors by carbachol results in an increase (stimulation factor 1.45) of apical protein transit which is independent of protein kinase C and PLD activities, but related to PA formation (1.7-fold increase) via phospholipase(s) C and diacylglycerol kinase activation; 4) an elevation of the cAMP level enhances apical protein transit by a PA-independent mechanism; 5) a trans-Golgi network or post-trans-Golgi network step of the transit is the target for the regulatory events. In conclusion, we have shown that PA can be produced by two independent signaling pathways; whatever the pathway followed, a close relationship between the amount of PA and the level of secretion was observed.

Chelerythrine, a protein kinase C inhibitor, interacts with cyclic nucleotide phosphodiesterases

Chelerythrine, a potent inhibitor of protein kinase C, was evaluated for its effect on cyclic nucleotide phosphodiesterases (PDE) isolated from bovine aorta. Chelerythrine activated basal PDE2 and inhibited activated PDE2, PDE4 and PDE5. The effect of chelerythrine (10 microM) was also investigated on vasorelaxation induced by a beta-adrenoceptor agonist or a PDE3 inhibitor. Chelerythrine attenuated the isoprenaline-mediated relaxation whereas it potentiated the relaxation induced by SK & F 94120 (5-(4)acetamidophenyl)pyrazin-2(1 H)-one, a PDE3 inhibitor). The present study demonstrates that chelerythrine, at a concentration generally reported in the literature to inhibit protein kinase C, interacts with cyclic nucleotide phosphodiesterases and consequently modulates vasorelaxation. These results cast some doubt on the use of chelerythrine as a specific inhibitor of protein kinase C.

Phospholipase D2, a distinct phospholipase D isoform with novel regulatory properties that provokes cytoskeletal reorganization

BACKGROUND

Activation of phospholipase D (PLD) is an important but poorly understood component of receptor-mediated signal transduction responses and regulated secretion. We recently reported the cloning of the human gene encoding PLD1; this enzyme has low basal activity and is activated by protein kinase C and the small GTP-binding proteins, ADP-ribosylation factor (ARF), Rho, Rac and Cdc42. Biochemical and cell biological studies suggest, however, that additional and distinct PLD activities exist in cells, so a search was carried out for novel mammalian genes related to PLD1.

RESULTS

We have cloned the gene for a second PLD family member and characterized the protein product, which appears to be regulated differently from PLD1: PLD2 is constitutively active and may be modulated in vivo by inhibition. Unexpectedly, PLD2 localizes primarily to the plasma membrane, in contrast to PLD1 which localizes solely to peri-nuclear regions (the endoplasmic reticulum, Golgi apparatus and late endosomes), where PLD activity has been shown to promote ARF-mediated coated-vesicle formation. PLD2 provokes cortical reorganization and undergoes redistribution in serum-stimulated cells, suggesting that it may have a role in signal-induced cytoskeletal regulation and/or endocytosis.

CONCLUSIONS

PLD2 is a newly identified mammalian PLD isoform with novel regulatory properties. Our findings suggest that regulated secretion and morphological reorganization, the two most frequently proposed biological roles for PLD, are likely to be effected separately by PLD1 and PLD2.

Does protein kinase C play a pivotal role in the mechanisms of ischemic preconditioning?

This communication reviews the evidence for the pivotal role of protein kinase C in ischemic myocardial preconditioning. It is believed that several intracellular signalling pathways via receptor-coupled phospholipase C and its "cross-talk" with phospholipase D converge to activation of protein kinase C isotypes which is followed by phosphorylation of until now (a number of) unknown target proteins which produce the protective state of ischemic preconditioning. After briefly introducing the general biochemical properties of protein kinase C, its isotypes and the limitations of the methodology used to investigate the role of protein kinase C, studies are discussed in which pharmacological inhibition and activation and (immunore) activity and/or isotypes measurements of protein kinase C isotypes were applied to assess the role of activation of protein kinase C in ischemic myocardial preconditioning. It is concluded that definitive proof for the involvement of protein kinase C in preconditioning requires future studies which must focus on the isotype(s) of protein kinase C that are activated, the duration of action, cellular translocation sites and the identity and stability (of covalently bound phosphate) of phosphorylated substrate proteins.