Protein kinase C-dependent stimulation of phospholipase D in phospholipase C-treated fibroblasts

Sphingosine-like stimulatory effects of propranolol on phospholipase D activity in NIH 3T3 fibroblasts

Propranolol and sphingosine exhibit several common biochemical effects, including inhibition of phosphatidic acid phosphohydrolase and protein kinase C (PKC) activities. In NIH 3T3 fibroblasts, sphingosine has also been shown to stimulate phospholipase D (PLD)-mediated hydrolysis of both phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) (Kiss Z and Anderson WB, J Biol Chem 265: 7345-7350, 1990). The present study demonstrates that in [14C]palmitic acid-labeled NIH 3T3 fibroblasts, propranolol (50-100 microM) and sphingosine had similar stimulatory effects on PLD-mediated synthesis of phosphatidylethanol in the presence of ethanol. In [14C]choline- and [14C]-ethanolamine-labeled fibroblasts, both compounds also stimulated the hydrolysis of both [14C]PtdCho and [14C]PtdEtn. However, while sphingosine preferentially stimulated PtdEtn hydrolysis, propranolol had greater effects on PtdCho hydrolysis. At each time point examined (15-45 min), lower concentrations (25-50 microM) of propranolol and 100 nM phorbol 12-myristate 13-acetate (PMA) synergistically enhanced PtdEtn hydrolysis; a higher concentration (100 microM) of propranolol inhibited this PMA effect only when the incubation time was 45 min. On the other hand, propranolol (10-100 microM) had either no effect or it inhibited PMA-induced PtdCho hydrolysis after treatments for 15 or 45 min, respectively. These potentiating and inhibitory actions of propranolol on the hydrolysis of PtdCho and PtdEtn were similarly elicited by sphingosine. The present study identified the PLD system as another common target for the pharmacological actions of sphingosine and propranolol.

The zinc chelator 1,10-phenanthroline enhances the stimulatory effects of protein kinase C activators and staurosporine, but not sphingosine and H2O2, on phospholipase D activity in NIH 3T3 fibroblasts

Protein kinase C (PKC), an enzyme which is believed to mediate the stimulatory effects of the PKC activator phorbol 12-myristate 13-acetate (PMA) on phospholipase D (PLD) activity, has a zinc-dependent structure required for phorbol ester binding. Accordingly, zinc or zinc chelators would be expected to promote or inhibit, respectively, the stimulatory effects of PMA on PLD-mediated phospholipid hydrolysis. Instead, treatment of [14C]choline- and [14C]ethanolamine-labelled NIH 3T3 fibroblasts with the high-affinity zinc chelator 1,10-phenanthroline (0.2-1 mM) for 20-30 min was found to enhance the stimulatory effects of PMA on PLD-mediated hydrolysis of phosphatidylcholine and phosphatidylethanolamine. In [14C]palmitic acid-labelled fibroblasts, in the presence of ethanol, phenanthroline also enhanced the stimulatory effect of PMA on the synthesis of phosphatidylethanol, a marker of PLD activity. Addition of zinc (250 microM) to phenanthroline-treated fibroblasts reversed the stimulatory effects of the chelator. The potentiating effects of phenanthroline were also partially reversed by cadmium, whereas iron, lead, copper, magnesium and calcium were without effects. Of the other activators of PLD tested, phenanthroline also enhanced the stimulatory effects of platelet-derived growth factor and staurosporine, but not that of sphingosine and H2O2, on the hydrolysis of both phospholipids. These results suggest that regulation of PLD by PKC activators and staurosporine involves a common intermediate step, which is inhibited by a chelatable cellular pool of zinc.