Phospholipase D activation in hepatocyte growth factor-stimulated rat hepatocytes mediates the expressions of c-jun and c-fos: Involvement of protein tyrosine kinase, protein kinase C, and Ca2+

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Abstract
Hepatocyte growth factor (HGF) activated phospholipase D (PLD) in primary‐cultured rat hepatocytes, as assessed by the formation of phosphatidylbutanol (PBut), a specific and stable product of PLD activity in the presence of 0.3% butanol. PLD hydrolyzes phosphatidylcholine to choline and phosphatidic acid (PA), which is further metabolized to diacylglycerol (DG) by PA phosphohydrolase (PAP). In HGF‐stimulated hepatocytes, butanol prevented the formation of PA and DG. A PAP inhibitor, propranolol, inhibited DG production with a reciprocal increase of PA, implying that PLD played a role in the formation of not only PA but DG. Inhibitors for protein kinase C (PKC), Ro31‐8425, H‐7, and calphostin C, reduced HGF‐induced PLD activation. A protein tyrosine kinase (PTK) inhibitor, genistein but not its inactive analogue daidzein, inhibited PLD activation by HGF. Moreover, depletion of extracellular Ca2+ by omission of Ca2+ or by chelating residual Ca2+ with ethyleneglycol‐bis(β‐aminoethyl ether)‐ N,N,N′,N′‐tetraacetic acid (EGTA) abolished HGF‐induced PLD activation. HGF, phorbol myristate acetate (PMA) and a DG analog, oleylacetylglycerol (OAG), activated the expression of c‐jun and c‐fos messenger RNAs (mRNAs). Ro31‐8425, calphostin C, and genistein, which prevented HGF‐induced PLD activation, inhibited induction of these immediate early genes. Butanol and propranolol at concentrations which effectively inhibited the formation of DG, suppressed HGF‐induced expression of c‐jun and c‐fos mRNAs. However, HGF‐induced mitogen‐activated protein kinase (MAPK) activation was not affected by both butanol and propranolol. These results suggest that PTK, PKC, and Ca2+ regulate HGF‐induced PLD activation, and that DG produced by PLD pathway may play a role in the induction of immediate early genes, which is activated in MAPK‐independent manner, in rat hepatocytes.

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