PROTEIN-TYROSINE KINASE-ACTIVITY REGULATES NITRIC-OXIDE SYNTHASE INDUCTION IN RAT HEPATOCYTES

Regulation of induced nitric oxide synthase (NOS) in isolated rat hepa tocytes is poorly understood. The specific protein tyrosine kinase inh ibitor genistein was used to determine if NOS induction is dependent o n protein tyrosine kinase activation. Genistein inhibited tumor necros is factor-alpha (TNF-alpha)-stimulated induction of NOS activity and N OS protein in a dose-dependent manner. Genistein also impaired TNF-alp ha-induced NOS mRNA accumulation, suggesting protein tyrosine kinase r egulation of NOS induction occurred at the level of transcription-tran slation. Like TNF-alpha, genistein inhibited induction of NOS protein by a second proinflammatory cytokine, interleukin-1 beta, suggesting s imilar activation mechanisms by proinflammatory cytokines. NOS inducti on by other stimuli, including phorbol 12-myristate 13-acetate and the superoxide-generating system xanthine/xanthine oxidase, was also inhi bited by genistein. Finally, cytokine-stimulated protein tyrosine kina se activity in hepatocytes was demonstrated by increased tyrosine phos phorylation of five high molecular mass protein bands. Genistein inhib ited this cytokine-induced phosphotyrosine increase. The commonality o f genistein inhibition suggests that protein tyrosine kinase activity is critical for NOS induction by a variety of stimuli.