The modulation of events associated with tumor promotion and epidermal differentiation by the fatty acids 12(S)-HETE and 13(S)-HOD

Dietary fat may modulate tissue levels of fatty acid metabolites, and these metabolites may have biological effects. It was shown that in mouse epidermis, linoleic acid was metabolized to 9(S)- or 13(S)-hydroxyoctadecadienoic acid (HODE) and arachidonic acid to 15(S)- or 12(S)-hydroxyeicosatetraenoic acid (HETE). Epidermal content of these compounds did not change significantly as dietary linoleic acid increased. Of these compounds, only 12(S)-HETE stimulated the in vitro activity of protein kinase C (PKC), an important signal transduction molecule. In keratinocytes grown in culture, evidence suggested that 12(S)-HETE affects keratinocyte signal transduction, with down-stream effects on gene transcription related to cellular differentiation. Exogenous 12(S)-HETE suppressed keratin 1 (K1) mRNA and protein expression after 24 hours of treatment. The signal transduction pathway mediated by 12(S)-HETE was investigated and it was found that cAMP, cGMP, and tyrosine kinase were not involved. The possible involvement of PKC was examined by pre-treatment with the PKC inhibitors RO-31-8220 or bryostatin-1. Results suggested that PKC may modulate K1 expression. To understand the exact role of PKC in these events, direct assay of cellular PKC activity and isoform distribution was performed. Results from these experiments indicated that 12(S)-HETE stimulates cytosolic PKC activity. This increase appears to be due to PKC$delta,$ the primary cytosolic PKC isoform found in these cells, because the sub-cellular distribution of PKC$delta$ protein correlated with activity. In conclusion, this study has established that the fatty acid 12(S)-HETE is found in mouse epidermis and stimulates epidermal PKC activity in vitro. In cultured keratinocytes, 12(S)-HETE stimulated PKC$delta$ activity rapidly and to a high level. Under these conditions, 12(S)-HETE modulated the expression of K1 mRNA and protein. Thus, a biologically occurring fatty acid has been shown to affect cell signal transduction and gene transcription.