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Inhibition of protein kinase C and proto‐oncogene expression by crocetin in NIH/3T3 cells

Crocetin, a carotenoid isolated from the seeds of Gardenia jasminoides, was found to be a potent inhibitor of tumor promotion induced by 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) in mouse skin. When mouse fibroblast NIH/3T3 cells were treated with TPA alone, protein kinase C (PKC) translocated from the cytosolic fraction to the particulate fraction. Pretreatment with 60 and 120 μM crocetin for 15 min inhibited the TPA‐induced PKC activity in the particulate fraction by 50% and 66%, respectively, but did not affect the level of PKC protein. Crocetin also reduced the level of TPA‐stimulated phosphorylation of cellular proteins. Cells pre‐treated with crocetin (120 μM) had 55% less PKC [3H]phorbol dibutyrate‐binding capacity. Suppression of TPA (100 ng/mL)‐induced c‐jun and c‐fos gene expression was also observed in the mouse fibroblast cells pre‐treated with crocetin (30, 60, and 120 μM). Our results provided a basis for understanding the inhibitory effect of crocetin on TPA‐mediated tumor promotion. © 1996 Wiley‐Liss, Inc.