Curcumin suppresses AP1 transcription factor-dependent differentiation and activates apoptosis in human epidermal keratinocytes

The diet-derived cancer preventive agent, curcumin, inhibits skin cancer cell proliferation and tumor formation. However, its effect on normal human keratinocyte differentiation, proliferation, and apoptosis has not been adequately studied. Involucrin (hINV) is a marker of keratinocyte differentiation and a useful model for the study of chemopreventive agent action. We show that curcumin suppresses the differentiation agent-dependent activation of hINV gene expression and that an AP1 transcription factor DNA binding site in the hINV gene is required for this regulation. A protein kinase C, Ras, MEKK1, MEK3 signaling cascade controls hINV expression by regulating AP1 factor level. Curcumin treatment inhibits the novel protein kinase C-, Ras-, and MEKK1-dependent activation of hINV promoter activity and reduces the differentiation agent-dependent increase in AP1 factor level and DNA binding. This reduction requires proteasome function. In addition, curcumin treatment reduces cell number, which is associated with a reduced cyclin and cdk1 levels. Curcumin treatment also suppresses the Bcl-xL level, leading to reduced mitochondrial membrane potential and increased cleavage of procaspases and poly(ADP-ribose) polymerase. These studies provide important insights regarding the mechanism whereby curcumin acts as a chemopreventive agent in normal human epidermis.