Keratinocyte differentiation marker suppression by arsenic: Mediation by AP1 response elements and antagonism by tetradecanoylphorbol acetate

Culture models of target cells are anticipated to help elucidate the mechan ism by which inorganic arsenic acts as a carcinogen in humans. Present work characterizes the response of human keratinocytes, a target cell type, to arsenic suppression of their differentiation program. Four representative d ifferentiation marker mRNAs (involucrin, keratinocyte transglutaminase, sma ll proline-rich protein 1, and filaggrin) were suppressed by both arsenate and arsenite in normal, spontaneously immortalized (premalignant), and mali gnant keratinocytes with EC50 values in the low micromolar range. The suppr ession was almost completely reversed 9 days after removal of arsenate from the culture medium. In the case of the involucrin gene, suppression was me diated primarily by two functional AP1 response elements in the gene promot er. Both glucocorticoid and serum stimulation of differentiation occurred t o a similar extent in the presence and absence of arsenic, indicating neith er stimulation was a specific target of arsenic action and neither agent co uld overcome arsenic suppression. In contrast, 12-O-tetradecanoylphorbol-13 -acetate prevented the suppression of keratinocyte transglutaminase, sugges ting that arsenic acts upstream of protein kinase C. (C) 2001 Academic Pres s.