Regulation of guanylate-binding protein expression in interferon-gamma-treated human epidermal keratinocytes and squamous cell carcinoma cells

Interferon-gamma is a potent inducer of growth arrest and squamous differen tiation of human epidermal keratinocytes in vitro, In order to understand t he proximate events regulating interferon-gamma action we studied the relat ionship between interferon-gamma-mediated induction of a cytoplasmic guanyl ate-binding protein and the expression of growth and differentiation marker genes in normal and transformed keratinocytes, Induction of guanylate-bind ing protein mRNA by interferon-gamma was detectable at 4 h, was transcripti on dependent, and preceded changes in the expression of markers of growth a rrest (E2F-1 mRNA downregulation) and differentiation (SQ37 mRNA induction) . The Ec(50) Value for guanylate-binding protein induction (4 units interfe ron-gamma per mi) was lower than previously reported for SQ37 (40 units int erferon-gamma per mi). Guanylate-binding protein mRNA appeared to be only m oderately downregulated by modulators of the squamous phenotype such as ret inoic acid and transforming growth factor-beta 1. In addition, mRNA levels of E2F-1 or SQ37 were not altered in several squamous carcinoma cell lines treated with interferon-gamma, In contrast, guanylate-binding protein mRNA was highly induced in all these cell lines following interferon-gamma treat ment. Further analysis of the signal transduction pathway mediating interfe ron-gamma responses using protein kinase inhibitors indicated that guanylat e-binding protein induction in normal human epidermal keratinocyte cells wa s most likely protein kinase C independent. Our data suggest that more than one postreceptor interferon-gamma signaling pathway exists in keratinocyte s and that at least one of these pathways is defective in squamous carcinom a cells. Furthermore, our data demonstrated that the failure of the squamou s carcinoma cells to undergo interferon-gamma-induced growth arrest and dif ferentiation is not due to an inherent defect in interferon-gamma receptor activation, but most likely is due to a defect in a non-guanylate-binding p rotein-dependent signaling pathway.