Regulation of NF-κB Activity and Keratinocyte Differentiation by the RIP4 Protein: Implications for Cutaneous Wound Repair

Receptor-interacting proteins (RIPs) are important regulators of cell proliferation and differentiation. As RIP4 is a crucial modulator of epidermal differentiation, we analyzed the expression of different rip genes in healing skin wounds. Rip4 expression was strongly downregulated in keratinocytes of the hyperproliferative epithelium at the wound edge early after injury and only returned to basal levels after completion of wound repair. Rip3 expression was strongly induced as early as 1 day after wounding. In contrast, rip and rip2 expression remained unaltered. To determine the factors that regulate rip4 gene expression in keratinocytes, human HaCaT keratinocytes were used as a model system. We found that scratch wounding as well as treatment with whole serum, phorbol esters, the growth/differentiation factors epidermal growth factor, transforming growth factor-β, and activin A, or the proinflammatory cytokines tumor necrosis factor-α and IL-1β strongly suppressed rip4 expression in these cells. In contrast, the steroid dexamethasone and all-trans retinoic acid slightly stimulated rip4 expression. Suppression of rip4 expression in keratinocytes using small interfering RNA technology reduced the activation of NF-κB, and enhanced the expression of epidermal differentiation markers in these cells. These data suggest important and unique functions of different RIP proteins in keratinocytes of normal and wounded skin.