The autocrine TNFα signalling loop in keratinocytes requires atypical PKC species and NF-κB activation but is independent of cholesterol-enriched membrane microdomains

Tumor necrosis factor α (TNFα) is involved in the pathogenesis of many inflammatory skin diseases. Epidermal keratinocytes produce and respond to TNFα via the cognate type 1 receptor (TNFR1). Little is known about regulation of TNFα signalling in this cell type. In this study, we report that in keratinocytes TNFα upregulates its own mRNA synthesis in an autocrine manner. This response peaks at approximately 1 h of stimulation with TNFα but sustained elevated levels of TNFα mRNA are observed for up to 24 h after stimulation and are dependent on the presence of the soluble cytokine. This autocrine response is mediated by the signalling cascade comprising TNFR1, atypical protein kinase C (aPKC) species and the transcription factor NF-κB, but is not dependent on the integrity of cholesterol-enriched membrane microdomains (lipid rafts). TNFα-stimulated keratinocytes produced the membrane-bound form of TNFα. It is conceivable that the described autocrine signalling loop contributes to the proinflammatory TNFα effect in the skin. The discovery of the crucial roles of aPKC and NF-κB might have consequences for the development of more selective anti-TNFα therapies for inflammatory skin diseases.