TNF-alpha stimulates Akt by a distinct aPKC-dependent pathway in premalignant keratinocytes

Tumor necrosis factor-alpha (TNF-alpha) is an important proinflammatory cytokine involved in the pathogenesis of inflammatory skin diseases and cutaneous squamous cell carcinoma. Some of these effects are mediated by the stimulatory effect of this cytokine on the Akt signalling pathway, which renders keratinocytes less susceptible to proapoptotic stimuli and enhances cell growth. We have recently shown that TNF-alpha-induced Akt activation may promote the early stages of skin cancer. In this work, we demonstrate that in the premalignant keratinocyte cell line HaCaT, TNF-alpha activates Akt, ERK1/2 and p38. The specific peptide blocking the activity of the atypical protein kinase C (aPKC) species zeta and iota/lambda abrogated the effects of TNF-alpha on Akt and ERK1/2 but increased the activation of p38. The TNF-alpha-dependent phosphorylation of Akt-ERK1/2 was slightly decreased by NF kappaB inhibition and in the presence of p38 blockers. Akt/ERK signalling but not p38 activation was abolished in the presence of the iron chelator desferroxamine that blocks formation of hydroxyl ( OH) radicals. Thus, the TNF-alpha signalling in keratinocytes seems to bifurcate into an aPKC-, NFkB- and OH-dependent pathway resulting in the activation of survival and mitogenic pathways mediated by Akt and ERK1/2, and a signalling pathway conveyed by p38 that contributes to Akt activation but is suppressed by aPKC. Our data may be utilized in the development of more selective anti-TNF-alpha therapeutic strategies.