The signal transduction pathways leading to apoptosis of human keratinocytes responding to UVB irradiation are complex and not completely understood. Previously, we reported that in UVB-irradiated keratinocytes, p38 MAPK instigates Bcl-2-associated X protein (Bax) activation and mitochondrial apoptosis. However, the molecular mechanism underlying the pro-apoptotic function of p38 MAPK remained unclear. Here, we show that in UVB-treated human primary keratinocytes the activation of p38 MAPK is

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... cessary to upregulate Noxa, a BH3-only pro-apoptotic dominantly induced by UVB and required for apoptosis. Whereas p53-silencing was marginally cytoprotective and poorly affected Noxa expression, p38 MAPK inhibition in p53-silenced keratinocytes or in p53 À/À cells could still efficiently prevent Noxa induction and intrinsic apoptosis after UVB, indicating that p38 MAPK signals mainly through p53-independent mechanisms. Furthermore, p38 MAPK was required for the induction and activation of hypoxia-inducible factor 1 (HIF-1) in response to UVB, and HIF-1 knockdown reduced Noxa expression and apoptosis. In UVB-irradiated keratinocytes, Noxa targeted the anti-apoptotic myeloid cell leukemia sequence 1 (Mcl-1) for degradation, and small-interfering RNA (siRNA)-mediated knockdown of Noxa or p38 MAPK inhibition restored levels of Mcl-1 and abolished apoptosis. Thus, the pro-apoptotic mechanisms orchestrated by p38 MAPK in human keratinocytes in response to UVB involve an HIF-1/Noxa axis, which prompts the downregulation of anti-apoptotic Mcl-1, thereby favoring Bax-mediated mitochondrial apoptosis of UVB-damaged keratinocytes. Abbreviations: Ask-1, apoptosis signaling kinase 1; Bax, Bcl-2-associated X protein; Bcl-2, B-cell lymphoma; Bcl-x L , B-cell lymphoma-extra large; Bim, Bcl-2-interacting mediator of cell death; BNIP3, Bcl-2/adeNovirus E1B 19 kDa protein-interacting protein 3; HIF-1, hypoxia-inducible factor 1; MAPK, mitogen-activated protein kinase; Mcl-1, myeloid cell leukemia sequence 1; NHK, normal human keratinocyte; PARP, poly (ADP-ribose) polymerase; PUMA, p53 upregulated modulator of apoptosis; SBC, sunburn cell; SCC, squamous cell carcinoma Van Laethem A, Van Kelst S, Lippens S et al. (2004) Activation of p38 MAPK is required for Bax translocation to mitochondria, cytochrome c release and apoptosis induced by UVB irradiation in human keratinocytes. FASEB J 18:1946-8 Villunger A, Michalak EM, Coultas L et al. (2003) p53-and drug-induced apoptotic responses mediated by BH3-only proteins puma and noxa. Science 302:1036-8 Youle RJ, Strasser A (2008) The BCL-2 protein family: opposing activities that mediate cell death. Nat Rev Mol Cell Biol 9:47-59 Ziegler A, Jonason AS, Leffell DJ et al. (1994) Sunburn and p53 in the onset of skin cancer. Nature 372:773-6