1. ¹Laboratory of Dermatology, University of Leuven, Leuven, Belgium
2. ²Division of Biochemistry, University of Leuven, Leuven, Belgium

Correspondence: Dr Marjan Garmyn, Laboratory of Dermatology, University of Leuven, GHB, O&N II, Herestraat 49, Box 724, Leuven B-3000, Belgium. E-mail: marjan.garmyn@med.kuleuven.be

³Present address: Laboratory of Neurogenetics, Department of Human Genetics, Flanders Interuniversity Institute for Biotechnology (VIB) and University of Leuven, School of Medicine, Leuven, Belgium

Received 8 March 2006; Revised 24 June 2006; Accepted 17 July 2006; Published online 24 August 2006.

Abstract

Upon irradiation with a high dose of UVB, keratinocytes undergo apoptosis as a protective mechanism. In previous work, we demonstrated the existence of an early-activated UVB-induced apoptotic pathway in growth factor-depleted human keratinocytes, which can be substantially delayed by the exclusive supplementation of IGF-1. We now show that in human keratinocytes, IGF-1 inhibits the onset of UVB-triggered apoptosis through a transcriptional independent, AKT-mediated mechanism, involving BAD serine 136 phosphorylation. Our results show that the early UVB-induced apoptosis in growth factor-depleted human keratinocytes is exclusively triggered through the mitochondrial pathway. It is accompanied by BAX translocation, cytochrome c release, and procaspase-9 cleavage, but not by procaspase-8 or BID cleavage. In human keratinocytes, IGF-1 supplementation inhibits these events in a transcription-independent manner. Both IGF-1 supplementation and the transduction of a membrane-targeted form of AKT result in a shift of the BH3-only protein BAD from the mitochondria to the cytoplasm, paralleled by an increase of AKT-specific Ser136 phospho-BAD bound to 14-3-3 protein. These data indicate that AKT-induced BAD phosphorylation and its subsequent cytoplasmic sequestration by 14-3-3 is a major mechanism responsible for the postponement of UVB-induced apoptosis in human keratinocytes.