1. ¹Division of Dermatology, Faculty of Health Sciences, Linköping University, Linköping, Sweden
2. ²Division of Pathology II, Faculty of Health Sciences, Linköping University, Linköping, Sweden

Correspondence: Dr Cecilia A. Bivik, Division of Dermatology, Faculty of Health Sciences, Linköping University, Linköping S-581 85, Sweden. E-mail: cecbi@ibk.liu.se

³These authors contributed equally to this work.

Received 23 May 2005; Revised 23 September 2005; Accepted 21 October 2005; Published online 9 March 2006.

Abstract

We demonstrate UVA/B to induce apoptosis in human melanocytes through the mitochondrial pathway, displaying cytochrome c release, caspase-3 activation, and fragmentation of nuclei. The outcome of a death signal depends on the balance between positive and negative apoptotic regulators, such as members of the Bcl-2 protein family. Apoptotic melanocytes, containing fragmented nucleus, show translocation of the proapoptotic proteins Bax and Bid from the cytosol to punctate mitochondrial-like structures. Bcl-2, generally thought to be attached only to membranes, was in melanocytes localized in the cytosol as well. In the fraction of surviving melanocytes, that is, cells with morphologically unchanged nucleus, the antiapoptotic proteins Bcl-2 and Bcl-X_L were translocated to mitochondria following UVA/B. The lysosomal proteases, cathepsin B and D, which may act as proapoptotic mediators, were released from lysosomes to the cytosol after UVA/B exposure. Proapoptotic action of the cytosolic cathepsins was confirmed by microinjection of cathepsin B, which induced nuclear fragmentation. Bax translocation and apoptosis were markedly reduced in melanocytes after pretreatment with either cysteine or aspartic cathepsin inhibitors. No initial caspase-8 activity was detected, excluding involvement of the death receptor pathway. Altogether, our results emphasize translocation of Bcl-2 family proteins to have central regulatory functions of UV-induced apoptosis in melanocytes and suggest cathepsins to be proapoptotic mediators operating upstream of Bax.