UVA/B-Induced Apoptosis in Human Melanocytes Involves Translocation of Cathepsins and Bcl-2 Family Members

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.

JNK mediates UVB-induced apoptosis upstream lysosomal membrane permeabilization and Bcl-2 family proteins

“…We have recently shown that cathepsins are released from lysosomes to the cytosol during UV-induced apoptosis in human melanocytes and that these proteases are involved in the intrinsic pathway by mediating pro-apoptotic effects upstream translocation of Bax to mitochondria [7]. Here, we present consistent data from Western blot analyses on [14].…”

“…The present and previous [7] results obtained after UVB irradiation suggest the redistribution of Bax to mitochondria to be the apoptosis determinant event.…”

“…4D). To explore the role of Mcl-1 in UVB-induced apoptosis, In a previous study, we have shown that Bax translocation from the cytosol to mitochondria is a prerequisite for apoptosis induction following UVB exposure [7]. Immunocytochemistry of Bax showed a diffuse cytosolic staining pattern and normal shaped nuclei in control cells, while in UVB irradiated cells, the Bax staining pattern changed into a punctate organellerestricted pattern, which paralleled detection of fragmented nuclei.…”

“…Control cells were treated identical but not irradiated. The irradiation dose was selected to achieve a frequency of apoptosis of 30-40 % with a minimum number of necrotic cells [7].…”

“…In a previous study, using human epidermal melanocytes, we have demonstrated lysosomal membrane permeabilization (LMP) to be essential in UV-induced apoptosis signaling [7]. UV exposure caused release of cathepsins from lysosomes to the cytosol, where they acted as pro-apoptotic mediators upstream translocation of Bax to mitochondria and mitochondrial membrane permeabilization.…”