Variable Bax antigenicity is linked to keratinocyte position within epidermal strata and UV-induced apoptosis

Pro- and anti-apoptotic members of the Bcl-2 family are fundamental in the control of apoptosis. Among them, Bax plays a key role in apoptosis induction by mediating the release of apoptogenic factors from mitochondria to the cytosol. In this report, we investigated, by immunohistofluorescence, the in vivo distribution of Bax in normal human epidermis before and 24 h after exposure to solar-simulated radiation. Bax expression was evaluated with three different, Western blot pretested, anti-Bax antibodies (Ab) and correlated with markers of keratinocyte differentiation and apoptosis using anti-beta(1) integrin and anti-active caspase-3 Abs respectively. Using anti-Bax N20 and A-3533 polyclonal Ab, we found that, whereas undifferentiated keratinocytes of the basal proliferative compartment contained Bax in the cytosol, the differentiated suprabasal cells had Bax mainly in the nucleus. This immunoreactivity pattern was not modified by skin irradiation. Interestingly, the well known apoptosis-related Bax redistribution to mitochondria in response to a cell death signal, could be detected only with yet another, the 2D2 monoclonal Ab. This relocalization occurred specifically in apoptotic, active caspase-3 positive cells of irradiated epidermis. Our data highlight the differentiation- and apoptosis-associated changes in the pattern of Bax subcellular and cellular distribution as uncovered by different anti-Bax Abs and suggest that Bax undergoes successive activation that progresses in parallel with keratinocyte differentiation and apoptosis.