Differential apoptotic pathways in human keratinocyte HaCaT cells exposed to UVB and UVC

Abstract

The induction of apoptosis in keratinocytes by ultraviolet (UV)-irradiation is considered to be a protective function against skin cancer. UV-induced DNA damage is a crucial event in UVB- and UVC-mediated apoptosis. However, the differences between the UVB- and UVC-induced apoptotic pathways remain unclear. Here we examine the differential mechanisms by which UVB and UVC irradiations induce keratinocyte apoptosis using human keratinocyte HaCaT cells. Differences in the production of (6-4)photoproducts ((6-4)PPs) and cyclobutane pyrimidine dimers (CPDs) were measured following irradiation with UVB and UVC at doses causing the same extent of apoptotic cell death. In addition, main apoptotic features, such as caspase activation and its regulation, were compared between UVB- and UVC-induced apoptosis. Exposures of 500 J/m(2) UVB and 100 J/m(2) UVC resulted in apoptosis to almost the same extent. At these apoptotic doses, the amounts of both (6-4)PPs and CPDs were significantly larger in the case of UVC irradiation than UVB irradiation; in parallel, the release of cytochrome c and Smac/DIABLO and the activation of caspases-9 following UVC irradiation were greater than after UVB irradiation. Importantly, caspase-8 activation occurred only in UVB-irradiated cells. Furthermore, the activation of caspase-8 was not inhibited by caspases-9 and -3 specific tetrapeptide inhibitors, indicating that the caspase-8 cleavage is not due to feedback from activation of caspases-9 and -3. Thus, these results clearly suggest that the reason apoptosis is induced to the same extent by UVB irradiation as by UVC irradiation, despite the lower production of photoproducts in DNA by UVB irradiation, is attributable to the additional activation of the caspase-8 pathway. Thus, UVB irradiation induces apoptosis through both mitochondrial (intrinsic) and caspase-8 activation (extrinsic) pathways, while UVC induces apoptosis only via the intrinsic pathway.

p63 regulates the caspase-8-FLIP apoptotic pathway in epidermis

“…7 This hypothesis is in agreement with the reported sensitivity of p53-mutated HaCat cells to UVB irradiationmediated apoptosis. 30 In Figure 1, TA isoforms are more potent in Luciferase assays, especially the b and g isoforms, but we think that this should not be taken as an indication that p63 targets in apoptotic pathways S Borrelli et al procaspase-8 is more of a target of TA than DN isoforms: in fact, we have noticed this behaviour routinely, even in promoters characterized as bona fide targets of DNp63a. 13,14 It is likely that this effect is due to the properties of the transient assays, which are presumably not including chromatin constraints.…”

The Anti-apoptotic and Anti-oxidant Effect of Eriodictyol on UV-Induced Apoptosis in Keratinocytes

“…36) In addition, the activation of initiator caspase-9 was also observed in UV-irradiated HaCaT cells. 37) Our results showed that when HaCaT and NHEK cells were irradiated with UV, the cleavage of procaspase-3 and PARP was clearly detected 8 h after irradiation. These time courses of caspase-3 activation after UV irradiation show good correlations with the time courses of the appearance of apoptotic cells.…”

Protective Effects of Catalase Overexpression on UVB-induced Apoptosis in Normal Human Keratinocytes

“…Reduction of UVB-induced apoptosis through inhibition of receptor clustering by keeping cells at low temperature (4 -10°C) during UVB irradiation or prevention of DISC formation by expressing a dominant negative FADD supports this notion (11,12). Correlation of apoptosis levels with DNA damage severity and reduction of apoptosis by enhancement of DNA repair enzymes (13,14) substantiates the idea that DNA damage induced by UVB irradiation activates the apoptotic pathway (6,15). While UVB is known to be an inducer of ROS formation (16,17) leading to apoptosis (18), the source of this ROS production is not precisely known.…”