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Sustained release of Smac/DIABLO from mitochondria commits to undergo UVB-induced apoptosis
1Genome & Drug Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-0022, Japan.
Abstract:
Apoptotic response of keratinocytes to UVB irradiation has physiological significance on photocarcinogenesis. Here, we show that the sustained release of Smac/DIABLO from mitochondria is an important event for the onset of apoptosis in keratinocytes exposed to UVB irradiation. In human keratinocyte HaCaT cells, UVB irradiation at 500 J/m(2), but not at 150 J/m(2), induces apoptosis. Significant activations of caspases-9 and -3, and slight activation of caspase-7 were observed only in 500 J/m(2) UVB irradiated HaCaT cells. Correspondingly, the cleavage of PARP, a substrate of caspases-3 and -7, was detected in cells irradiated at 500 J/m(2) UVB, but not at 150 J/m(2). However, with both 150 and 500 J/m(2) UVB irradiation, cytochrome c, an activator of caspase-9 via the formation of apoptosome, was released from mitochondria to the cytosol at the same extent. In contrast, significant amounts of Smac/DIABLO are released from mitochondria to the cytosol only with 500 J/m(2) UVB irradiation, and that the level of XIAP is decreased. These results suggest that the extent of Smac/DIABLO efflux from mitochondria is a determinant whether a cell will undergo apoptosis or survival.
Insights
The sustained release of mitochondrial Smac/DIABLO is crucial for UVB-induced keratinocyte apoptosis. Its efflux level determines cell survival or death after UV exposure, impacting photocarcinogenesis.
Area of Science:
- Cell Biology
- Dermatology
- Molecular Biology
Background:
- UVB irradiation is a major cause of skin damage and photocarcinogenesis.
- Keratinocyte apoptosis plays a key role in the skin's response to UV radiation.
- Mitochondrial pathways are critical regulators of programmed cell death.
Purpose of the Study:
- To investigate the role of Smac/DIABLO release from mitochondria in UVB-induced keratinocyte apoptosis.
- To determine the factors influencing keratinocyte survival or apoptosis following UVB exposure.
Main Methods:
- Human keratinocyte (HaCaT) cells were exposed to varying doses of UVB irradiation (150 J/m² and 500 J/m²).
- Apoptosis markers, including caspase activation (caspase-9, -3, -7) and PARP cleavage, were assessed.
- Mitochondrial release of cytochrome c and Smac/DIABLO was measured.
- Levels of the inhibitor of apoptosis protein XIAP were quantified.
Main Results:
- UVB irradiation at 500 J/m², but not 150 J/m², induced significant apoptosis, caspase activation, and PARP cleavage.
- Cytochrome c release was similar at both UVB doses, indicating mitochondrial outer membrane permeabilization.
- Smac/DIABLO was significantly released from mitochondria only at 500 J/m² UVB.
- XIAP levels decreased in response to higher UVB doses, correlating with Smac/DIABLO release.
Conclusions:
- Sustained Smac/DIABLO release from mitochondria is a critical determinant of keratinocyte apoptosis following UVB irradiation.
- The extent of Smac/DIABLO efflux, rather than cytochrome c release alone, dictates cell fate (apoptosis vs. survival).
- These findings highlight Smac/DIABLO's pivotal role in UVB-induced photocarcinogenesis.