G Protein βγ Subunits Augment UVB-induced Apoptosis by Stimulating the Release of Soluble Heparin-binding Epidermal Growth Factor from Human Keratinocytes

UV radiation induces various cellular responses by regulating the activity of many UV-responsive enzymes, including MAPKs. The βγ subunit of the heterotrimeric GTP-binding protein (Gβγ) was found to mediate UV-induced p38 activation via epidermal growth factor receptor (EGFR). However, it is not known how Gβγ mediates the UVB-induced activation of EGFR, and thus we undertook this study to elucidate the mechanism. Treatment of HaCaT-immortalized human keratinocytes with conditioned medium obtained from UVB-irradiated cells induced the phosphorylations of EGFR, p38, and ERK but not that of JNK. Blockade of heparin-binding EGF-like growth factor (HB-EGF) by neutralizing antibody or CRM197 toxin inhibited the UVB-induced activations of EGFR, p38, and ERK in normal human epidermal keratinocytes and in HaCaT cells. Treatment with HB-EGF also activated EGFR, p38, and ERK. UVB radiation stimulated the processing of pro-HB-EGF and increased the secretion of soluble HB-EGF in medium, which was quantified by immunoblotting and protein staining. In addition, treatment with CRM179 toxin blocked UV-induced apoptosis, but HB-EGF augmented this apoptosis. Moreover, UVB-induced apoptosis was reduced by inhibiting EGFR or p38. The overexpression of Gβ 1 γ 2 increased EGFR-activating activity and soluble HB-EGF content in conditioned medium, but the sequestration of Gβγ by the carboxyl terminus of G protein-coupled receptor kinase 2 (GRK2ct) produced the opposite effect. The activation of Src increased UVB-induced, Gβγ-mediated HB-EGF secretion, but the inhibition of Src blocked that. Overexpression of Gβγ increased UVB-induced apoptosis, and the overexpression of GRK2ct decreased this apoptosis. We conclude that Gβγ mediates UVB-induced human keratinocyte apoptosis by augmenting the ectodomain shedding of HB-EGF, which sequentially activates EGFR and p38.