Insulin-like Growth Factor-1-mediated AKT Activation Postpones the Onset of Ultraviolet B-induced Apoptosis, Providing More Time for Cyclobutane Thymine Dimer Removal in Primary Human Keratinocytes

Insulin-like growth factor-1 (IGF-1) acts as a potent survival factor in numerous cell lines, primarily through activation of the AKT signaling pathway. Although some targets of this pathway have known anti-apoptotic functions, its relationship with the improved survival of cells after exposure to environmental stresses, including UVB, remains largely unclear. We report that in growth factor-deprived keratinocytes, IGF-1 significantly and consistently delayed the onset of UVB-induced apoptosis by >7 h. This delay allowed IGF-1-supplemented keratinocytes to repair significantly more cyclobutane thymine dimers than their growth factor-deprived counterparts. This increase in cyclobutane thymine removal resulted in enhanced survival if the amount of DNA damage was not too high. To increase cell survival after UVB irradiation, IGF-1 supplementation was required only during this initial time period in which extra repair was executed. Finally, we show that IGF-1 mediated this delay in the onset of UVB-induced apoptosis through activation of the AKT signaling pathway. We therefore believe that the AKT signaling pathway increases cell survival after a genotoxic insult such as UVB irradiation not by inhibiting the apoptotic stimulus, but only by postponing the induction of apoptosis, giving the DNA repair mechanism more time to work.