RAD51 and RAD51 paralog inhibition sensitizes nonreplicating quiescent keratinocytes to UV radiation.

UV radiation (UVR) leads to the formation of potentially lethal and chromosome-destabilizing double-strand breaks (DSBs) in DNA. However, most studies on UVR-induced DSB formation and repair have used cells that are actively progressing throughout the cell cycle. To explore how nonreplicating, quiescent cells deal with UVR-induced DSBs, we used small-molecule inhibitors of various DSB repair pathways and unexpectedly observed a major role for the recombination protein RAD51 in promoting quiescent HaCaT keratinocyte survival. We further observed that both DSB formation and RAD51 function occur independently of nucleotide excision repair, which generates potentially unstable single-stranded DNA gaps. However, RAD51 inhibition also sensitizes quiescent cells to agents that are known to inhibit transcription, thus suggesting a role for transcription in UVR-induced DSB formation. Interestingly, siRNA-mediated knockdown of RAD51 did not sensitize cells to UVR to the same extent as pharmacological inhibition did. In contrast, knockdown of several Rad51 paralogs negatively impacted cell viability after UVR exposure, similar to pharmacological inhibition, suggesting that pharmacological inhibitors of RAD51 may also target one or more RAD51 paralogs. In summary, these findings highlight the important roles of RAD51 and its paralogs in promoting quiescent cell survival in response to UVR.