Targeting DNA damage response pathways to activate the STING innate immune signaling pathway in human cancer cells

Activating stimulator of interferon genes to turn immunologically refractive cold tumor hot is an exciting therapeutic approach to increase the clinical responsiveness of some human cancers to immune checkpoint inhibitors. DNA damaging drugs and PARP inhibitors are two types of agents that have demonstrated this potential. Inhibitors of Chk1 or Wee1 induce DNA damage in cancer cells in predominantly the S-phase population. Increased cytoplasmic single-stranded and double-stranded DNA (dsDNA) from this DNA damage resulted in increased tank-binding kinase 1 (TBK1) phosphorylation in a range of cancer cell lines. However, despite robust increases in pTBK1, no downstream consequences of TBK1 phosphorylation were observed (namely no increase in pIRF3/7, interferon regulatory factor (IRF)-dependent gene expression or a type I IFN response). In combination with cytotoxic chemotherapy such as gemcitabine or camptothecin (CPT), Chk1 inhibition increased cytoplasmic dsDNA compared with the cytotoxic alone but attenuated the cytotoxic chemotherapy-induced increase in IRF1 protein and STAT1 phosphorylation through inhibition of nuclear RelB translocation. Despite increased cytoplasmic DNA and TBK1 activation, inhibition of Chk1, ataxia telangiectasia and Rad3-related protein, or Wee1 failed to activate a type I IFN response. We discuss the potential underlying mechanisms for this lack of IRF-dependent gene response and how this might influence the clinical strategies of combining Chk1 or Wee1 inhibitors with immune checkpoint inhibitors.