PARP1 and cancer: Measurement of PARP enzyme activity did, however, indicate that treatment with SIK2 inhibitors further decreased olaparib-induced suppression of PARP enzyme activity (Figure 3A) in cancer cells with detectable PARP protein levels (Supplemental Figure 2, B and C), consistent with the possibility that inhibition of PARP enzyme activity underlies, at least in part, the synergistic effect of SIK2 and PARP inhibition.