Rucaparib induces mitochondrial fragmentation and apoptosis in prostate cancer cells by targeting Drp1

Mitochondrial dynamics, particularly the balance between fission and fusion, are critical in regulating cellular metabolism, apoptosis, and cancer progression. Dysregulation of this balance contributes to tumor survival and therapeutic resistance in castration-resistant prostate cancer (CRPC). Rucaparib, a clinically approved poly (ADP-ribose) polymerase (PARP) inhibitor, is primarily known for its role in DNA damage repair; however, its impact on mitochondrial function remains largely unexplored. In this study, we demonstrate that Rucaparib induces significant cytotoxicity and apoptosis in PC-3 CRPC cells in a time- and concentration-dependent manner, characterized by increased Bax/Bcl-2 ratio, cytochrome c release, and caspase-3 activation. Mechanistically, Rucaparib disrupts mitochondrial integrity by reducing mitochondrial membrane potential (MMP), inhibiting Complex IV activity, and depleting ATP levels. Confocal imaging and biochemical assays reveal that Rucaparib triggers mitochondrial fragmentation by promoting phosphorylation of dynamin-related protein 1 (Drp1) at Ser616 and enhancing its translocation to mitochondria. This process is accompanied by elevated intracellular Ca²⁺ levels and activation of calcium/calmodulin-dependent protein kinase II (CaMKII), suggesting a Ca²⁺/CaMKII/Drp1 signaling axis. Importantly, pharmacological inhibition of CaMKII with KN-93 reverses Drp1 mitochondrial translocation, restores mitochondrial morphology, and partially rescues ATP production, confirming the functional role of CaMKII in Rucaparib-induced mitochondrial dysfunction. These findings uncover a previously unrecognized mechanism of Rucaparib action beyond DNA repair inhibition, highlighting its ability to target mitochondrial dynamics and bioenergetics through Ca²⁺/CaMKII/Drp1 signaling. Our results provide new insights into the multifaceted anticancer mechanisms of Rucaparib and suggest that modulation of mitochondrial fission may offer a promising therapeutic avenue for CRPC.