Arp2/3 Inhibition Synergizes With PARP Inhibitors by Impairing Homologous Recombination in Gastric Cancer Cells

Background: Poly(ADP-ribose) polymerase inhibitors (PARPi) have shown limited efficacy in gastric cancer, primarily due to the rarity of inherent homologous recombination (HR) deficiency. The nuclear Arp2/3 complex was recently implicated in DNA double-strand break repair via HR. Here, we investigate whether pharmacological inhibition of Arp2/3 can induce a functional HR-deficient state to synergize with PARPi in gastric cancer cells. Methods: HR repair efficiency was assessed using a direct repeat green fluorescent protein (DR-GFP) reporter assay in HEK-293T cells following Arp2/3 inhibition with CK666, siRNA-mediated knockdown, or CRISPR/Cas9-mediated knockout of ACTR2/ACTR3. HGC-27 and AGS gastric cancer cells were treated with CK666 alone and in combination with olaparib or niraparib. Cell viability and synergy were evaluated by Cell Counting Kit-8 (CCK-8) and clonogenic assays (n = 3 independent experiments). DNA damage and apoptosis were assessed by immunofluorescence for gamma H2A histone family member X (γH2AX) foci (≥100 cells per condition) and Western blot analysis for cleaved PARP. Results: Arp2/3 inhibition with CK666 significantly impaired HR repair efficiency in the DR-GFP assay (p < 0.01). This effect was recapitulated by genetic knockdown or knockout of ACTR2/ACTR3. CK666 alone exerted minimal cytotoxicity, but synergized with both olaparib and niraparib to suppress proliferation (Combination Index <1), reducing the half-maximal inhibitory concentration (IC50) of PARPi by approximately threefold. Combination treatment markedly enhanced γH2AX foci formation (p < 0.0001) and increased the level of cleaved PARP compared with either monotherapy. Conclusions: Inhibition of Arp2/3 compromises HR repair capacity, thereby creating a synthetic lethal interaction with PARP inhibition in gastric cancer cells. This combination strategy represents a promising therapeutic approach to broaden the clinical utility of PARPi in HR-proficient tumors and may also enable dose reduction to mitigate associated toxicities.