Osteoclast-Cancer Cell Metabolic Symbiosis Renders PARP Inhibitor Therapy Resistance in Bone Metastasis

ABSTRACT Seventy percent of patients with late-stage breast cancer develop distal bone metastases; however, the mechanism by which the metabolic microenvironment affects resistance to therapy remains unknown. We investigated the metabolic bone microenvironment and identified glutathione metabolism as the top pathway in osteoclasts, which provides feedback to tumor cells to help neutralize oxidative stress and generate PARP inhibitor (PARPi) therapy resistance. GPX4, the critical enzyme responsible for glutathione oxidation, was upregulated during PARPi therapy through stress-induced ATF4-dependent transcriptional programming. The increased absorption of glutamine and the upregulation of GPX4 expression work in concert to enhance glutathione metabolism in cancer cells. Human clinical sample analysis of paired primary breast tumor and bone metastasis samples revealed that GPX4 was significantly induced in bone metastases. Combination therapy utilizing PARPi and zoledronate, which blocks osteoclast activity and thereby reduces the microenvironmental glutamine supply, generates a synergistic effect in reducing bone metastasis. Thus, our results identified an essential metabolic symbiosis between bone-resident cells and metastatic cancer cells during PARPi therapy. SIGNIFICANCE Osteoclast-derived glutamine is taken up by tumor cells to synthesize glutathione and neutralize the ROS generated by PARPi. This is the first example of “metabolic symbiosis” in therapeutic resistance of bone metastasis.