(P 79) NOVEL THERAPEUTIC POTENTIAL THROUGH HYPERACTIVATION OF THE ANTICIPATORY UNFOLDED PROTEIN RESPONSE PATHWAY IN BONE CANCER TREATMENT

Objective: Bone cancer ranks as the third leading cause of cancer-related deaths in children and teenagers. There are several subtypes, including osteosarcoma (OS), Ewing sarcoma (ES), and chondrosarcoma (CS), each originating from different cell types within the bone. These malignant bone sarcomas present significant treatment challenges, particularly upon relapse and metastasis due to chemoresistance. Therefore, there is a clinical need for the discovery of newer effective therapies for recurrent or relapsed disease. This study investigates the potential activity of ErSO, a small compound known to selectively kill estrogen receptor alpha (ERα)-positive breast cancer cells through hyperactivation of UPR in the treatment of bone cancer.

Methods: To determine the activity of ErSO across various bone cancer subtypes, sensitivity to ErSO was measured in 13 OS cell lines, 3 ES cell lines, and 2 CS cell lines. The mechanism of action of ErSO was specifically investigated in OS cell lines, the most prevalent type of bone cancer, using western blot and qPCR analysis, and treatment with various UPR inhibitors. To elucidate unknown factors influencing ErSO sensitivity in bone cancer, ErSO-resistant clones were developed from ErSO-sensitive OS cell lines, and the previously determined associated molecular target of ErSO was confirmed in these clones. The activity of ErSO in metastatic OS was investigated in mouse xenograft models. Additionally, the activity of ErSO in combination with traditional chemotherapeutic agents, doxorubicin and cisplatin, was measured.

Results: Cell viability tests with ErSO revealed that bone cancer cell lines could be categorized into ErSO-sensitive and ErSO-insensitive groups. Notably, in bone cancer cells, ErSO sensitivity did not correlate with ERα expression, unlike in breast cancer cell lines. In ErSO-sensitive OS cell lines, ErSO induced UPR activation, particularly through the PERK and ATF6 pathways, with cytotoxicity specifically linked to ATF6 activation. Interestingly, while TRPM4 expression was observed in all ErSO-sensitive cell lines, some TRPM4-expressing bone cell lines were found to be ErSO-insensitive. Comparisons between parental cells and resistant clones consistently showed reduced ATF6 activation in resistant clones. In mouse models of experimental lung metastatic models using ErSO-sensitive human OS, single injections of ErSO dramatically reduced lung metastasis progression. Furthermore, combination treatment with ErSO and traditional chemotherapeutic agents demonstrated an additive or synergistic effect in ErSO-sensitive OS cell lines.

Conclusion: ErSO was found to be an effective agent against certain types of bone cancer, acting independently of ERα expression through ATF6 activation. However, factors other than TRPM4 expression contribute to ErSO sensitivity in bone cancer. These findings highlight the potential anti-cancer activity of ErSO in treating bone cancer, even in patients with lung metastasis, and underscore the need for further exploration of its mechanisms and determinants of sensitivity.