Abstract 679: Targeting LC3-SQSTM1 (p62) signaling axis to treat apoptosis-resistant and metastatic breast cancers

Abstract

Development of therapy resistance to conventional treatments of breast cancer is a major health concern. Of these, triple negative breast cancers are among the most aggressive and therapy resistant. Early breast tumors, which are sensitive to the apoptosis inducing effects of radiation and chemotherapy, become resistant to apoptosis inducing therapies in their advanced stages. Therefore, there is an urgent need to develop alternative approaches to induce non-apoptotic death in apoptosis resistant breast tumors. In this connection, we showed that certain sulfhydryl (-SH) reactive compounds containing α,β-ketones cause misfolding of nascent proteins, endoplasmic reticulum (ER) stress and non-apoptotic cytoplasmic vacuolation death of breast cancer cells. Importantly, we found that 15-deoxy-Δ 12,14-prostaglandin J2 (15d-PGJ2), a cyclooxygenase-2 derived prostaglandin, induced non-apoptotic cell death in apoptosis resistant and rapidly growing MDA-MB-231 breast cancer cells. In contrast, identical doses of 15d-PGJ2 failed to induce similar cytoplasmic vacuolation and cell death in slow growing benign human mammary epithelial cells (HME). Other cyclopentenone prostaglandins such as PGA1 and PGA2 or PGD2, the precursor of 15d-PGJ2, were less potent in inducing cytoplasmic vacuolation death suggesting the uniqueness of 15d-PGJ2 in mediating non-apoptotic cell death in MDA-MB-231 cells. In addition to induction of ER stress marker (Bip), 15d-PGJ2 treatment also induced microtubule-associated protein 1 light chain 3 (LC3) and sequestosome1 (p62) proteins in MDA-MB-231 but not in normal HME cells. Interestingly, our results show that 15d-PGJ2 treatment further increased p62 protein to levels far higher than the concentrations found in untreated breast cancer cells, that are already elevated, compared to HME cells. This suggests that autophagy may be inhibited by 15d-PGJ2. We also found that Manumycin A (Man A), a structurally different molecule with -SH reactive unsaturated α,β-ketone group also induced cytoplasmic vacuolation leading to cell death similarly to 15d-PGJ2 in MDA-MB-231 cells by inducing LC3, p62 and ER stress marker Bip. To study whether Man A might be effective in reducing breast tumor burden, we used a nude xenograft model. Tumors receiving Man A showed remarkable reduction in tumor growth and contained extensive cytoplasmic vacuoles and pyknotic nuclei. Our results showed for the first time that in treatment resistant rapidly dividing breast cancer cells with a high requirement for new protein synthesis, the apoptotic and autophagy fate is avoided, but the cells can be killed by a facile non-autophagic and non-apoptotic mechanism, triggered by specific -SH reactive inducers of protein misfolding and aggregation through a possible LC3-SQSTM1(p62) signaling axis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 679.