Abstract P2-26-02: Valproic acid induces apoptosis and mitochondrial perturbations in breast cancer cells

Abstract Introduction: Breast cancer is the second leading cause of death among women after lung cancer. Despite the increase and improvement in preventive screening, breast cancer still remains a threat, both for pre- and postmenopausal women, due to the rapid development of resistance to drug treatments, which recalls the complicity of multiple factors. Breast cancer prognosis can be correlated with specific histological subtypes and their receptor status. Depending on stage and invasiveness, ER + and PR + breast cancers are treated with surgery and adjuvant chemotherapy, which includes endocrine-based agents, such as SERMS (Tamoxifen), SERDs (Fulvestrant) and aromatase inhibitors (Letrazole and Anastrazole). Resistance to endocrine therapy can be explained by various mechanisms, such as mutations of the estrogen receptor, epigenetic modifications of the same receptor, alterations in hormone synthesis and metabolism, aberrant activation of signaling transduction pathways (up-regulation of PI3K, MAPK, CDKs). To reduce pharmacological resistance, it is possible to use histone deacetylase inhibitors, such as valproic acid, as the epigenetic alterations play an important role in the oncogenesis of breast cancer. Epigenetic modifications of genes are a mechanism used by cancer cells to silence the expression of tumor suppressor genes, driven by epigenetic mutations of regulator factors. The valproic acid, used as an antiepileptic drug and for the treatment of bipolar disorders, is also a selective inhibitor of class I and class II HDAC isoforms, involved in the progression of different types of tumors. In this study, we tested the effects of valproic acid on the signaling involved in apoptosis and in the generation of Reactive Oxygen Species (ROS) in MCF-7 breast cancer cells. Methods: MTT assay was performed to evaluate cell proliferation. Cell cycle, ROS levels and apoptosis were analyzed by flow cytometry, while protein levels were detected by Western Blotting (WB). Results: Cell treatment with valproic acid reduced cell proliferation and induced G0/G1 cell cycle arrest. In addition, the drug enhanced the generation of ROS by the mitochondria and addressed breast cell death by apoptosis. Indeed, the valproic acid was found to downregulate the anti-apoptotic marker Bcl-2 and to upregulate the expression of pro-apoptotic markers, such as Bax and Bad, leading to release of cytochrome C into the cytosol and PARP cleavage. The drastic reduction in mitochondrial membrane potential and in mitochondrial mass was observed following treatment with valproic acid, indeed the ratio between the two parameters, index of functionality per mitochondrion, resulted to be lowered, thus emphasizing the metabolic perturbation in the cells. Conclusion: In summary, our results have demonstrated that in estrogen receptor positive breast cancer cells the valproic acid is a suitable drug to arrest cell growth, to address apoptosis and mitochondrial perturbations, all factors that are important in determining cell fate and health. This study encourages that valproic acid, as an epigenetic drug, is a promising class of antineoplastic agent to be further explored also in combination with other classical chemotherapeutics in breast tumors. Citation Format: Alessandro Paolì, Martina Forestiero, Giuseppina Daniela Naimo, Loredana Mauro, Maria Luisa Panno, Francesca Giordano. Valproic acid induces apoptosis and mitochondrial perturbations in breast cancer cells [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-26-02.