Triple-negative breast cancer (TNBC) is a highly malignant type of breast cancer and lacks effective therapy. Targeting cysteine-dependence is an emerging strategy to treat the mesenchymal TNBC. However, many TNBC cells are non-mesenchymal and unresponsive to cysteine deprivation. To overcome such resistance, three selective HDAC6 inhibitors (Tubacin, CAY10603, and Tubastatin A), identified by epigenetic compound library screening, can synergize with cysteine deprivation to induce cell death in the non-mesenchymal TNBC. Despite the efficacy of HDAC6 inhibitor, knockout of HDAC6 did not mimic the synthetic lethality induced by its inhibitors, indicating that HDAC6 is not the actual target of HDAC6 inhibitor in this context. Instead, transcriptomic profiling showed that tubacin triggers an extensive gene transcriptional program in combination with erastin, a cysteine transport blocker. Notably, the zinc-related gene response along with an increase of labile zinc was induced in cells by the combination treatment. The disturbance of zinc homeostasis was driven by PKCγ activation, which revealed that the PKCγ signaling pathway is required for HDAC6 inhibitor-mediated synthetic lethality. Overall, our study identifies a novel function of HDAC6 inhibitors that function as potent sensitizers of cysteine deprivation and are capable of abolishing cysteine-independence in non-mesenchymal TNBC.

No abstract

Triple-negative breast cancer (TNBC) is an aggressive type of breast cancer with a high risk of recurrence following therapeutic treatments. Targeted cysteine therapy via inhibition of cysteine uptake by erastin effectively induces mesenchymal TNBC cells to ferroptosis. However, a small residual population of cancer cells exhibited the erastin-resistance and survived after the erastin treatment. This phenomenon is likely analogous to the event of tumor recurrence in patients after therapy. To characterize this resistance, we established the erastin-resistant/recurrent TNBC cell models in vitro by multi-cycles of erastin challenge. By an epigenetic compound library screen, the erastin-resistance be abolished by adjuvant epigenetic compounds. Intriguingly, the erastin-recurrent TNBC cells failed to grow in anchorage-independent conditions, implying a loss of tumorigenicity. By transcriptomic profiling analysis, the recurrent cells displayed many gene expression alterations and attenuated signaling processes, including K-Ras signaling. Three members of the cystatin gene family, including CST4, were significantly downregulated in recurrent cells. Knocking out of CST4 by CRISPR/Cas9 significantly suppressed the tumorigenic potential and K-Ras signaling. Our findings suggested that targeted cysteine therapy could be a valid treatment for mesenchymal TNBC with a low probability of tumor recurrence.

No abstract

Triple-negative breast cancer (TNBC) accounts for 15% ~ 20% of overall breast cancer cases and exhibits earlier age of onset, high metastasis, and aggressiveness with poor clinical outcomes shown by higher relapse and lower survival rates than other types of breast cancer. TNBC contributes to the major mortality of breast cancer patients. Treatments of TNBC patients are still limited to surgery, chemotherapy, or radiation since the absence of cell receptors makes targeted hormonal therapies impossible. Targeting cysteine-dependence is an emergent strategy to treat the mesenchymal TNBC. However, many TNBC cells are non-mesenchymal and irresponsive to cysteine deprivation. To overcome such resistances, three selective HDAC6 inhibitors (Tubacin, CAY10603, and Tubastatin A), identified by epigenetic compound library screening, can synergize with cysteine deprivation to induce cell death in the non-mesenchymal TNBC. Despite the efficacy of the HDAC6 inhibitor, knockout of HDAC6 did not mimic the synthetic lethality induced by its inhibitors, particularly tubacin, indicating that HDAC6 is not the actual target of HDAC6 inhibitor in this context. Instead, transcriptomic profiling showed that tubacin triggers an extensive gene transcriptional program in combination with erastin, a cysteine transport blocker. Notably, the zinc-related gene response and an increase of labile zinc were induced in cells by the combination treatment. We found that the disturbance of cellular zinc homeostasis was driven by PKCγ activation, which revealed that the PKCγ signaling pathway is required for HDAC6 inhibitor-mediated synthetic lethality. Overall, our study identifies a novel function of HDAC6 inhibitors that function as potent sensitizers of cysteine deprivation. HDAC6 inhibitors are capable to overcome cysteine independence in non-mesenchymal tumor cells, which allows the application of targeted cysteine-dependence therapy in various subtypes of breast cancer. Citation Format: Tahiyat Alothaim, Morgan Charbonneau, Xiaohu (Mark) Tang. HDAC6 inhibitors sensitize non-mesenchymal triple-negative breast cancer cells to cysteine deprivation [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-001.