Breast cancer treatment has evolved drastically with the addition of immunotherapy and novel targeted drugs to the current treatment options. However, achieving long-term responses with minimal adverse events remains challenging. Cancer testis antigens (CTAs) offer novel opportunities for drug development thanks to their tumor specificity, immunogenicity, pro-tumorigenic functions, and negative prognostic connotations. We previously reported that lactate dehydrogenase C (LDHC) plays a key role in regulating genomic stability and that targeting LDHC significantly improved treatment response to DNA damage response drugs in breast cancer. Here, we explored the molecular mechanisms associated with LDHC silencing in two basal-like breast cancer cell lines, MDA-MB-468 and BT-549, and a Her2-enriched breast cancer cell line, HCC-1954. Transcriptomic analyses identified the cell line-dependent differential activation of the pro-survival STAT3 pathway following LDHC depletion. While LDHC silencing significantly compromised cell survival in basal-like breast cancer cells in conjunction with a downregulation of STAT3 signaling, the opposite effect was observed in Her2-enriched breast cancer cells, which demonstrated the enhanced activation of the pro-survival STAT3 signaling pathway. The inhibition of STAT3 not only reversed the unfavorable effect of LDHC silencing in the Her2-enriched cancer cells but also demonstrated significant anti-cancer activity when used as a single agent. Our findings suggest that the LDHC-STAT3 signaling axis plays a role in regulating breast tumor cell survival in a subtype-dependent manner. Thus, LDHC-targeted therapy could be a viable therapeutic approach for a subset of breast cancer patients, particularly patients with basal-like breast cancer, whereas patients carrying Her2-enriched tumors may likely benefit more from monotherapy with STAT3 inhibitors.

Background: Lactate Dehydrogenase C (LDHC) is a promising therapeutic target due to its highly tumor-specific expression, immunogenicity, and oncogenic functions. We previously showed that LDHC silencing in triple-negative breast cancer (TNBC) cells enhances treatment response to DNA-damage response-related drugs, supporting its therapeutic potential. However, no selective LDHC inhibitors exist, highlighting the need for innovative targeting strategies. Methods: We assessed the physicochemical properties and evaluated the delivery efficiency, anti-tumor activity, and safety of four cell-penetrating peptides (CPPs)—R10, 10R-RGD, cRGD-10R, and iRGD-10R—for siRNA-mediated LDHC silencing in TNBC. Clonogenic assays were used to evaluate effects on olaparib sensitivity, and TNBC zebrafish xenografts were utilized to study in vivo anti-tumor activity. Results: All CPP:siRNA complexes formed uniform nanocomplexes (129–168 nm) with low polydispersity indices (<0.25) and positive zeta potentials (+6.47 to +29.6 mV). Complexes remained stable in human serum for 24 h and showed no significant cytotoxicity in TNBC and non-cancerous cell lines. The 10R-RGD and cRGD-10R:siLDHC complexes achieved 40% LDHC protein knockdown, reduced TNBC clonogenicity by 30–36%, and enhanced olaparib sensitivity. Treatment of TNBC zebrafish xenografts with 10R-RGD or cRGD-10R:siLDHC complexes significantly reduced tumor growth by approximately 50% without major toxicity. Conclusions: These results demonstrate that CPP-mediated siRNA delivery enables selective LDHC silencing with tumor growth inhibition in triple-negative breast cancer models. This approach represents a novel, effective, and safe proof-of-concept therapeutic strategy to target LDHC, with potential translational relevance as a standalone therapy or in combination with common anti-cancer drugs.