Abstract 3072: Novel approach to target trastuzumab resistant, HER2+ breast cancer: Activation of PKCdelta

Abstract The ERBB2/HER2 oncogene is amplified in 15% of breast cancers leading to overexpression of the HER2 protein. Trastuzumab based therapy has improved survival outcomes for patients with localized and regional HER2+ tumors. However, the prognosis for patients with HER2+ metastatic breast cancer remains poor, with survival rates as low as 30%. Resistance to anti-HER2 targeted therapy is a key factor contributing to disease progression. Previously, we showed that Notch1 is a driver of trastuzumab resistance and PKCα attenuates Notch signaling to prevent resistance. There are many isoforms of PKC in breast cancer cells. This study investigated the contributions of at least 6 PKC isoforms in the context of trastuzumab sensitive and resistant breast cancer cell lines (isogenic BT474 sensitive and resistant and de novo resistant HCC1954). PKCδ mRNA and protein were expressed higher than any other PKC isoform and further increased in resistant compared to sensitive cells. The activity of PKCs overall was less in resistant compared to sensitive cells. Exogeneous addition of TPA activated PKC and potently inhibited proliferation, cell cycle progression, and mammosphere formation only in resistant cells and not in isogenic sensitive cells. Knockdown of PKCδ rescued these growth phenotypes, suggesting that TPA-mediated activation of PKCδ was sufficient to inhibit growth of trastuzumab resistant cells lines. Based on these encouraging findings, we tested other PKC modulators to identify potentially safer alternatives to TPA. Among the seven compounds (TPPB, Prostratin, iPr-ILV, Ac-ILV, 1, 2-dioctanoyl-sn-glycerol, 1-oleoyl-2-acetyl-sn-glycerol and Bryostatin) screened for growth phenotypes of HCC1954 and BT474 resistant cell lines, TPPB was as potent as TPA (IC50=14nM TPPB vs. 0.45nM TPA). TPPB-mediated growth inhibition was dependent on PKCδ as knocked down of PKCδ using an RNAi approach almost completely rescued the inhibition of growth. An RNA sequencing approach was used to determine the mechanism by which TPA-mediated activation of PKCδ inhibits proliferation and cancer stem cell survival of trastuzumab resistant breast cancer cell lines. The most significant pathway activated by TPA was the TNFα- NFKβ pathway, suggesting that the mechanism by which activated PKCδ inhibits growth of trastuzumab resistant breast cancer could be through activation of a pro-inflammatory signaling cascade. In particular, the S100 family of cytokines were significantly upregulated by TPA, and their expression was reversed by PKCδ knockdown. These results suggest that even though TPA has been shown to be an effective agent to activate PKCδ, TPPB could possibly be a safer alternative. Future studies will determine mechanisms by which activating PKCδ selectively inhibits growth of trastuzumab resistant breast cancer in vivo and if PKCδ is a novel vulnerability for trastuzumab resistant disease. Citation Format: Maria Mejia, Debra Wyatt, Mitchell F. Denning, Kelvin Billingsley, Clodia Osipo. Novel approach to target trastuzumab resistant, HER2+ breast cancer: Activation of PKCdelta [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3072.