Abstract 721: Mevalonate pathway mediates acquired anti-HER2 treatment resistance in HER2+ breast cancer

Background: Compelling evidence suggests that a more complete blockade of the HER receptor layer and its signaling, by combining anti-HER2 drugs such as Trastuzumab (T) and Lapatinib (L), is highly effective. However, resistance remains a challenge. To understand resistance mechanisms, we established a broad panel of L, T, and L+T resistant cell line models. Initial mRNA expression profiling identified upregulation or restoration of the mevalonate (MVA) pathway in some models where HER signaling is completely and sustainably blocked, suggesting a possible role of the MVA pathway in resistance. This pathway is a biosynthetic process for cholesterol and isoprenoid intermediates, particularly farnesyl and geranylgeranyl pyrophosphates (FPP and GGPP). Coordinating gene expression of this pathway is regulated by the master transcription factor, SREBP. Statins, cholesterol-lowering drugs, block this pathway via inhibiting the rate-limiting enzyme, HMG-CoA reductase. While a role of the MVA pathway in tumor initiation and progression was reported, its role in anti-HER2 resistance has remained elusive. Methods: SKBR3, AU565, and UACC812 parental HER2+ cells and their T, L, and L+T resistant derivatives (TR, LR, and LTR) were used. SREBP activity was determined by a reporter luciferase assay. Cell growth was assayed by methylene blue staining. Apoptosis was determined by Annexin V staining and the protein level of cleaved PARP. Molecular signaling analysis was performed by western blotting (WB) and reverse phase protein array (RPPA). Results: Reporter assays showed the inhibition of the SREBP activity upon LT treatment and its restoration at resistance. Blocking the MVA pathway with statins led to a marked growth inhibition or apoptosis in LR/LTR models, in which the HER signaling remains sustainably inhibited, while cognate parental cells and TR cells, in which HER is (re)activated, were slightly inhibited. Prevention of statin-induced growth inhibition or apoptosis by exogenous MVA indicated the specific role of this pathway in resistance. Cholesterol or its precursor squalene could not rescue growth inhibition. In contrast, both FPP and GGPP reversed the growth inhibition or apoptosis in SKBR3 and AU565 LR/LTR models, while in the UACC812LTR model only GGPP rescued. WB showed that mTOR and estrogen receptor (ER) are downstream effectors mediating the MVA pathway to anti-HER2 resistance. Proteomic analysis revealed additional novel MVA effectors which are currently under biochemical and functional validation. Conclusion: The MVA pathway plays a key role as an escape pathway by activating alternative signaling, including mTOR and ER pathways, in acquired resistance to potent HER2 inhibition in a cholesterol-independent but FPP/GGPP-dependent manner. Targeting the MVA pathway or its downstream effectors might provide a novel therapeutic strategy to overcome anti-HER2 resistance. Citation Format: Huizhong Hu, Lukas Simon, Lanfang Qin, Agostina Nardone, Chad Shaw, Gary Chamness, Laura Heiser, Nicholas Wang, Joe W. Gray, C. Kent Osborne, Rachel Schiff. Mevalonate pathway mediates acquired anti-HER2 treatment resistance in HER2+ breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 721. doi:10.1158/1538-7445.AM2015-721