Abstract P5-05-01: Targeting the mevalonate pathway to overcome acquired anti-HER2 treatment resistance

Background: Compelling preclinical and clinical 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 is still common and remains a challenge. To understand resistance mechanisms and further to identify novel therapeutic strategies, 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. The MVA pathway is commonly considered as a biosynthetic process primarily for cholesterol and isoprenoid intermediates, particularly farnesyl and geranylgeranyl pyrophosphates (FPP and GGPP, respectively). Statins, widely-used cholesterol-lowering drugs, block this pathway via inhibition of the rate-limiting enzyme, HMG-CoA reductase. While accumulating evidence also suggests a role of the MVA pathway in tumor initiation and progression, its role in anti-HER2 resistance remains elusive. Methods: SKBR3, AU565, and UACC812 parental HER2+ cells and their T, L, and L+T resistant (TR, LR, and LTR respectively) derivatives were used in this study. Cell growth after treatment with statins in the presence or absence of MVA, cholesterol, squalene, FPP, or GGPP was measured by methylene blue staining. Apoptosis was determined by Annexin V staining and the protein level of cleaved PARP. Parallel analysis of molecular signaling was done by western blotting. Results: Blocking the MVA pathway with lipophilic statins, simvastatin or atorvastatin, 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 only slightly inhibited. Interestingly, only lipophilic statins (which can be taken up by cancer cells), but not hydrophilic statins such as pravastatin (whose primary target is liver cells), conveyed the inhibitory effect. Prevention of statin-induced apoptosis by adding exogenous MVA indicated that the cell death caused by statin treatment was via its specific blockade of the MVA pathway. 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. Interestingly, mTOR was identified as the downstream signaling target of the MVA pathway in SKBR3 and AU565LTR models, while in the UACC812LTR model, the growth inhibition by statin was due to substantial estrogen receptor (ER) protein reduction. 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 could provide a novel therapeutic strategy to overcome anti-HER2 resistance. Citation Format: Huizhong Hu, Lukas M Simon, Agostina Nardone, Chad A Shaw, Gary C Chamness, Laura M Heiser, Nicholas Wang, C Kent Osborne, Rachel Schiff. Targeting the mevalonate pathway to overcome acquired anti-HER2 treatment resistance [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-05-01.