Abstract 2295: Regulation of Her-2 in long-term letrozole resistant cells by HSP-90

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Development of aromatase inhibitors (AIs) has significantly improved the treatment outcome of hormone responsive post-menopausal breast cancer. However, not all tumors respond and some eventually acquire resistance. Using our MCF-7Ca xenograft model, we also observed that although, AI letrozole provides a longer control over tumor growth than tamoxifen, tumors eventually began to grow. A cell line was isolated from these Long-Term Letrozole Treated tumors (LTLT-Ca). These cells and had lower expression of ER and aromatase compared to parental MCF-7Ca cells. On the other hand, Her-2/MAPK pathway was upregulated. Inhibition of Her-2 using trastuzumab resulted in reversal of resistance and restoration of sensitivity to estrogens, antiestrogens and AIs, Histone deacetylase (HDAC) inhibitor, entinostat also reversed AI resistance. Similarly, stopping letrozole treatment also resulted in reversal of letrozole resistance. In all these studies, we found that downregulation of Her-2 was associated with reversal of letrozole resistance. In order to determine the mechanism of regulation of Her-2 in LTLT-Ca cells, we compared parental MCF-7Ca cells to LTLT-Ca for basal levels of Her-2 protein, protein stability, mRNA and mRNA stability. Cell lysates or mRNA were then collected at various time points after trating with Actinomycin D to inhibit RNA synthesis and cycloheximide to inhibit translation. Although, our previous data shows that letrozole resistant tumors do not have amplification of the Her-2 gene (FISH). LTLT-Ca cells have higher basal level of Her-2 protein and mRNA (4.5-fold higher mRNA; p<0.001). We also found that Her-2 protein in LTLT-Ca cells is more stable and has a longer half-life than MCF-7Ca cells. However, Her-2 mRNA in LTLT-Ca did not have a longer half-life. In LTLT-Ca cells, more association between Her-2 and HSP-90 was observed compared to parental MCF-7Ca cells using IP. When hormone sensitive MCF-7Ca cells were placed in estrogen-deprived conditions, there was increased association of Her-2 with HSP-90. Stability of Her-2 protein was regulated by HSP-90. Inhibition of HSP-90 with HSP-90 inhibitor 17-DMAG downregulated Her-2 and also reduced the half-life of Her-2. Furthermore, 17-DMAG also effectively reduced viability of parental MCF-7Ca and LTLT-Ca cells (in 72 hours) with IC50 of 2.5μM and 1.7μM respectively. In addition, combination of letrozole with HSP-90 inhibitors such as 17-DMAG, HSP-990 and AUY9122 inhibited the growth of both LTLT-Ca and MCF-7Ca cells significantly better than single agent alone. Our results indicate that HSP-90 plays a role in regulation of Her-2 protein, which modulates resistance to letrozole. This suggests use of HSP-90 inhibitors may inhibit the upregulation of Her-2 and thereby overcome subsequent resistance to aromatase inhibitors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2295. doi:10.1158/1538-7445.AM2011-2295