Estrogen Receptor-α36Is Involved In Pterostilbene Induced Apoptosis In Breast Cancer In Vitro And In Vivo

Objective:Pterostilbene (trans-3,5-dimethoxy-4’-hudroxystilbene) is an anti-oxidant primarily found in blueberries and has also been considered as a agent against breast cancer regardless of conventional estrogen receptor (ER-a66) status through inducing both caspase dependent and caspase independent apoptosis. However, the pterostilbene induced apoptosis rate in ER-a66negative breast cancer cells was much higher than ER-a66positive breast cancer cells. Estrogen receptor ER-a36(ER-a36), a variant of ER-a66, is widely expressed in ER-a66negative breast cancer and its high expression could mediate ER-a66positive breast cancer patients resistant to Tamoxifen therapy. However, whether the antiproliferation activity of pterostilbene in breast cancer cells was.related.with.ER-a36.expression.remains.unknown. Here, we report the role of ER-a36in breast cancer apoptosis induced by pterostilbene.Methods:RT-PCR and western blot showed the expression of ER-α36in Mb231、 Mb231/Si36、MCF-7and MCF-7/ER36breast cancer. We use MTT trial to determine whether the sensitivity of breast cancer cells to pterostilbene was depended on ER-a36expressions. Apoptosis and our in vivo finding show ER-a36plays an important role in pterostilbene-induced apoptosis in breast cancer cells.Results:Mb231/Si36with negative expression of ER-a36exhibited dramatically decreased sensitivity to pterostilbene, compared to parental Mb231cells. Moreover, MCF-7/ER36cells with ER-a36overexpressed were more sensitive to pterostilbene than MCF-7cells. Compared to parental Mb231cells, the apoptotic percentage of Mb231/Si36with ER-a36knocked down induced by pterostilbene was decreased (26.6±1.9%versus13.6±3.9%, p=0.006). The apoptosis induced by pterostilbene was promoted in MCF-7/ER36cells with higher expression of ER-a36, compared to the parental MCF-7cells (25.7±2.2%versus10.5±1.5%,p=0.006). Pterostibene treatment significantly reduced the growth rate of the Mb231tumors compared to physiological saline treatment. However, there was no significantly reduction in the growth rate of Mb231/Si36tumors after pterostilbene treatment compared to vehicle control group. Immunohistochemical staining investigates whether the reduction of tumor growth is associated with the downregulation of ER-a36expression. A large area of necrosis was found in Mb231xenograft tumors after pterostilbene treatment. However, the areas of necrosis in Mb231/Si36tumors after pterostilbene treated was small and scattered, which was similar to that of vehicle.Conclusions:ER-a36is a potential therapeutic target in ER-a36-positive breast cancer that has resistance to TAM. And, pterostilbene could be considered as a selective inhibitor targeting ER-a36in the future therapy against ER-a36-positive breast cancer.