TETRANDRINE INDUCED INHIBITION OF SIGNAL TRANSDUCER AND ACTIVATOR OF TRANSCRIPTION (STAT) 3 CAUSES THE REDUCTION OF CELL SURVIVAL, PROLIFERATION, AND ANGIOGENESIS IN TNBC

Natural agents may be promising to combat aggressive behavior of the triple-negative breast cancer (TNBC). STAT3 is a protein that is highly expressed in breast cancer tissues compared to non-malignant breast tissues. Our objective for the present study is to analyze the anti-tumorogenic effects of a Chinese herbal drug, tetrandrine, in TNBC progression through inhibition of STAT3 phosphorylation. Purpose (a): The most successful therapies for breast cancer target the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (Her-2). Hormonal therapies are not useful in combating triple negative breast cancer (TNBC), which lacks these targeted hormonal receptors. In fact, some of these patients that undergo hormone deprivation and/or Herceptin therapy acquire resistance. The triple negative breast cancer (TNBC) phenotype, which lacks the presence of Her-2, ER, and PR are even more aggressive and resistant. Therefore, there is an urgent clinical need to identify novel agents that can kill tumor cells with no additional toxicity to normal cells and this would have great impact on treatment of such patients. Tetrandrine, a bis-benzylisoquinoline alkaloid isolated from the root of Stephania tetrandra, is a calcium channel blocker used in Chinese medicine for the treatment of silicosis and arthritis. Studies have shown that tetrandrine also has anti-tumor and anti-growth activities. Our objective is to study the effects of tetrandrine on STAT3 signaling that plays an important role in cell proliferation, survival, chemoresistance and angiogenesis. STAT3 protein is highly expressed in breast cancer tissues compared to non-malignant breast tissues. We hypothesize that tetrandrine treatment inhibits the phosphorylation of STAT3 and its associated downstream signaling lead to the reduction of cell survival, proliferation, and angiogenesis in TNBC cells. Methods (b): TNBC cell lines, MDA-MB-231 and HCC70, and non-tumorigenic epithelial cell line MCF-10A were cultured in ATCC recommended medium. MTT assays were carried out to determine the effect of tetrandrine on cell viability. Additionally, cells were subjected to various concentrations of tetrandrine and Western blotting was performed for analysis of protein expression and phosphorylation. Results (c): Our data indicate that tetrandrine selectively inhibits the growth of MDA-MB-231 and HCC70 cells compared to non-tumorigenic MCF-10A cells. In the MTT assay, Tetrandrine concentrations ranging from 0 to 40μM gradually decreased MDA-MB-231, HCC70 and MCF-10A cell viability, corresponding to IC50 values of 25, 20 and 75 μM (n = 8), respectively, after 48 hours of treatment. Our results show that tetrandrine inhibited the phosphorylation of STAT3 in a concentration dependent manner. Furthermore, the inhibition of STAT3 activation by tetrandrine led to the suppression of proteins involved in proliferation (cyclin D1), survival (Bcl-2, Bcl-xL, and Mcl-1), and angiogenesis (VEGF). This effect correlated with the inhibition of proliferation and apoptosis in TNBC cells. Conclusions (d): Our preliminary results suggest that tetrandrine inhibits the proliferation of TNBC cells through inhibition of constitutive STAT3 phosphorylation and it’s associated down stream signaling and has therapeutic potential in the treatment of TNBC.