Drug resistance in breast cancer : effect of Bcl-2 level on the responsiveness of two mammary carcinoma cell lines to chemotherapeutic agents

• Main Author: Sartor, Jason Robert
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/12215

The Bcl-2 pro-survival protein is an important regulator of apoptosis and has been implicated in mechanisms of drug resistance by virtue of the expression levels of this protein in the cell. In these studies we sought to determine the effect of the inherent Bcl-2 level in breast cancer cell lines on drug sensitivity and susceptibility to apoptosis, a s well a s attempt to sensitize these cell lines to chemotherapy by decreasing the amount of Bcl-2 in the cell. Initial studies involved the down-regulation of Bcl-2 level in the high- Bcl-2-expressing MCF - 7 and low-Bcl-2-expressing MDA - 435/LCC6 cell lines in vitro through the use of the Bcl-2 antisense oligonucleotide Genasense™ (G3139). Our findings indicated that Bcl-2 downregulation via Bcl -2 antisense was sufficient to induce apoptosis in vitro as measured by a series of apoptotic endpoints, but that changes in Bcl-2 expression levels were not sufficient to sensitize the cell lines to chemotherapy drugs. Analysis of the effect of Bcl-2 down regulation via estrogen inhibition in high and low Bcl-2-expressing clonal populations produced from Bcl-2 transfected MDA - 435/LCC6 and MCF - 7 cell lines indicated that this was not sufficient to sensitize these lines to chemotherapy drugs. Xenotransplantation of these same cell lines into immune-compromised mice indicated that Bcl-2 downregulation using Genasense was sufficient to impair tumor growth alone, and to a greater degree when combined with chemotherapy; however, the results of these studies established that the cytotoxicity of combined agents was additive and not synergistic. Interestingly, and in contrast to data presented in vitro, Bcl-2 levels did appear to confer some degree of resistance in vivo to Bcl - 2 antisense as a single agent or when combined with chemotherapy drugs. Taken together, these results established that in the cell lines used in this study, reduction of the level of Bcl-2 resulted in increased apoptosis in vitro and impaired tumor growth in vivo, although it did not sensitize cells to chemotherapy agents in either environment. These results suggest that while Bcl-2 clearly constitutes an attractive target due to its role in regulating apoptosis in breast cancer cells, additional mechanisms are likely important in the control of apoptosis arising from exposure to chemotherapy drugs.