Raf-1 and Bcl-2 induce distinct and common pathways that contribute to breast cancer drug resistance

Overexpression of Bcl-2 plays a role in the development of drug resistance in leukemia and other apoptosis-prone tumors. Raf isoforms are serine/threonine kinases that act as signal transducers in cascades initiated by many growth factors and mitogens. Raf isoform activation has been linked to drug resistance in leukemia. In this study we investigated effects of Bcl-2 and Raf-1 on doxorubicin-induced growth inhibition of MCF-7 breast cancer cells. In the absence of doxorubicin, overexpression of Bcl-2 or a constitutively active form of Raf-1 in MCF-7 cells did not affect proliferation rate. Overexpression of Bcl-2 increased resistance of MCF-7 cells to doxorubicin in 2-day, 5-day, and 8-week assays. Analysis of doxorubicin sensitivity of individual MCF/Bcl-2 clones showed that doxorubicin resistance was positively correlated with level of Bcl-2 overexpression. Overexpression of constitutively active Raf-1 also increased resistance to doxorubicin. Induction of Raf-1 activity in MCF-7 cells overexpressing Bcl-2 resulted in greater doxorubicin resistance than induction of Raf-1 activity in MCF-7 cells lacking Bcl-2 overexpression. Furthermore, levels of P-glycoprotein mRNA were increased in MCF-7 cells overexpressing a constitutively active Raf-1. MCF-7 cells overexpressing constitutively active Raf-1 were also more resistant to paclitaxel, which, like doxorubicin, is a substrate of P-glycoprotein. These observations suggest both independent and overlapping roles for Raf-1 and Bcl-2 oncogenes in the resistance to growth inhibition by doxorubicin.