Forced expression of Hsp27 Reverses P-Glycoprotein (ABCB1) Mediated Drug Efflux and MDR1 Gene Expression in Adriamycin Resistant Human Breast Cancer Cells

Mutant p53 accumulation has been shown to induce multidrug resistance (MDR1) gene and ATP binding cassettes (ABCs)-based drug efflux in human breast cancer cells. In the present work we have found that transcriptional activation of the oxidative stress-responsive heat shock factor 1 (HSF-1) and expression of heat shock proteins, including Hsp27 which is normally known to augment proteosomal p53 degradation, are inhibited in adriamycin (Doxorubicin, Dox) resistant MCF-7 cells (MCF-7/adr). Such an endogenous inhibition of HSF-1 and Hsp27 in turn results in p53 mutation with gain of function in its transcriptional activity and accumulation in MCF7/adr. Also, lack of HSF-1 enhances nuclear factor κB (NF-κB) DNA binding activity together with mutant p53, induces MDR1 gene and Pglycoprotein (P-gp, ABCB1) resulting in multidrug resistant phenotype. Ectopic expression of Hsp27, however, significantly depleted both mutant p53 and NF-κB (p65), reversed the drug resistance by inhibiting MDR1/P-gp expression in MCF-7/adr cells, and induced cell death by increased G2/M population and apoptosis. We conclude from these results that HSF-1 inhibition and depletion of Hsp27 is a trigger, at least in part, for the accumulation of transcriptionally active mutant p53, which can either directly or NF-κBdependently induce MDR1/P-gp phenotype in MCF-7 cells. Upon Hsp27 over expression, this pathway is abrogated, and the acquired multi drug resistance is significantly abolished so that MCF7/adr cells are sensitized to Dox. Thus clinical alteration in Hsp27 or NF-κB level will be a potential approach to circumvent drug-resistance in breast cancer. Development of multidrug resistant phenotype is a major obstacle to the successful treatment of breast cancer (1,2). There are two major pathways by which cancer cells acquire drug resistance, drug efflux and direct suppression of apoptosis. Drug efflux is due to increased plasma membrane accumulation of various ATPbinding cassette (ABC) transporters, including ABCB1, also known as P-glycoprotein (Pgp), which extrude the internalized drugs from the cancer cells (3-5). Various approaches have been reported to overcome the drug efflux, including pharmacological inhibition of ABCs and modulation of endogenous regulators of MDR1 (6). Drug resistance is also acquired via direct suppression of apoptotic pathways due to accumulation of mutant p53 (mutp53) with “gain of function” (7,8) and increased expression of anti-apoptotic proteins such as BCl-2 (4). In addition to abrogating the pro-apoptotic function of wild type p53 (wtp53), these p53 missense mutations have been shown to have unusual gain of function properties both in vitro and in vivo (9-11). Recent studies have established a link between these two pathways of drug resistance (12).