Cell adhesion mediated drug resistance (CAM-DR): Role of integrins and resistance to apoptosis in human myeloma cell lines

Integrin-mediated adhesion influences cell survival and may prevent programmed cell death. Little is known about how drug-sensitive tumor cell lines survive initial exposures to cytotoxic drugs and eventually select for drug-resistant populations. Factors that allow for cell survival following acute cytotoxic drug exposure may differ from drug resistance mechanisms selected for by chronic drug exposure. We show here that drug-sensitive 8226 human myeloma cells, demonstrated to express both VLA-4 (alpha(4)beta(1)) and VLA-5 (alpha(5)beta(1)) integrin fibronectin (FN) receptors, are relatively resistant to the apoptotic effects of doxorubicin and melphalan when pre-adhered to FN and compared with cells grown in suspension, This cell adhesion mediated drug resistance, or CAM-DR, was not due to reduced drug accumulation or upregulation of anti-apoptotic Bcl-2 family members. As determined by flow cytometry, myeloma cell lines selected for drug resistance, with either doxorubicin or melphalan, overexpress VLA-4. Functional assays revealed a significant increase in alpha(4)-mediated cell adhesion in both drug-resistant variants compared with the drug-sensitive parent line. When removed from selection pressure, drug-resistant cell lines reverted to a drug sensitive and alpha(4) -low phenotype. Whether VLA-4-mediated FN adhesion offers a survival advantage over VLA-5-mediated adhesion remains to be determined, in conclusion, we have demonstrated that FN-mediated adhesion confers a survival advantage for myeloma cells acutely exposed to cytotoxic drugs by inhibiting drug-induced apoptosis. This finding may explain how some cells survive initial drug exposure and eventually express classical mechanisms of drug resistance such as MDR1 overexpression. (C) 1999 by The American Society of Hematology.