Reduction in drug-induced DNA double-strand breaks associated with β1 integrin–mediated adhesion correlates with drug resistance in U937 cells

We previously showed that adhesion of myeloma cells to fibronectin (FN) by means of β1 integrins causes resistance to certain cytotoxic drugs. The study described here found that adhesion of U937 human histiocytic lymphoma cells to FN provides a survival advantage with respect to damage induced by the topoisomerase (topo) II inhibitors mitoxantrone, doxorubicin, and etoposide. Apoptosis induced by a topo II inhibitor is thought to be initiated by DNA damage. The neutral comet assay was used to determine whether initial drug-induced DNA damage correlated with cellular-adhesion–mediated drug resistance. Cellular adhesion by means of β1 integrins resulted in a 40% to 60% reduction in mitoxantrone- and etoposide-induced DNA double-strand breaks. When the mechanisms regulating the initial drug-induced DNA damage were examined, a β1 integrin–mediated reduction in drug-induced DNA double-strand breaks was found to correlate with reduced topo II activity and decreased salt-extractable nuclear topo IIβ protein levels. Confocal studies showed changes in the nuclear localization of topo IIβ; however, alterations in the nuclear-to-cytoplasmic ratio of topo IIβ in FN-adhered cells were not significantly different. Furthermore, after a high level of salt extraction of nuclear proteins, higher levels of topo IIβ–associated DNA binding were observed in FN-adhered cells than in cells in suspension. Together, these data suggest that topo IIβ is more tightly bound to the nucleus of FN-adhered cells. Thus, FN adhesion by means of β1 integrins appears to protect U937 cells from initial drug-induced DNA damage by reducing topo II activity secondarily to alterations in the nuclear distribution of topo IIβ.