We previously showed that adhesion of myeloma cells to
fibronectin (FN) by means of beta1
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 beta1
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
beta1 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 IIbeta
protein levels. Confocal studies showed changes in the nuclear localization of
topo IIbeta; however, alterations in the nuclear-to-cytoplasmic ratio of
topo IIbeta in FN-adhered cells were not significantly different. Furthermore, after a high level of
salt extraction of
nuclear proteins, higher levels of
topo IIbeta-associated
DNA binding were observed in FN-adhered cells than in cells in
suspension. Together, these data suggest that
topo IIbeta is more tightly bound to the nucleus of FN-adhered cells. Thus, FN adhesion by means of beta1
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 IIbeta.