Histone deacetylases (HDACs) modify nucleosomal histones, have a key role in the regulation of gene transcription, and may be involved in cell-cycle regulation, differentiation and human cancer. Purified recombinant human HDAC1 protein was used to screen a cDNA expression library, and one of the clones identified encoded DNA topoisomerase II (Topo II), an enzyme known to have a role in transcriptional regulation and chromatin organization. Coimmunoprecipitation experiments indicate that HDAC1 and HDAC2 are associated with Topo II in vivo under normal physiological conditions. Complexes containing Topo II possess HDAC activities, and complexes containing HDAC1 or HDAC2 possess Topo II activities. HDAC and Topo II modify each other's activity in vitro and in vivo. Our results indicate the existence of a functionally coupled complex between these two enzymes and offer insights into the potential mechanisms of action of both enzymes.

We previously showed that adhesion of myeloma cells to fibronectin (FN) by means of ␤1 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-adhesionmediated drug resistance. Cellular adhesion by means of ␤1 integrins resulted in a 40% to 60% reduction in mitoxantroneand etoposide-induced DNA doublestrand breaks. When the mechanisms regulating the initial drug-induced DNA damage were examined, a ␤1 integrinmediated reduction in drug-induced DNA double-strand breaks was found to correlate with reduced topo II activity and decreased salt-extractable nuclear topo II␤ protein levels. Confocal studies showed changes in the nuclear localization of topo II␤; however, alterations in the nuclear-to-cytoplasmic ratio of topo II␤ in FN-adhered cells were not significantly different. Furthermore, after a high level of salt extraction of nuclear proteins, higher levels of topo II␤-associated DNA binding were observed in FN-adhered cells than in cells in suspension. Together, these data suggest that topo II␤ is more tightly bound to the nucleus of FN-adhered cells. IntroductionStudies have found that cellular adhesion by means of ␤1 integrins inhibits cell death induced by DNA cross-linking agents and topoisomerase (topo) II inhibitors. 1,2 The mechanisms of resistance associated with ␤1-mediated adhesion are unknown. It is thought that DNA cross-linking agents and topo II inhibitors initiate cellular death by inducing DNA damage. Thus, cellular adhesion by means of ␤1 integrins could confer resistance by either decreasing drug-induced DNA damage or increasing cellular tolerance to such damage. To address this issue, we examined the effects of ␤1-mediated cellular adhesion to fibronectin (FN) on DNA damage induced by pharmacologic inhibitors of topo II. If ␤1 integrin-mediated adhesion reduces DNA damage induced by this class of drugs, then alterations in the putative target (topo II) may represent one mechanism of cellular-adhesion-mediated drug resistance (CAM-DR).Topo II is an adenosine triphosphate (ATP)-dependent enzyme that reversibly cuts double-stranded DNA and is transiently linked to the 5Ј end of the break site by phosphotyrosyl bonds. Mammalian cells contain 2 isoforms of topo II (topo II␣ and topo II␤, which are 170 kd and 180 kd, respectively). These 2 isoforms are encoded by separate human genes and differ with respect to molecular mass, sequence specificity for DNA cleavage, regulation of expression, and tissue distribution. 3,4 Many topo II inhibitors stabilize this normally transiently bound DNA-protein complex and form what is referred to as the cleavable complex. 5 Stabilization o...

Purpose: A phase II trial of the novel camptothecin karenitecin (BNP1350) was conducted to determine its efficacy and tolerability in patients with metastatic melanoma. Patients were biopsied to determine topoisomerase expression at baseline and response to therapy. Patients and Methods: Eligible patients had metastatic melanoma with up to three prior chemotherapy and/or any number of immunotherapy regimens.Treatment consisted of an i.v. infusion of 1mg/m 2 karenitecin daily for 5 days with cycles repeated every 3 weeks. Fine-needle aspiration biopsies were done before treatment and on day 3 to determine topoisomerase expression from patients' tumors. Results: Forty-three patients were evaluable for response and toxicity. Most patients (72%) had stage M1C disease and were previously exposed to chemotherapy (56%). The investigational agent was well tolerated with limited gastrointestinal side effects or fatigue. The major toxicity seen was reversible noncumulative myelosuppression. One patient had a complete response after 11months of therapy. No partial responses were seen, but 33% of the patients had disease stabilization lasting z3 months. Topoisomerase I, IIA, and IIB expression and localization were determined in a subset of patients.Topoisomerase I expression was highest, followed by topoisomerase IIB and topoisomerase IIA. Conclusion: Karenitecin was a well-tolerated investigational agent in this phase II study; side effects were generally mild and mostly hematologic. Karenitecin has significant activity in metastatic melanoma. Melanoma metastases express high levels of topoisomerase I. We did not observe any compensatory increase in topoisomerase II upon treatment with karenitecin.