Abstract
Topoisomerase II (topo II), an enzyme essential for cell viability, is present in mammalian cells as the α- and β-isoforms. In human leukemia HL-60/S or HL-60/doxorubicin (DOX)0.05 cells, the levels of topo IIα- or β-protein were similar in either asynchronous exponential or synchronized cultures. Although topo IIα was hypophosphorylated in HL-60/DOX0.05 compared with HL-60/S cells, both overall and site-specific hyperphosphorylation of topo IIβ was apparent in HL-60/DOX0.05 compared with HL-60/S cells. The phosphorylation of topo IIα and not β was enhanced in the S and G2 + M phases of HL-60/S cells. In contrast, an increase in the phosphorylation of topo IIβ compared with α was apparent in the G1 and S phases of HL-60/DOX0.05 cells. The cytotoxicity and depletion of topo IIα or β in cells treated with drug for 1 h revealed that mole-for-mole, amsacrine was 2-fold more effective than etoposide in killing HL-60/S or HL-60/DOX0.05 cells and in depleting the β versus α topo II protein. Present results demonstrate that: 1) hyperphosphorylation of topo IIβ in HL-60/DOX0.05 cells may be a compensatory consequence of the hypophosphorylation of topo IIα to maintain normal topo II function during proliferation, and 2) enhanced sensitivity of HL-60/S or HL-60/DOX0.05 cells to amsacrine may be due to the preferential interaction and depletion of topo IIβ.
Footnotes
- Received July 12, 1999.
- Accepted September 15, 1999.
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Send reprint requests to: Dr. Ram Ganapathi, Taussig Cancer Center M38, Cleveland Clinic Foundation, 9500 Euclid Ave., Cleveland, OH 44195. E-mail: ganapar{at}cc.ccf.org
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This work was supported by U.S. Public Health Service Grant CA35531 and Grant CA74939 from the Department of Health and Human Services (to K.A.H., D.R.G., M.A., Y.Y., M.K.G., and R.G.), and by the Imperial Cancer Research Fund (I.D.H.).
- The American Society for Pharmacology and Experimental Therapeutics
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