Multiple isoenzymes of the Na+,K+-ATPase (α, α+, and α3) have been identified by molecular cloning (Shull, G. E., J. Greeb, and J. B. Lingrel. 1986. Biochemistry. 25: 8125-8132; and Schneider, J. W., R. W. Mercer, and E. J. Benz, Jr. 1987. Clin. Res. 35: 585A. [Abstr.]). At least one of these, the α3 chain, represents a novel form for which protein products and enzymatic activities are just beginning to be defined in rodents. We have recently demonstrated that expression of α3 is largely confined to neuromuscular tissues of fetal and adult rats (Schneider, J. W., R. W. Mercer, M. Gillmore-Hebert, M. F. Utset, C. Lai, A. Greene, and E. J. Benz, Jr. 1988. Proc. Natl. Acad. Sci. USA. 85: 284-288). We now report that certain human leukemia cell lines including HL60, HEL, and Molt 4 express mRNA for both α and α3 isoforms of Na+,K+-ATPase; mRNA was not detected in several other cell lines, including K562 and U937; no cell lines expressed α+ mRNA. In uninduced HL60 cells, α3 mRNA comprised 20-30% of total Na+,K+-ATPase mRNA. Furthermore, in HL60 and HEL cells, both α and α3 mRNA declined after induction of maturation by DMSO, retinoic acid, or hemin. However, the reduction in α3 mRNA was far more dramatic. α3 mRNA virtually disappeared, but α mRNA declined by only ~ 50%. In contrast, when maturation of HL60 cells along the monocyte/macrophage lineage was induced by exposure to phorbol esters, α3 mRNA remained abundant. Moreover, mRNA for the β subunit of the Na+,K+-ATPase increased dramatically. Our results demonstrate that the α3 isoform, formerly thought to be confined to neuromuscular tissues, is expressed in restricted lineages of hematopoietic origin. These leukemia cell lines should provide a useful model for analyzing regulation of the α3 isoform gene and characterization of α3 isoform activities.
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