Induction of sodium pump beta 1-subunit mRNA expression during hepatocellular growth transitions in vitro and in vivo

Abstract

Previous suggestions (Hubert, J. J., Schenk, D. B., Skelly, H., and Leffert, H. L. (1986) Biochemistry 25, 4156-4163) of tissue-specific isoforms or nonexistence of hepatic Na,K-ATPase beta 1-subunits were reevaluated by quantifying beta 1-subunit mRNA levels in quiescent and proliferating liver. RNA was extracted from caudate liver lobes of sham or 67% hepatectomized adult rats and from primary cultures of adult rat hepatocytes that simulate developmental and regenerating growth transitions. Northern blot analysis with a 32P-labeled full-length Na,K-ATPase beta 1-cDNA probe (Mercer, R. W., Schneider, J. W., Savitz, A., Emmanuel, J., Benz, T.J., and Levenson, R. (1986) Mol. Cell. Biol. 6, 3884-3890) revealed four (approximately 2.7, 2.4, 1.7-1.8, and 1.5 kilobases) low abundance mRNA species in quiescent tissue, freshly isolated hepatocytes, and cultured hepatocytes derived from lag or late stationary phase (1-2 days or 11-12 days postplating, respectively). In contrast, proliferating liver from 4 h post-67% hepatectomized rats or cultured hepatocytes in logarithmic growth phase contained levels of beta 1-subunit mRNA which exceeded quiescent levels by 4-35-fold. Membrane Na,K-ATPase activity also increased 2-3-fold during liver regeneration 12-24 h after partial hepatectomy. When proliferation in vitro was augmented by transforming growth factor-alpha, a hepatocyte mitogen, or reinitiated in late stationary phase by a change to fresh culture medium containing rat serum, beta 1-subunit mRNA expression was restimulated 4-20-fold. Parallel measurements of alpha-tubulin mRNA induction showed relatively nonsynchronous or invariant changes during hepatocyte proliferative transitions; similar results were obtained after Northern blots with a sodium pump alpha I-subunit cDNA probe. No detectable hybridization signals were observed when either rat kidney or hepatocyte RNAs from freshly isolated and cultured cells or regenerating tissues were probed for the sodium pump 3.4-kilobase mRNA beta 2-isoform. These observations suggest that enhanced hepatic beta 1-subunit gene expression is linked specifically to growth-associated increases in Na,K-ATPase activity, hepatocyte proliferation, and mitogen activation.