Increased Hepatic Na,K-ATPase Activity during Hepatic Regeneration Is Associated with Induction of the β1-Subunit and Expression on the Bile Canalicular Domain

Abstract

Cellular and molecular mechanisms regulating the activity of the sodium pump or Na,K-ATPase during proliferation of hepatocytes following 70% liver resection have not been defined. Na,K-ATPase may be regulated by synthesis of its ␣-and ␤-subunits, by sorting to either the sinusoidal or apical plasma membrane domains, or by increasing membrane lipid fluidity. This study investigated the time course of changes during hepatic regeneration for Na,K-ATPase activity, lipid composition and fluidity, and protein content of liver plasma membrane subfractions. As early as 4 h after hepatic resection, Na,K-ATPase activity was increased selectively in the bile canalicular fraction. It reached a new steady state at 12 h and remained elevated for 2 days. Although hepatic regeneration was associated with a reduced cholesterol/phospholipid molar ratio and increased fluidity, measured with two different probes, these changes in lipid metabolism were in the sinusoidal membrane domain. The Na,K-ATPase ␤ 1 -subunit, but not the ␣ 1 -subunit, was increased selectively at the bile canalicular surface as shown by immunoblotting of liver plasma membrane subfractions and the morphological demonstration at both the light and electron microscopic levels. Furthermore, cycloheximide blocked the rise in ␤ 1 -subunit mRNA levels. Since the time course for ␤ 1 -subunit accumulation was similar to that for activation of Na,K-ATPase activity, this change implicated the ␤ 1 -subunit in activating sodium pump activity.

Tissue-specific Expression of the Na,K-ATPase β3 Subunit

“…Fluidization of the membrane increased Na,KATPase activity in the canalicular fraction in control rats (where ␤1 was absent), but not in that from rats undergoing regeneration (where ␤1 was present). Thus there appear to be functional differences in canalicular Na,K-ATPase depending on whether or not ␤1 is expressed there (8). We can now speculate that the missing ␤ subunit in normal adult rats is ␤3 and that it is responsible for the difference.…”

“…The polarized distribution of reactivity for ␤1 correlated with the prior histochemistry, suggesting that the presence of ␤1 makes the enzyme active. Most recently, this group reported that the increased ␤1 seen during hepatic regeneration was specifically localized to the canalicular domain and lateral surfaces of the hepatocyte (8). Fluidization of the membrane increased Na,KATPase activity in the canalicular fraction in control rats (where ␤1 was absent), but not in that from rats undergoing regeneration (where ␤1 was present).…”

“…Although the major ATPase characteristics are assigned to the ␣ subunit, the ␤ subunit is required for folding and transport of ␣,␤-heterodimers to the plasma membrane (2). Nonetheless, there have been a number of reports in which little or no ␤1 or ␤2 subunit or mRNA was detected despite significant levels of ␣ or its mRNA (3-6), or alternatively, in which ␣ and ␤ mRNA or protein did not appear to colocalize in tissue sections (7)(8)(9).…”

Decreased hepatocyte membrane potential differences and GABAa-β3 expression in human hepatocellular carcinoma

“…Also, like the GABAergic system, changes in Na/K ATPase activity and in particular the ␣1 and perhaps ␤1 subunits have been described in association with changes in hepatocyte proliferative activity. [10][11][12][13][14] The objectives of this study were to (1) document PD levels in HCC and adjacent nontumor tissues, (2) describe the nature and extent of GABA receptor and Na/K ATPase expression in these tissues, and (3) determine what impact enhanced GABA A receptor expression has on malignant hepatocyte growth in vivo.…”

Effect of altered Na+entry on expression of apical and basolateral transport proteins in A6 epithelia

“…These observations suggest that the enzyme may not traffic exclusively as ␣␤-heterodimers. Moreover, it has been shown that increases in a single subunit of the Na ϩ -K ϩ -ATPase not only occur but have also been implicated in increasing pump activity (33). Our observations would suggest that enzyme function may be increased by alteration in basolateral expression of ␣-subunit alone, which implies that there may be an excess of ␤-subunit already present in or near the membrane.…”