Long-term adaptation of renal cells to hypertonicity: role of MAP kinases and Na-K-ATPase

Abstract

Renal cells in culture have low viability when exposed to hypertonicity. We developed cell lines of inner medullary collecting duct cells adapted to live at 600 and 900 mosmol/kgH(2)O. We studied the three modules of the mitogen-activated protein (MAP) kinase family in the adapted cells. These cells had no increase in either extracellular signal-regulated kinase, c-Jun NH(2)-terminal kinase, or p38 MAP kinase protein or basal activity. When acutely challenged with further increments in tonicity, they had blunted activation of these kinases, which was not due to enhanced phosphatase activity. In contrast, the cells adapted to the hypertonicity displayed a marked increment in Na-K-ATPase expression (5-fold) and ouabain-sensitive Na-K-ATPase activity (10-fold). The changes were reversible on return to isotonic conditions. Replacement of 300 mosmol/kgH(2)O of NaCl by urea in cells adapted to 600 mosmol/kgH(2)O resulted in marked decrement in Na-K-ATPase and failure to maintain the cell line. Replacement of NaCl for urea in cells adapted to 900 mosmol/kgH(2)O did not alter either Na-K-ATPase expression, or the viability of the cells. The in vivo modulation of Na-K-ATPase was studied in the renal papilla of water-deprived mice (urinary osmolality 2,900 mosmol/kgH(2)O), compared with that of mice drinking dextrose in water (550 mosmol/kgH(2)O). Increased water intake was associated with a ~30% decrement in Na-K-ATPase expression (P < 0.02, n = 6), suggesting that this enzyme is osmoregulated in vivo. We conclude that whereas MAP kinases play a role in the response to acute changes in tonicity, they are not central to the chronic adaptive response. Rather, in this setting there is upregulation of other osmoprotective proteins, among which Na-K-ATPase appears to be an important component of the adaptive process.

Cells adapted to high NaCl have many DNA breaks and impaired DNA repair both in cell culture and in vivo

“…However, cells adapt to elevated NaCl, proliferating in culture (12,13) and functioning without apoptosis in the renal inner medulla in vivo (20,21) where NaCl normally is always high. The question addressed in the following studies is whether the adaptation to high NaCl involves restoration of the DNA damage response and repair of the DNA damage.…”

“…The known protective responses include accumulation of organic osmolytes (30), up-regulation of heat shock proteins (12,13), and activation of Na͞K ATPase (12). In cell culture, high NaCl causes DNA damage (8,9), but also induces transient cell cycle arrest (14,16,18), which was believed to provide time for repair of the DNA.…”

“…Premature cessation of the cell cycle arrest causes apoptosis (31). After the cell cycle delay, cells in culture can adapt to hypertonicity and resume apparently normal growth (12,13) (Fig. 1).…”

“…However, cells can be adapted to high NaCl in culture by gradually increasing NaCl over several days. Such adapted cells proliferate indefinitely in high NaCl medium (12,13), which provides a convenient model. The initial response to acute elevation of NaCl is cell cycle arrest that lasts for several hours (14).…”

The expression of the γ subunit of Na-K-ATPase is regulated by osmolality via C-terminal Jun kinase and phosphatidylinositol 3-kinase-dependent mechanisms

“…It is very likely that the stress imposed on renal cells by hypertonicity brings about a coordinated response in which a number of cellular process are activated, many of which are critical to cell viability. Among the proteins that seem important to this process are the heat shock proteins (6), cycloxygenase 2 (7), and, as we recently reported, Na-K-ATPase (8). This study examined the up-regulation of the ␣ and ␤ subunits of the protein.…”