MODULATION OF PHOSPHOENOLPYRUVATE CARBOXYKINASE MESSENGER-RNA LEVELS BY THE HEPATOCELLULAR HYDRATION STATE

Exposure of isolated perfused rat livers to hypo-osmotic (225 mosmol/l ) perfusion media for 3 h led to a decrease of about 60% in mRNA level s for phosphoenolpyruvate carboxykinase (PEPCK) compared with normo-os motic (305 mosmol/l) perfusions. Conversely, PEPCK mRNA levels increas ed about 3-fold during hyperosmotic (385 mosmol/l) perfusions. The ani sotonicity effects were not explained by changes in the intracellular cyclic AMP (cAMP) concentration or by changes of the extracellular Na or Cl- activity. Similar effects of aniso-osmolarity on PEPCK mRNA le vels were found in cultured rat hepatoma H4IIE.C3 cells, the experimen tal system used for further characterization of the effect. Whereas du ring the first hour of anisotonic exposure no effects on PEPCK mRNA le vels were detectable, near-maximal aniso-osmolarity effects were obser ved within the next 2-3 h. PEPCK mRNA levels increased sigmoidally wit h the osmolarity of the medium, and the anisotonicity effects were mos t pronounced upon modulation of osmolarity between 250 and 350 mosmol/ l. The aniso-osmolarity effects on PEPCK mRNA were not affected in pre sence of Go 6850, a protein kinase C inhibitor. cAMP increased the PEP CK mRNA levels about 2.3-fold in normo-osmotic media, whereas insulin lowered the PEPCK mRNA levels to about 8%. The effects of cAMP and ins ulin were also observed during hypo-osmotic and hyperosmotic exposure, respectively, but the anisotonicity effects were not abolished in pre sence of the hormones. The data suggest that hepatocellular hydration affects hepatic carbohydrate metabolism also over a longer term by mod ulating PEPCK mRNA levels. This is apparently unrelated to protein kin ase C or alterations of cAMP levels. The data strengthen the view that cellular hydration is an important determinant for cell metabolic fun ction by extending its regulatory role in carbohydrate metabolism to t he level of mRNA.