VERY-LOW PROTEIN DIETS DECREASE CAMP-MEDIATED CYTOSOLIC AND NUCLEAR RESPONSES IN ISOLATED RAT HEPATOCYTES

Very low protein diets disrupt the cAMP signal transduction cascade in rat hepatocytes. After 3 days of feeding a 0.5% protein diet to weanl ing rats, quantity of the RI regulatory subunit of cAMP-dependent prot ein kinase (PKA) is decreased in the cytosol of the hepatocyte. During the second week of feeding very low protein diets, this reduction in RI quantity is sustained, but is coupled with an increase in hormone-s timulated cAMP production. We have investigated the effects of these c hanges in the cAMP signal transduction pathway on two downstream respo nses to cAMP: activation of glycogen phosphorylase and phosphorylation of cAMP response element binding protein (CREB). Isolated hepatocytes from mts fed 15% protein (control) or 0.5% protein (malnourished) die ts for 3 or 14 days were treated with glucagon, dibutyryl-cAMP, an RI- specific agonist, or an RII-specific agonist pair. Activation of glyco gen phosphorylase in hepatocytes from malnourished rats was lower in r esponse to RI-agonist at days 3 and 14, whereas RII-stimulated activat ion did not change. Stimulation of glycogen phosphorylase by glucagon was lower in hepatocytes from malnourished rats at day 3, but not at d ay 14, when hormone-stimulated cAMP production is increased. After tre atment with glucagon and RI agonist, quantity of phosphorylated CREB ( pCREB) was lower in hepatocytes from malnourished rat; however, stimul ation relative to the baseline quantity of pCREB was maintained. These data suggest that decreases in the quantity of cytosolic PKA-RI after consumption of very low protein diets impair down-stream responses to PKA activation in both the cytosol and nucleus of rat hepatocytes. (C ) Elsevier Science Inc. 1997.