ATP depletion causes a reversible redistribution and inactivation of a subpopulation of galactosyl receptors in isolated rat hepatocytes

Abstract

Isolated rat hepatocytes, treated with metabolic energy poisons such as NaN3 in the absence of exogenous ligand, lose surface galactosyl (Gal) receptor activity (Clarke, B. L., and Weigel, P. H. (1985) J. Biol. Chem. 260, 128-133). We have used 125I-labeled asialo-orosomucoid and affinity-purified anti-receptor IgG to quantitate, respectively, the activity and the amount of Gal receptor protein. Cells were treated with NaN3 at 37 degrees C and the surface or total (surface and intracellular) binding of these two probes was measured at 4 degrees C, respectively, in intact cells or in cells permeabilized with digitonin. As a function of NaN3 concentration, both surface receptor activity and protein decreased in parallel by 50-80%. Virtually all of the lost surface receptor protein was found inside the cell, but only about 50% of all cellular Gal receptors were active. As determined by equilibrium binding studies, this decreased receptor activity reflected an overall loss of ligand binding sites with little change in binding affinity of the remaining Gal receptors for asialo-orosomucoid. When ATP was restored, normal surface receptor activity and number completely recovered even in the absence of protein synthesis. We conclude that a subpopulation of Gal receptors constitutively recycles and undergoes an inactivation/reactivation cycle. In the absence of ligand, these receptors are normally internalized and then inactivated. Loss of cellular ATP blocks receptor reactivation, prevents the reappearance of receptors at the cell surface and redistributes Gal receptors as inactive receptors accumulate intracellularly.