Recycling of the asialoglycoprotein receptor in isolated rat hepatocytes. ATP depletion blocks receptor recycling but not a single round of endocytosis

Continuous endocytosis of ¹²⁵I-asialo-orosomucoid (ASOR) mediated by the galactosyl receptor in rat hepatocytes is a cyclic process. ¹²⁵I-ASOR-receptor complexes are internalized, processed, and the ligand is degraded while the receptor is returned to the cell surface for reutilization. Since a true cycle has a thermodynamic requirement for the input of external energy, we examined the effects of changes in intracellular ATP levels on the function of the receptor cycle. Hepatocytes were depleted of ATP to various extents prior to endocytosis by incubating cells at 15°C in the presence of 2 mM NaF and 0-20 mM NaN₃. A luciferase-luciferin bioluminescence assay was used to quantitate the amount of cellular ATP. ATP-depleted cells were allowed to bind ¹²⁵I-ASOR to 0°C, washed through discontinuous Percoll gradients, and only viable cells were isolated and incubated at 37°C to initiate a synchronous single round of endocytosis. The extent of internalization of this surface-bound ¹²⁵I-ASOR was unaffected by an ATP depletion to less than 1% of the control level. The rate of internalization of surface-bound ligand was unaffected until the ATP levels decreased to 30% or less; at >98% ATP depletion the initial rate decreased by a maximum of 55% and the kinetics became biphasic. In contrast, continuous endocytosis in the presence of excess ASOR was inhibited by only a 25% decline in cellular ATP content and demonstrated a very sharp threshold response to changing ATP levels. Continuous endocytosis, which requires receptor recycling, was completely inhibited when the total cellular ATP level decreased by only 40%. We conclude that the internalization phase of endocytosis is not dependent on ATP but that the processing and/or externalization phases of the complete receptor cycle are either directly or indirectly dependent on ATP and very sensitive to changes in cellular ATP content.