It has been suggested that there are multiple pathways for the cellular internalization of
insulin. To investigate these pathways we have examined the effects of three perturbations of endocytosis on the
insulin internalization process and have compared these effects with those obtained using an
asialoglycoprotein,
asialofetuin (Afet), and
epidermal growth factor (
EGF). Freshly isolated hepatocytes were incubated with radiolabeled
ligands and internalization measured under conditions of
anoxia to deplete cellular
ATP, in the presence of
phenylarsine oxide (PAO) to inhibit endocytosis, and in the presence of
monensin to interfere with endosomal acidification. Afet internalization essentially was blocked by all three treatment processes, while
insulin internalization was inhibited approximately 40% in the presence of
anoxia, and 54% in the presence of PAO.
Monensin exhibited differential effects on internalization of high and low
insulin concentrations. The effects of the treatment processes on
EGF internalization were intermediate to those seen with Afet and
insulin. These results suggest that
insulin and
EGF utilize routes of internalization exhibiting different energy requirements that may correspond to coated pit, non-coated pit, and fluid-phase internalization pathways. The observations with Afet internalization remain consistent with utilization of the coated pit pathway.