Insulin and α2‐macroglobulin‐methylamine undergo endocytosis by different mechanisms in rat adipocytes: II. Comparison of intracellular events

A previous ultrastructural study showed that gold‐labeled insulin (Au‐I) and the non‐hormonal ligand gold‐labeled alpha‐2‐macroglobulin‐methylamine (Au‐α2MGMA) underwent endocytosis by dissimilar cell surface structures on rat adipocytes. The present ultrastructural study compared the intracellular routes taken by these two ligands in adipocytes. Intracellular Au‐α2MGMA was initially found within apparent coated vesicles but Au‐I was not, consistent with the previous demonstration that Au‐α2MGMA underwent endocytosis by coated pits whereas Au‐I was internalized by uncoated micropinocytotic invaginations. Early in the endocytic pathway, the two ligands were segregated within separate small vesicles and tubulovesicles. Au‐α2MGMA was concentrated in a small number of these structures whereas Au‐I was sparsely distributed among a relatively large number. Subsequently, the two endocytic pathways converged as the ligands intermingled within pale multivesicular bodies and lysosome‐like structures. Au‐I was less efficiently transferred to lysosomes than Au‐α2MGMA since a greater proportion of intracellular Au‐I remained associated with small vesicles and tubulovesicles. This study indicates that early intracellular events in the endocytic pathways of insulin and α2MGMA are distinct. These findings are discussed in light of the fundamentally dissimilar biological roles of these two molecules and the possible involvement of the endocytic pathway in the insulin signaling mechanism.