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

Abstract

A previous ultrastructural study showed that gold-labeled insulin (Au-I) and the non-hormonal ligand gold-labeled alpha-2-macroglobulin-methylamine (Au-alpha 2MGMA) 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-alpha 2MGMA was initially found within apparent coated vesicles but Au-I was not, consistent with the previous demonstration that Au-alpha 2MGMA 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-alpha 2MGMA 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-alpha 2MGMA 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 alpha 2MGMA 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.

An Update on Non-clathrin-coated Endocytosis

“…14 Studies on insulin entering cells via smooth nonclathrin-coated vesicles, found ligand subsequently co-localised with a marker entering cells via clathrincoated pits. 15 Further studies likewise detected markers, initially entering from coated and non-coated regions, subsequently in common intracellular vesicles. 8,14,16 However, in some situations the intracellular destination of ligand internalised via non-coated regions of the PM differed from that of ligand internalised via coated pits.…”

Role of Caveolae and Caveolins in Health and Disease

“…Initial interest in caveolae as sites for insulin signaling came from ultrastructural studies of rat adipocytes, which showed that gold-labeled insulin ligand clustered within plasmalemmal caveolae (78,79). Further studies showed that caveolae are highly enriched in adipocytes and that both caveolin-1 mRNA and protein levels increase over 25-fold during the differentiation of mouse 3T3-L1 fibroblasts into adipocytes (an insulin-dependent phenomenon), also suggestive of a role for these microdomains in insulin signaling (215).…”

Induction of caveolin during adipogenesis and association of GLUT4 with caveolin-rich vesicles.

“…Jarett and co-workers (22) showed that in adipocytes the in-sulin receptor is predominantly localized to plasmalemmal micropinocytotic invaginations (caveolae) that occupied a significant percentage (~13%) of the total plasma membrane. In contrast, coated pits (a more conventional route for endocytosis of receptors) occupied about 0.4 % of the plasma membrane, only about one fifth of the density seen for coated pits in fibroblasts.…”

“…Some of these proteins are receptors for ligands (such as folate) that seem to be internalized through caveolae, the process termed potocytosis (1). Other roles of caveolae may include transmembrane signaling, since they contain a number of cellsurface receptor and signal transducing proteins, as judged both by morphological (9,22,36,38,49) and biochemical criteria (9,29,30,42).…”