Sortilin Is Essential and Sufficient for the Formation of Glut4 Storage Vesicles in 3T3-L1 Adipocytes

Abstract

Impaired translocation of the glucose transporter isoform 4 (Glut4) to the plasma membrane in fat and skeletal muscle cells may represent a primary defect in the development of type 2 diabetes mellitus. Glut4 is localized in specialized storage vesicles (GSVs), the biological nature and biogenesis of which are not known. Here, we report that GSVs are formed in differentiating 3T3-L1 adipocytes upon induction of sortilin on day 2 of differentiation. Forced expression of Glut4 prior to induction of sortilin leads to rapid degradation of the transporter, whereas overexpression of sortilin increases formation of GSVs and stimulates insulin-regulated glucose uptake. Knockdown of sortilin decreases both formation of GSVs and insulin-regulated glucose uptake. Finally, we have reconstituted functional GSVs in undifferentiated cells by double transfection of Glut4 and sortilin. Thus, sortilin is not only essential, but also sufficient for biogenesis of GSVs and acquisition of insulin responsiveness in adipose cells.

Protein sorting gone wrong – VPS10P domain receptors in cardiovascular and metabolic diseases

“…Following insulin stimulation, Glut4 moves from intracellular storage vesicles to the cell surface to facilitate uptake of glucose in muscle and adipose tissues. In line with its role as an intracellular sorting factor, sortilin was subsequently shown to interact with the Glut4 protein and to trigger its inclusion in storage vesicles 50,51 . In adipocytes, this sortilin activity proved essential to confer insulin responsiveness of Glut4 storage vesicles 52 .…”

Ins (endocytosis) and outs (exocytosis) of GLUT4 trafficking

“…In particular, sortilin has recently been shown to play an important role in the formation of the specialized GLUT4 IRC in adipocytes [15]. During 3T3L1 differentiation from the fibroblast to adipocyte state, the IRC is formed concomitant with the induction of sortilin expression prior to the expression of GLUT4 [15].…”

“…In particular, sortilin has recently been shown to play an important role in the formation of the specialized GLUT4 IRC in adipocytes [15]. During 3T3L1 differentiation from the fibroblast to adipocyte state, the IRC is formed concomitant with the induction of sortilin expression prior to the expression of GLUT4 [15]. Moreover, when GLUT4 is over-expressed prior to the induction of sortilin, the GLUT4 protein undergoes rapid degradation, whereas over expression of sortilin stabilizes the GLUT4 protein, increases the formation of the IRC and promotes insulin-stimulated glucose uptake [15].…”

“…During 3T3L1 differentiation from the fibroblast to adipocyte state, the IRC is formed concomitant with the induction of sortilin expression prior to the expression of GLUT4 [15]. Moreover, when GLUT4 is over-expressed prior to the induction of sortilin, the GLUT4 protein undergoes rapid degradation, whereas over expression of sortilin stabilizes the GLUT4 protein, increases the formation of the IRC and promotes insulin-stimulated glucose uptake [15]. Conversely siRNA-mediated sortilin knockdown decreases both the formation of the IRC and insulin-regulated glucose uptake.…”

Glucose deprivation attenuates sortilin levels in skeletal muscle cells

“…Gastrocnemius, soleus, and EDL skeletal muscles from each hindleg were then removed, washed in PBS(-), frozen in liquid as well as several Rab proteins [5]. Sortilin is proposed to be essential and sufficient for the formation of GSVs [6,7]. Under basal conditions, GSVs reside in the intracellular regions; and some stimuli such as insulin induce its translocation and fusion to the cell surface to initiate glucose uptake; however, the reduction of sortilin deprives this characteristic of GSVs and decreases insulin-dependent glucose uptake [6].…”