Elevated circulating glucose represents a hallmark of type 2 diabetes mellitus that leads to numerous diabetes complications, such as neuropathy, retinopathy, amputations, etc. (Kroemer, Lopez-Otin, Madeo, & de Cabo, 2018). In mammals, clearing of blood glucose is achieved by insulin-dependent translocation of the glucose transporter isoform 4, Glut4, to the plasma membrane of fat and skeletal muscle cells. Multiple lines of independent evidence including various transgenic models (Charron et al., 1999, Graham and Kahn, 2007, Minokoshi et al., 2003) and in vivo NMR studies (Shulman, 2000) have proved that Glut4-mediated glucose uptake represents a rate-controlling step of not just blood glucose levels, but of overall insulin-stimulated glucose disposal. If we find the way to deliver more Glut4 to the plasma membrane in response to insulin, we are almost certain to significantly improve the overall metabolic control and clinical manifestations of diabetes.

Rapid development of the Glut4 field in 1990s and 2000s clarified many but not all aspects of Glut4 regulation and traffic. In particular, we still do not understand how exactly insulin triggers translocation of Glut4. The best studied signaling pathway involves the insulin receptor, its substrates (IRSs), phosphatidylinositol-3 kinase (PI-3 kinase), the serine/threonine kinase, Akt, the Rab GTPase-activating protein TBC1D4 (a.k.a. AS160), and Rab10 (Fazakerley et al., 2022, Klip et al., 2019, Taniguchi et al., 2006). The mechanism of TBC1D4/Rab10 action is still not completely clear. These proteins may induce formation of membrane vesicles containing Glut4, their translocation to or fusion with the plasma membrane. Importantly, TBC1D4/Rab10-mediated signaling accounts for approximately 50% of the total effect of insulin on glucose uptake in adipocytes (Klip et al., 2019) leaving room for alternative or additional effectors.

Other PI 3 kinase-dependent and independent mechanisms of Glut4-mediated glucose uptake have been described (reviewed in Bogan, 2012). For example, insulin activates a PI 3 kinase-independent TC10α-PIST pathway that causes the proteolytic cleavage of the IRV anchor TUG by the protease USP25m (Bogan, 2012, Bogan et al., 2003, Habtemichael et al., 2018). In addition, inhibitors of RNA and protein synthesis, mimic the effect of insulin on Glut4 translocation and glucose uptake in adipocytes without engaging conventional signaling proteins, such as Akt, TBC1D4, or TUG (Meriin, Zaarur, Bogan, & Kandror, 2022). It is not fully understood whether various insulin signaling pathways converge on the same target and/or regulate different vesicle trafficking steps, so that the integration of multiple non-overlapping pathways of insulin signaling into one general model remains, at present, a challenge.

In the past, most research efforts directed towards understanding insulin resistance have focused on potential defects in the “canonical” insulin signaling pathway. In particular, Ser/Thr phosphorylation of IRS1 counter regulatory to insulin signaling has been observed in multiple experimental contexts (Boura-Halfon & Zick, 2009). However, these upstream IRS phosphorylation events do not necessarily correlate with decreased glucose uptake (Copps et al., 2010, Hoehn et al., 2008). More generally, attenuated insulin signaling may not account for reduced insulin-stimulated glucose uptake in insulin resistance and diabetes (Fazakerley et al., 2018, Klip et al., 2019). Moreover, other studies show that type 2 diabetic patients and their first-degree relatives can have impaired glucose transport despite normal Akt phosphorylation (Karlsson et al., 2006, Kim et al., 1999, Ramos et al., 2021).

An alternative hypothesis is that the inability of insulin to stimulate glucose uptake is caused by the aberrant Glut4 recycling pathway and may or may not be accompanied by impaired signaling. In this review, we will focus on the cell biology of the “Glut4 pathway” in adipocytes. Specifically, we will analyze two major problems: (i) the mechanism of the IRV biogenesis and (ii) the nature of their insulin responsiveness. We will also discuss a possibility that these data can help us to identify primary diabetes-related abnormalities and to search for new therapeutic targets.