Insulin signalling and GLUT4 trafficking in insulin resistance

Abstract

Insulin-stimulated glucose uptake into muscle and adipose tissue is vital for maintaining whole-body glucose homeostasis. Insulin promotes glucose uptake into these tissues by triggering a protein phosphorylation signalling cascade, which converges on multiple trafficking processes to deliver the glucose transporter GLUT4 to the cell surface. Impaired insulin-stimulated GLUT4 translocation in these tissues underlies insulin resistance, which is a major risk factor for type 2 diabetes and other metabolic diseases. Despite this, the precise changes in insulin signalling and GLUT4 trafficking underpinning insulin resistance remain unclear. In this review, we highlight insights from recent unbiased phosphoproteomics studies, which have enabled a comprehensive examination of insulin signalling and have transformed our perspective on how signalling changes may contribute to insulin resistance. We also discuss how GLUT4 trafficking is disrupted in insulin resistance, and underline sites where signalling changes could lead to these trafficking defects. Lastly, we address several major challenges currently faced by researchers in the field. As signalling and trafficking alterations can be examined at increasingly high resolution, integrative approaches examining the two in combination will provide immense opportunities for elucidating how they conspire to cause insulin resistance.

Keywords: GLUT4; glucose transport; insulin resistance; insulin signalling; phosphoproteomics; trafficking.

Figure 1.. Overview of insulin signalling.

Insulin initiates a protein phosphorylation cascade to regulate cellular metabolism (reviewed in [125] and [126]), with the kinase Akt as a central node in this cascade. The events leading to Akt activation — ‘proximal insulin signalling' — begin with insulin binding to its receptor, activating its receptor tyrosine kinase activity and triggering its auto-phosphorylation. Phosphorylated tyrosine residues bind the insulin receptor substrate proteins (IRS1 and IRS2), which are also phosphorylated, recruiting the lipid kinase PI3K to generate PIP3 at the plasma membrane. PIP3 then recruits additional kinases, including Akt, PDK1, and mTORC2, through PIP3-binding domains. PDK1 and mTORC2 sequentially phosphorylate Akt at T308 and S473, leading to full activation. In ‘distal insulin signalling', Akt phosphorylates multiple proteins to enact complex cellular changes. A primary function of insulin in skeletal muscle and adipose tissue is promoting the translocation of GLUT4-storage vesicles (GSVs) to the plasma membrane (adipose) and sarcolemma and transerve tubule membrane (muscle), with the best characterised regulator being the Akt substrate TBC1D4. Other Akt substrates include the kinases GSK3α and GSK3β, which promote glycogen synthesis, and PRAS40 and TSC2, which promote protein synthesis by relinquishing their inhibition of the kinase mTORC1. Insulin also activates Akt-independent signalling axes such as the RAS-ERK pathway [22], though Akt signalling is generally considered the major mediator of insulin's acute metabolic actions.

Figure 2.. Defective and emergent signalling in insulin resistance.

Phosphoproteomics has revealed that insulin resistance can be accompanied by both impaired insulin signalling responses (‘defective signalling') and the emergence of enhanced or novel insulin signalling responses (‘emergent signalling'). Both of these processes could contribute to insufficient insulin-stimulated GLUT4 translocation in insulin resistance.

Figure 3.. GLUT4 trafficking pathway.

There are multiple steps in the GLUT4 trafficking pathway that could be impaired in insulin resistance: (1) Newly synthesised or internalised GLUT4 could be mis-sorted away from GSVs [4,5,48]; (2) Translocation of GSVs to the periphery of the cell in response to insulin could be reduced, such that there is less GLUT4 available for insertion into the membrane [57]; (3) Tethering, docking and fusion of GSVs may be reduced such that less GLUT4 is inserted into the membrane [58]; (4) Dispersal and density of GLUT4 in the membrane may be reduced [60]. Either one or a combination of these defects could result in reduced plasma membrane GLUT4 in response to insulin.