Structural Biochemistry/Diabetes/Regulation of glucose

Glucose is a primary subtract for energy production and essential in metabolism. Because energy is necessary to perform body functions, it is important to maintain a constant blood concentration of glucose. The concentration of glucose increases during the intake of food. To cope with unpredictable food availability, mammals adopted a highly regulated system to maintain the concentration of blood glucose at approximately 4-5mM. The presence of glucose stimulates the release of insulin from pancreatic beta-cell, which suppresses hepatic glucose production and promotes glucose uptake by muscle and adipose tissue, or commonly known as fat. The excess glucose is stored as glycogen in liver and skeletal muscles and as triglyceride in adipose. Data shows the presence of insulin enhances the uptake of glucose by moving GLUT4, a transport protein, from intracellular to the surface of the membrane. The function of insulin is agreed upon, but the vesicle trafficking pathway and insulin signaling interaction still remain controversial.

Glucose transporter and glucose uptake
The transport of sugar contains two steps. First, glucose must bind to a transporter at the surface of the cell. Then, a conformational change must occur to move the sugar across the cell membrane and release it.

The mammalian facilitative glucose transport family (GLUT) contains 14 proteins and their function is to transport sugar and maintain the glucose blood concentration. These protein contains 12 transmembrane domains, cytosolic amino and carboxyl termini. GLUT 4 is most common found in muscle and adipose cell. Most data support the view GLUT4 accumulates selectively in intracellular vesicles during the absence of insulin. When insulin becomes available, the GLUT4 storage vesicles (GSV) move into the plasma membrane rapidly and the uptake of glucose increases. Such regulation is cell type specific and the targeting of glucose transporter to these vesicles is GLUT isoform specific. Data shows thatonly about 1% of GLUT4 is presence at the surface of the adipose when without the sign of insulin. When the insulin is added to the environment, the presence of GLUT at the surface of the cell increases to around 40%.

Because GLUT4 is cell type specific, researchers have created fat-like (3T3-L1) and muscles-like (L6) cultured cell. These cultured cells allow researchers to manipulate the genetic expression and create a more physiological setting. However, the cultured cell appeared to be poor models for studying GLUT4. Glucose uptake is stimulated by 20-30 fold in a normal adipose cell but only 4 to 8 fold in the cultured cell. Although the result still indicates that the presence of GLUT4 in plasma membrane increase with the stimulation of insulin, the insulin response appear to be more blunt. Such result is most likely due to the reduce capacity to sequester GLUT4 in the basal. Abnormal regulation of GLUT 4 in adipose cells can contribute to insulin resistance by modulating the secretion of adipose derived hormones. The impairment of insulin is often observed in type 2 diabetes patients. Such result is shown by NMR spectroscopy.

Insulin signaling
=AS160=

GLUT4 trafficking is control by multiple insulin signaling pathway. AS160, which contain Ran GYPase activating protein domain (GAP), is an Akt substrate that was found to play an important role in signaling and trafficking insulin pathway. Insulin causes AS 160 phosophorylation at multiple sites and inactivates the GAP activity. Since Rab protein is involve in directing GLUT4 vesicle trafficking, the inactivation of GAP activity result in the impairment of translocation. in addition, to understand which component in AS 160 results in the inactivation of GLUT4 translocation, researchers compare the fraction of GLUT4 in the cell membrane when AS160 is presence, absence and GAP domain is mutated. During the complete absence of AS 160, the fraction of GLUT4 increases and the fraction of GLUT4 decreases when AS 160 is presence. However, when AS160 with mutated GAP domain is presence, the fraction of GLUT4 still remain low. Such result indicates that GAP domain is most likely responsible for insulin signaling. =Tbc1D1= Tbc1D1 is most abundant in muscle cell. Such protein is also regulated by insulin. However, insulin is not the only factor influences Tbc1D1. In fact, muscle contraction stimulates GLUT4 translocation, as the result, AMP activated protein kinase to phosphorylate Tbc1D1. It is also not clear whether AS160 serves as another factor that inhibits Tbc1D1. An AMPK-Tbc 1-D-1 is crucial in human. The TBC1D1 gene also shows so correlation with obesity in mice and humans.

Although patients with type 2 diabetes show sign of phosphorylation of AS160 and Tbc1D1, it is unclear how such result correlate to type 2 diabetes since drug treatments do not seem to improve the sensitivity to insulin. In addition, exercising is shown to improve insulin sensitive in some studies. It is also possible the increase GLUT4 was due to the enhance blood flow rather than the effects of insulin. Furthermore, the defect in GLUT4 targeting is also likely to contribute to insulin resistance. In other word, the effect of these physiological mechanisms are still poorly understand today.

Why is it important?
Understanding the pathway of GLUT4 and insulin signaling are important for understanding type 2 diabetes. The combination of overnutrition and genetic factors often lead to insulin resistance, which often results of type 2 diabetes. Insulin resistance causes failure to suppress hepatic glucose production and to uptake glucose into muscles. The failure to uptake glucose into muscles is due to dyregulated GLUT 4 trafficking. Insulin insufficient and lack of insulin secretion often lead to hyperglycemia. In addition, insulin insufficient is often correlated hypertension, dyslipidemia and other metabolism syndrome.

reference
Bogan JS. Annu Rev Biochem. 2012;81:507-32. Epub 2012 Apr 5. Review. PMID: 22482906 [PubMed - indexed for MEDLINE]