Sortilin Affects Glut4 Levels with Little Effect on Trafficking Kinetics

Abstract

Glucose Transporter Isoform 4 (Glut4) responds to insulin stimulation and takes up glucose into the cells, which, in turn, decreases blood sugar levels. The relevance of Glut4 trafficking pertains to Type II Diabetes, which is becoming an epidemic in the U.S. In Type II Diabetes Glut4 does not respond to insulin. In order to identify where the malfunction occurs, a scheme of the normal process must be developed. Many proteins interact with Glut4 as it cycles from endosomes to the plasma membrane or from endosomes to Glut4 storage vesicles (GSVs) to the plasma membrane. One of those proteins has been identified as sortilin. Sortilin assembles glucose storage vesicles (GSVs) in the cell. However, its effects on the kinetics of Glut4 trafficking have not been measured. Small hairpin RNA (shRNA) was used to knock down expression of sortilin in adipocytes, and sortilin cDNA was used to express the protein in fibroblasts. Then flow cytometry was used to record endocytic and exocytic rate constants (ken and kex) of Glut4, as well as the cycling pool size (ymax). It is hypothesized that sortilin participates with the slow cycling pool which interacts with the GSVs. However, results showed that sortilin is acting in a separate pathway, without significant effects on the Glut4 trafficking kinetics, but overall effects on total Glut4 expression.