Differential mitogenic and metabolic signaling of insulin receptor variants IR-A and IR-B

JIMENA GIUDICE; FEDERICO COLUCCIO LESKOW; DONNA J. ARNDT-JOVIN; THOMAS M. JOVIN; ELIZABETH JARES-ERIJMAN

Ventura Beach, USA

Congreso; Gordon Research Conference. Insulin-Like Growth Factors in Physiology & Disease; 2011

Gordon Research Conference

Insulin signaling is involved in glucose metabolism and cellular growth. Impaired response to insulin is the hallmark of diabetes while upregulated insulin activity occurs in many cancer types. Two splice variants of the insulin receptor (IR) exist in mammals: IR-A lacking exon 11, involved in mitogenic signaling, and the full length IR-B responsible for the metabolic response. Although all cell types express both IR isoforms to various degrees, IR-A predominates in fetal tissues and cancer cells, and IR-B in adult differentiated cells. In embryos, IR-A promotes growth due to its ability to bind insulin and insulin like growth factor II (IGF-II); in adults, IR-B is expressed predominantly in insulin-sensitive tissues regulating glucose homeostasis. IR-A has a higher affinity for insulin than IR-B. Furthermore IGF-II binds to IR-A, but not to IR-B, with an affinity close to that of insulin, whereas the affinity for IGF-I for both receptors is similar. When IR and IGF-IR are co-expressed the pro-receptors can generate IR/IGF-IR hybrids. These hybrid receptors promote insulin resistance in type II diabetes by decreasing the number of insulin binding sites. Although considerable biochemical data exist on insulin and IGF-II binding and signaling, little is known about the differential spatio temporal dynamics of IR isoforms and its possible effects on signaling when different ligands bind to them. To gain insight into the dynamics of the activation, internalization and signaling of IR isoforms by microscopy we combined 2 techniques: streptavidin-quantum dots conjugated with biotinylated ligand; and visible fluorescent proteins. Using confocal microscopy, and programmable array microscope (PAM), we visualized endocytosis of both isoforms of IR after ligand binding in living and fixed cells. By a cell-by-cell study we demonstrated a higher rate of endocytosis of IR-A compared to IR-B induced by insulin and this result correlates with interesting differences between the signaling triggered by the two IR: the isoform that internalizes more readily (IR-A), is activated in a higher and more persistent manner (measured as receptor autophosphorylation) by insulin, inducing the ERK 1/2 mitogenic pathway. In contrast, IR-B signals from the membrane through Akt to regulate glucose metabolism.