Characterization of insulin signaling in rat retina in vivo and ex vivo

Abstract

Insulin receptor (IR) signaling cascades have been studied in many tissues, but retinal insulin action has received little attention. Retinal IR signaling and activity were investigated in vivo in rats that were freely fed, fasted, or injected with insulin by phosphotyrosine immunoblotting and by measuring kinase activity. A retina explant system was utilized to investigate the IR signaling cascade, and immunohistochemistry was used to determine which retinal cell layers respond to insulin. Basal IR activity in the retina was equivalent to that in brain and significantly greater than that of liver, and it remained constant between freely fed and fasted rats. Furthermore, IR signaling increased in the retina after portal vein administration of supraphysiological doses of insulin. Ex vivo retinas responded to 10 nM insulin with IR β-subunit (IRβ) and IR substrate-2 (IRS-2) tyrosine phosphorylation and AktSer473 phosphorylation. The retina expresses mRNA for all three Akt isoforms as determined by in situ hybridization, and insulin specifically increases Akt-1 kinase activity. Phospho-AktSer473 immunoreactivity increases in retinal nuclear cell layers with insulin treatment. These results demonstrate that the retinal IR signaling cascade to Akt-1 possesses constitutive activity, and that exogenous insulin further stimulates this prosurvival pathway. These findings may have implications in understanding normal and dysfunctional retinal physiology.

Suppressors of Cytokine-Signaling Proteins Induce Insulin Resistance in the Retina and Promote Survival of Retinal Cells

“…3C). In concert with high metabolic requirements of retina (6,31), insulin signaling is constitutively active in retina, as indicated by detection of pAKT in freshly isolated primary retinal cells (Fig. 3B).…”

Beta-2-Adrenergic Receptor Regulates Insulin Signaling to Reduce Cell Death in Müller Cells

“…37 The first 24 amino acids are the pre form, which guides the peptide for cleavage in the endoplasmic reticulum. 38 Following cleavage, the proinsulin portion is left and form the A and B chains, both of which are stored in the beta cells of the pancreas.…”

“…37,39 The actions of insulin are initiated when the receptor becomes bound on the alpha subunits and this binding of receptor causes phosphorylation of tyrosine kinases on the beta subunits inducing a conformational change of the receptor. 37,39 The beta subunits of the insulin receptor become phosphorylated due to the binding of insulin, initiating a conformational change. 37,39 Following tyrosine phosphorylation of the beta-subunit of the insulin receptor, one of the 30 or more tyrosine or serine/threonine residues on the insulin receptor substrate (IRS) complex becomes phosphorylated.…”

“…37,39 The beta subunits of the insulin receptor become phosphorylated due to the binding of insulin, initiating a conformational change. 37,39 Following tyrosine phosphorylation of the beta-subunit of the insulin receptor, one of the 30 or more tyrosine or serine/threonine residues on the insulin receptor substrate (IRS) complex becomes phosphorylated. 37,39 Following insulin stimulation, phosphorylation of Akt on Ser 473 will occur downstream of the IRS complexes to inhibit apoptosis.…”

“…37,39 Following tyrosine phosphorylation of the beta-subunit of the insulin receptor, one of the 30 or more tyrosine or serine/threonine residues on the insulin receptor substrate (IRS) complex becomes phosphorylated. 37,39 Following insulin stimulation, phosphorylation of Akt on Ser 473 will occur downstream of the IRS complexes to inhibit apoptosis. Das et al (1984) suggested the possibility of insulin being produced in other tissues beyond the beta cells of the pancreas.…”