Diabetic retinopathy is characterized by early onset of neuronal cell death. We previously showed that insulin mediates a prosurvival pathway in retinal neurons and that normal retina expresses a highly active basal insulin receptor/Akt signaling pathway that is stable throughout feeding and fasting. Using the streptozotocin-induced diabetic rat model, we tested the hypothesis that diabetes diminishes basal retinal insulin receptor signaling concomitantly with increased diabetes-induced retinal apoptosis. The expression, phosphorylation status, and/or kinase activity of the insulin receptor and downstream signaling proteins were investigated in retinas of age-matched control, diabetic, and insulin-treated diabetic rats. Four weeks of diabetes reduced basal insulin receptor kinase, insulin receptor substrate (IRS)-1/2-associated phosphatidylinositol 3-kinase, and Akt kinase activity without altering insulin receptor or IRS-1/2 expression or tyrosine phosphorylation. After 12 weeks of diabetes, constitutive insulin receptor autophosphorylation and IRS-2 expression were reduced, without changes in p42/p44 mitogen-activated protein kinase or IRS-1. Sustained systemic insulin treatment of diabetic rats prevented loss of insulin receptor and Akt kinase activity, and acute intravitreal insulin administration restored insulin receptor kinase activity. Insulin treatment restored insulin receptor-␤ autophosphorylation in rat retinas maintained ex vivo, demonstrating functional receptors and suggesting loss of ligand as a cause for reduced retinal insulin receptor/Akt pathway activity. These results demonstrate that diabetes progressively impairs the constitutive retinal insulin receptor signaling pathway through Akt and suggests that loss of this survival pathway may contribute to the initial stages of diabetic retinopathy.

Cultured alveolar epithelial cells exhibit gap junction intercellular communication (GJIC) and express regulated levels of connexin (Cx) 43 mRNA and protein. Newly synthesized radiolabeled Cx43 protein equilibrates with phosphorylated Cx43 isoforms; these species assemble to form both connexons and functional gap junction plaques. The saponin 18α-glycyrrhetinic acid (GA) rapidly and reversibly blocks GJIC at low concentrations (5 μM). Extended exposure to 18α-GA at higher concentrations causes inhibition of GJIC and time- and dose-dependent reductions in both Cx43 protein and mRNA expression. The latter toxic effects are paralleled by disassembly of gap junction plaques and are reversed less readily than acute effects on GJIC. These observations demonstrate 18α-GA-sensitive regulation of intercellular communication in epithelial cells from the mammalian lung and suggest a role for Cx43 expression and phosphorylation in acute and chronic regulation of GJIC between alveolar epithelial cells.

Rapid hyperplastic growth of the remaining lung is initiated by partial pneumonectomy in many mammalian species. The response restores normal tissue structure and function. Although physiological control of compensatory lung growth is documented, little is known about the molecular mechanisms that underlie the process. The aim of this study was to investigate the role of mechanical signals in the induction of immediate-early gene (IEG) expression after pneumonectomy. Expression of c-fos and junB increased nine- and fourfold, respectively, in the right lung within 30 min after left pneumonectomy in rats. In contrast, changes in expression of c-jun and c-myc were not observed. When isolated lungs were subjected to elevated airway pressures in vitro, expression of c-fos and junB was induced in a time- and dose-dependent manner similar to that observed in vivo. Similarly, in vitro lung perfusion induced c-fos and junB expression in the absence of increasing lung inflation. These results support the premise that rapid changes in IEG expression after pneumonectomy are initiated by mechanical signaling in the remaining lung. Elevated IEG expression may contribute to initiation of compensatory lung growth.