Up-regulation Of Growth Hormone Releasing Hormone (GHRH) Receptors In The Ischemic Retina

Abstract

Purpose: Modulation of the growth hormone axis may have potential therapeutic applications for the treatment of ischemic retinopathies and retinal neovascularization. Growth hormone releasing hormone (GHRH) is a hypothalamic hormone involved in the regulation of growth hormone release from the pituitary gland. Recent evidence suggests that GHRH expression and activity has extra-pituitary effects. Here we have studied the localization and expression of GHRH receptors (GHRH-R) in normal and ischemic retina.

Methods: Western blotting and immunohystochemical analyses were employed to identify GHRH-R in retinas of streptozotocin-induced diabetic rats (STZ-rats) and control age-matched normoglycemic rats. Human post-mortem diabetic donor retinas and normoglycemic controls were also analyzed. Expression and immunolocalization of GHRH was also determined in mice subjected to oxygen induced retinopathy (OIR) and age-matched control mice. Retinal endothelial cells (RECs) were exposed to hypoxia (Hx, pO2=1%) for 6, 12 and 24 hours. The expression of GHRH-R was then assessed by Western blotting and compared to that measured in cells cultured in normoxic conditions (Nx, PO2=23%).

Results: In normal retinas, GHRH-R expression is very low and localized to ganglion cells. In the diabetic retinas of both rats and humans, GHRH-R are up-regulated, initially in ganglion cells and subsequently, more markedly, around blood vessels. We, therefore, measured GHRH-R expression in a model of ischemic retinopathy where retinal neovascularization takes place. Western blotting analysis showed up-regulated GHRH-R expression in the ischemic retina, at postnatal days 14 and 17, with immunohistochemistry revealing clear co-localization of GHRH-R with blood vessels. Exposure of RECs to hypoxia promoted the up-regulation of GHRH-R after 12 hours treatment, as measured by Western blotting.

Conclusions: Our studies reveal, for the first time, that GHRH-R are expressed in the retina and their expression levels are increased by ischemia/hypoxia particularly in blood vessels and in retinal endothelial cells. Our work suggests that modulation of GHRH signaling in retinal blood vessels may be of potential therapeutic value for the management of ischemic retinopathies and retinal neovascularization.

Keywords: 499 diabetic retinopathy • 543 growth factors/growth factor receptors • 572 ischemia