The functional role and therapeutic potential of somatostatin receptor subtype expression in human retinal epithelium

Abstract

Abstract: : Purpose:Human retinal epithelium (RPE) has been shown to express somatostatin receptors (sst1 and sst2). Analogues of somatostatin (SS) are currently being evaluated for the management of certain ocular diseases such as cystoid macular edema (CME), proliferative diabetic retinopathy (PDR) and exudative age–related macular degeneration (ARMD), where dysfunction of the RPE is thought to be important in disease pathogenesis. Our studies are aimed at further understanding the responses of RPE to SS and its analogues, and to evaluate the therapeutic potential of sst over–expression. Methods: RPE, extracted from cadaver eyes of two similar–aged donors, were cultured in vitro and each incubated in the presence or absence of the sst2–preferring analogue, octreotide. RNA, from treated or untreated cells, was pooled to reduce donor–specific effects and subsequently hybridized to Affymetrix U133A chips. The data was analysed by the Affymetrix microarray suite (5.0). To further study sst subtype–specific responses and to evaluate the therapeutic potential of sst subtype expression, RPE cells over–expressing human sst2 have been developed using a retroviral expression vector. Clonal lines have been produced by cell sorting and selected by ¹²⁵I–SS–14 binding studies to determine sst2 expression. Results:Thirty–six genes were found to be differentially responsive to octreotide treatment (> 2–fold change). The majority of genes were down–regulated; twelve genes were down–regulated by more than 5–fold (p< 0.003). These genes are implicated in various biological processes, including cell proliferation, cell differentiation, channel activity and phosphorylation status. The findings are being investigated by Q–PCR. In addition, we have developed a series of sst2 over–expressing cell lines and are studying the clones in assays of cell proliferation and production of angiogenic–promoting factors implicated in the pathogenesis of PDR and exudative ARMD. Future experiments will turn to the study of differentiated monolayers of the RPE clones. Conclusions:The exact modes of action of SS and its analogues are not yet fully understood, and in different disease environments their primary therapeutic effect may vary. Understanding the responses of RPE to SS and its analogues may help to define further targets and provide a more rational approach to treatment.

Keywords: retinal pigment epithelium • gene microarray • gene/expression