Role of Sigma Receptors in the Human Lens

Abstract

Abstract: : Purpose: Sigma receptors have no known homology with other membrane receptors, but appear to be located in the endoplasmic reticulum (ER). They are upregulated in tumours and fast growing cells. The human lens grows throughout life and uncontrolled growth after cataract surgery is responsible for Posterior Capsule Opacification (PCO). We have therefore investigated the expression of Sigma receptors in the Human Lens and the possible use of Sigma antagonists for controlling PCO. After initiating the study it became obvious that the lens cells were pigmenting and hence this was also investigated. Methods: Human capsular bags were prepared from donor eyes. They were maintained in serum–free Eagle’s minimum essential medium (EMEM) or EMEM supplemented with the Sigma receptor antagonists Rimcazole or BD1047 at 35°C in 5% CO₂. Cell growth and pigmentation were assessed by digital imaging and electron microscope (EM) techniques. Expression of the Sigma–1 receptor was assayed by RT–PCR. Western immunoblot techniques were employed to measure levels of tyrosinase (TYR) and tyrosinase–related proteins (TYRP1 and 2). The incorporation of 14–C tyrosine directly into lens proteins was also determined.Results: The Sigma–1 receptor was expressed in human lens epithelial and fibre cells. Compared to serum–free controls, cell growth on the capsule was inhibited by rimcazole (3 µM) and by the specific Sigma–1 antagonist BD1047 (3 µM). Surprisingly, both initiated pigmentation after 4–6 days exposure and typical black pigment granules (identified in cross section in the EM) were produced within cells on the anterior and posterior capsules. The following elements of the melanin synthesis pathway: TYR, and TYRP1&2 were all expressed in control cultures. 3 µM rimcazole stimulated incorporation of 14–C tyrosine into cellular proteins and increased production of TYR and TYRP1, while TYRP2 remained at control level.Conclusions:The normally transparent lens surprisingly contains all of the mechanisms required to produce pigment granules and it appears that Sigma receptors play a key role in modulating the activity of TYR and TYRP1. The ER–based receptors also play an important role in cell growth and survival. As some of the components of the melanin pathway have antioxidant properties, we suggest they may have a role in lens protection. We also propose that over stimulation of the pathway leads to pigment granule production (as in lens Cataracta Nigra).

Keywords: receptors • growth factors/growth factor receptors • posterior capsular opacification (PCO)