Role of Nitric Oxide Synthases and Nitrotyrosine in Retinoblastoma

Abstract

Abstract: : Purpose: There has been much interest recently in the role that nitric oxide (NO) plays in both tumour growth and dissemination. There are three distinct isoforms of Nitric Oxide Synthases (NOS) , encoded by three different genes. Two isoforms are constitutive and calcium/calmodulin dependent, the endothelial and neuronal types (eNOS and nNOS, respectively); the third is inducible (iNOS), and is not dependent upon calcium/calmodulin for its enzymatic action. The produced NO can react with free radicals such as superoxide anions to form peroxynitrate, a potent nitrating agent. Peroxynitrate can cause oxidation of DNA and membrane phospholipids in addition to nitration of free or protein–associated tyrosines producing Nitrotyrosine (NT). Thus, the occurrence of NT in tissues has been measured as a marker of peroxynitrite formation.Recent studies have investigated the expression and the activity of iNOS in various human cancers. There is not much information available about the expression of NOS in retinoblastoma. Therefore, we studied the expression of eNOS, iNOS and NT in a small series of retinoblastomas and also on a retinoblastoma cell line and correlated with invasion of the tumors. Methods: ENOS, iNOS and NT reactivity was evaluated by immunohistochemistry in 34 archival retinoblastoma specimens and from a human Y79 retinoblastoma cell line. The tumors were divided into 2 groups. Group A: (n=17) Tumors with no invasion and Group B: (n=17) Tumors with invasion of choroid/ optic nerve/orbit. Their expression was correlated with invasiveness of the tumors. Results: In–group A (n=17) tumors with no invasion, eNOS was positive in 100% (17/17), iNOS was positive in 82% (14/17) and NT in 100% (17/17) tumors. In–group B tumors (n=17) with invasion, eNOS was positive in 100% (17/17), iNOS was positive in 94% (16/17) and NT in 100%(17/17) tumours. The invasive cohort showed a significantly higher expression of iNOS (P<0.0001) and NT (P<0.020) than the non–invasive tumors. The Y79 cells were also seen to express eNOS, iNOS and NT. Non–neoplastic retina was positive for eNOS, iNOS and NT. Conclusions: Taken together, our results suggest that retinoblastomas can produce NO. The role of NO in the biology, and prognosis of retinoblastoma remains to be established.

Keywords: retinoblastoma • tumors • oncology