Hyperexpression Of Low-Density Lipoprotein Receptors And Hydroxy-Methylglutaryl Coenzyme A-Reductase In Human Pterygium

PURPOSE:pterygium is one of the most common diseases in ophthalmology.It is nonmalignant,slowgrowing proliferations of conjunctival connective tissue in proximity to the limbus,generally arising in response to UVB radiation.Pterygium results from the invasion of the comea by hyperproliferating limbal epithelial cells, together with their supporting blood vessels,and may therefore stretch and distort the cornea and induce astigmatism and low vision.Because the morbidity rate and postoperative recurrence rate of pterygium is high,many scholars pay attention to pterygium.Due to the lack of safe and effective medicine,the management of patients with pterygium has always been a tough job in ophthalmology.There is now increasing evidence that pterygium are tumorlike tissues ranging from mild dysplasia to carcinoma in situ and local invasiveness.Controversial data have been accumulated on the role of p53 oncogene,Dushku and Reid reported high expression of p53 in the epithelium overlying the pterygium and speculated on the existence of a p53 gene mutation.Oncogenic human papillomaviruses(HPVs),particularly types 16 and 18, which have been found in pterygia and limbal tumors.Recently,heparin-binding epidermal growth factor(HB-EGF),a potent mitogen,has been localized in pterygium tissue and has been demonstrated to be significantly induced by UVB in pterygium -derived epithelial cells.Pterygial fibroblasts exhibit characteristics of microsatellite instability and loss of heterozygosity and high levels of mitotic activity.All reported to be common findings in neoplastic tissues,supporting the hypothesis that pterygium may be a benign neoplastic lesion.In neoplastic tissue,modifications in cholesterol synthesis and metabolism are common findings.There is now increasing evidence that pterygium is tumorlike tissues and that cell growth and DNA replication are closely linked to cholesterol metabolism. It has been suggested that an increased demand for free cholesterol derived from both exogenous and endogenous sources is part of the cellular response to a mitogenic stimulus,and it is satisfied by an increased amount of HMG-CoA-reductase activity and expression in LDL receptors per cell.Moreover,this information is related to recent studies indicating that photodynamic therapy(PDT) with verteporfin may be useful for several neovascular conditions involving the anterior segment of the eye, such as corneal neovascularization and pterygium.Because cellular uptake of verteporfin by means of LDL-R is necessary for the achievement of the cytotoxic response to PDT,it would be interesting to know the level of expression of LDL receptors in pterygia.In this study,the expression of two main genes correlated to cholesterolmetabolism namely,the low-density lipoprotein receptor(LDL-R) gene and the hydroxyl -methylglutaryl-coenzyme A reductase(HMG-CoA-R) gene were investigated in primary pterygium,recurrent pterygium,and normal conjunctiva.METHODS:Primary pterygium,recurrent pterygium and normal conjunctiva samples were obtained from 30 eyes at the time of surgery during february of 2007 to february of 2008.Ten patients with primary pterygium and 10 patients with recurrent pterygium were included in the study.All patients had blood cholesterol levels within normal limits.Diagnosis of pterygium was based on clinical history and evaluation of signs and symptoms.Patients with primary pterygium had at least a 3-year history of a slow growing lesion,with a corneal extension of at least 4 mm. Patients with recurrent pterygium had accepted operation within 2 year.Pterygium morphology was clinically graded according to the system of Tan et al,based on the assessment of pterygium translucency:atrophic(T1),intermediate(T2),or fleshy(T3) pterygium.All pterygia collected in this study were T3.All patients with recurrent and primary pterygium had never used medicine.The control group was undergoing surgery for retinal detachment.Total RNA was extracted from the specimens by the guanidine isothiocyanate phenol-chloroform extraction method,Equal amounts of total RNA were reverse transcribed into cDNA,The cDNA was subsequently amplified by the Real Time Fluorescence Quantitative PCR in the presence of specific primers for low-density lipoprotein receptor(LDL-R) and forhydroxy methyl- glutaryl coenzyme A reductase(HMG-CoA-R).RESULTS:1.The RNA of pterygium was integrative enough to be used to further QRT-PCR analysis.2.QRT-PCR revealeda statistically significant increase of LDL-R mRNA levels:4.1 fold in primary pterygia and 2.7 fold in recurrent pterygium, compared with the control normal conjunctiva.The differences were statistically significant(P＜0.05).3.Also the mRNA levels of HMG-CoA-R were increased significantly in both primary and recurrent pterygia by 4.2 fold and 3.6 fold respectively,compared with normal conjunctivae,Differences were statistically significant(P＜0.05).CONCLUSIONS:1.QRT-PCR could specifically and accurately detect gene expression level in pterygium.The method is practical and feasible.2.This study indicates that pterygium have an altered metabolism of cholesterol-namely increased LDL-R and HMG-CoA-R mRNAs-as is characteristic of tumorlike tissues,and that the high expression of LDL receptors renders them enable to be treated by photodynamic therapy with intravenously injected verteporfin.