Plasminogen activator inhibitor-1 polymorphism is associated with increased risk for oral cancer

Summary

In light of the recently observed contribution of thrombosis-related factors to carcinogenesis, we investigated the possible association of plasminogen activator inhibitor-1 (PAI-1) with increased risk for oral cancer. In DNA samples of 104 patients with oral squamous cell carcinoma and 106 healthy controls of comparable ethnicity, age and sex, we studied the 4G/5G polymorphism in the PAI-1 gene, which affects its expression. The mutant 4G allele and carrier frequencies were significantly increased in patients compared to controls (65.9% versus 49.5%; 88.5% versus 69.8% respectively, P < 0.01). That increase was even higher in patients with a positive family history for thrombophilia or without one for cancer (P < 0.001). Interestingly, significant difference from controls was observed only in patients with cancer stages I and II. These findings suggest that the 4G allele, by resulting in higher PAI-1 expression, is a major contributing factor in early stages of oral oncogenesis. Possibly, increased PAI-1 promotes initial development of oral cancer through regulation of cell detachment and delays further tumor progression by inhibiting vascularization.

Introduction

Multiple genetic events are involved in carcinogenesis in the oral region, including activation of oncogenes and inactivation of tumor suppressor genes.1, 2 Recently, factors related to angiogenesis and thrombosis have been also implicated in the predisposition for development and advancement of oral squamous cell carcinoma.1, 3, 4, 5 A factor related to both thrombophilia and other types of cancer is plasminogen activator inhibitor-1 (PAI-1).6, 7, 8, 9, 10, 11, 12, 13

PAI-1 controls the activation of plasmin by regulating the activities of tissue plasminogen activators.⁹ Tissue plasminogen activator (t-PA) and urokinase plasminogen activator (u-PA) convert the inactive proenzyme plasminogen to its active form plasmin.⁹ Plasmin plays an important role in fibrinolysis as well as in the zymogen activation of matrix metalloproteinases (MMPs).9, 14 The MMPs constitute a large family of proteases, which have pivotal roles in development and tissue remodeling by degrading the major components of the extracellular proteins, including type I collagen.1, 9, 14 In oral cancer, MMP-2 and MMP-9 are produced by tumor cells at variable degrees.1, 15

In addition to its role in extracellular matrix proteolysis, PAI-1 is also involved in cellular adhesion and migration through its binding to vitronectin or integrins.⁹ Integrins are cation-dependent cell adhesion molecules, which compete with PAI-1 for binding to the extracellular matrix glycoprotein vitronectin.⁹ By mediating cell–cell and cell–matrix interactions, integrins play an important role in the maintenance of tissue integrity and in the regulation of cell proliferation, growth, differentiation, and migration.1, 9 Expression of integrins varies in different tumor types and these molecules are implicated in tumor progression and metastasis.¹ Excess of PAI-1 leads to hypofibrinolysis, down regulation of MMPs and decreased cellular adhesion.⁹

A deletion/insertion polymorphism (4G/5G) in the promoter region of the PAI-1 gene interferes with its transcription regulation.⁷ The 4G allele binds only an activator, while the 5G allele binds a repressor as well as an activator, therefore results in reduced transcription of PAI-1.¹⁶ It follows that the 5G variant is associated with reduced inhibition of the plasminogen activators and, consequently, increased plasminogen conversion to plasmin, increased activation of MMPs and decreased adhesive strength of cells for their substratum.7, 8, 16 The 4G/5G allele frequencies range in various populations from 31/69% to 63/37%, respectively.6, 17, 18, 19, 20, 21 Homozygosity of the 4G allele is considered to be a risk factor for deep vein thrombosis, myocardial infarction and high rate of miscarriage during pregnancy.6, 7, 8

Significantly higher levels of PAI-1 than in normal tissue have been reported in colorectal cancer, glioma and breast cancer, and some studies have found significant association of the 4G allele with increased risk for these types of cancer.10, 11, 22, 23, 24 Nevertheless, a strong association of the 5G allele with uPA/PAI-1 complex accumulation inside breast cancer cells was observed in conjunction with a less aggressive, better-differentiated tumor phenotype.22, 23

Increased PAI-1 expression has been correlated with advancement of oral squamous cell carcinoma, poor prognosis and tumor relapse.25, 26, 27 In this light, it would be interesting to investigate whether the 4G/5G polymorphism in the promoter area of the PAI-1 gene is associated with increased risk for oral cancer. In order to examine the effect of 4G/5G in Europeans, we evaluated the polymorphism in DNA samples of 104 patients with oral cancer and 106 healthy controls of comparable age and sex.