Loss of p16 and p27 is associated with progression of human gastric cancer

Abstract

We performed the immunohistochemical staining for six G1 check point cell cycle proteins to study their expression patterns and roles in the gastric carcinogenesis. We studied 76 cases of paraffin blocks that included the sections of 18 tubular adenomas (TA), 38 early gastric carcinomas (EGC) (20 cases of mucosal type, nine cases of submucosal type with no nodal metastasis, nine cases of submucosal type with nodal metastasis), 20 advanced gastric carcinomas (AGC) (ten cases with no nodal metastasis, ten cases with nodal metastasis). We found that abnormal expression of p16 and p27 increased with the progression of tubular adenomas to advanced gastric cancers. Inverse relationship between pRb and p16 proteins was found in a small portion of the gastric tumors. Expressions of pRb and cdk4 were consistently high in benign and malignant gastric tumors. Expression of cyclin D1 and cyclin E rather decreased with the tumor progression. In conclusion, losses of p16 and p27 seem to play a significant role during the gastric carcinogenesis, and the G1 checkpoint cell cycle proteins such as pRb, cdk4, cyclin D1, and cyclin E variably participate in the gastric carcinogenesis and metastasis by the mechanisms which are yet unknown; thus, further studies need to be performed to elucidate the mechanisms.

Introduction

There are accumulating evidences that the carcinogenic process frequently involves abnormalities in the expression of cyclins and other cell cycle-related genes especially during G1 phase regulation [1]. Unlike transit through the S, G2, and M phases, G1 progression normally relies on stimulation by mitogens and can be blocked by antiproliferative cytokines. The mitogens in the G1 phase of the cell cycle include cyclin D, E, A, and their catalytic partners such as cyclin-dependent kinase (cdk) 2, 4, and 6. The blockers inhibit the cell cycle progression of G1 phase, including inhibitors of cdk4 and cdk6, e.g. p16^INK4a, p15^INK4b, p18^INK4c, p19^INK4d and another group of inhibitor proteins such as p21^CIP1, p27^KIP1, and p57^KIP2. Unconstrained proliferation can be largely explained by the gain or loss of function of proteins that constitute the cell machinery itself [2]. The cell cycle control pathway governed by the D-type cyclins is the one most commonly mutated pathway in tumor cells [2]. In many studies for the alterations in cyclin D1 and cyclin E expressions, their increased expressions have been noted in many cancers of breast, head and neck, esophagus, lung, and liver [3], [4], [5]. The INK family of cdk inhibitors is frequently mutated in tumor cells. The changes in the cdk inhibitors such as p16 and p27 revealed that the homozygous deletions of their loci occurred in gliomas, mesotheliomas, nasopharyngeal carcinomas, acute lymphocytic leukemia, sarcomas, and bladder and ovarian cancers [5]. Rb is another tumor suppressor gene and its product, Rb protein, binds to E2F which inhibits G1→S progression. Inactivation and reduction of its growth-suppressive activity lead to retinoblastoma and adult cancers such as small cell lung cancer [5]. In studies on gastric carcinomas, it has been reported that p27^KIP1 decreased significantly in relation to tumor progression, whereas gene amplification and overexpression of cyclin E were associated with biological malignancy of gastric cancers [6], [7], with inverse relationship between the expressions of p27 and cyclin E [7]. However, overview of the changes in the expression of G1 check point cell cycle proteins including cyclin D1, p16, cdk, and Rb protein has not been reported well in the gastric carcinomas and during their carcinogenesis.

We performed an immunohistochemical staining study for six essential G1 check point cell cycle proteins, including cyclin D1, E, cdk4, p16, p27, and Rb protein, to correlate alterations in their immunohistochemical expression with regard to the tumor progression and to delineate their roles in the gastric carcinogenesis.