Reduced expression of cell-cycle regulator p27^Kip1 correlates with a shortened survival in non-small cell lung cancer

Abstract

Background: The cell cycle progression is governed by a family of cyclin-dependent kinases, which are regulated by associated cyclins and by phosphorylation. p27, a cyclin-dependent kinase inhibitor, regulates the progression from G1 into the S phase by binding and inhibiting cyclin/cdks. Although p27 mutations in human tumors are extremely rare, a reduced expression of p27 might to lead to a progression of cancer cells. Methods: We examined tissues that had been surgically excised from 161 unselected Japanese patients with non-small cell lung cancer, and investigated the p27 protein expression by immunohistochemistry. Results: A reduced expression of the p27 protein was found in 63 cases (39.0%). Statistical correlation was found between the reduced p27 expression and advanced stage, although no correlation was found between the level of p27 expression and the gender, T factor, N factor or histological differentiation. The 5-year survival rate in the reduced group was 35.4%, which was statistically poorer than the 63.2% rate in the normal group (P=0.0016), in patients with complete resection. In a multivariate analysis, the level of p27 expression was found to be an independent prognostic indicator. Conclusions: We demonstrated the expression of p27 protein to be a biological prognostic indicator which can indicate the subsets of patients with either a good or poor prognosis, in patients who underwent surgical resection.

Introduction

Lung cancer is the leading cause of cancer death in males, and the third leading cause in females in Japan, and more than 50 000 patients died in 1998. The number of patients with lung cancer is increasing remarkably in Japan, as well as in many developed nations. Presently, surgery is the only method to cure lung cancer, however more than 50% of all patients who undergo a complete resection demonstrate recurrence. There is a considerable need for reliable prognostic markers to assist clinicians in making management decisions.

Recent studies have shown that cyclins and cyclin dependent kinase (CDK) complexes play an important regulatory role in the cell cycle progression [1], [2], [3], [4]. The activity of cyclin-CDK complexes is regulated by two families of proteins that generally inhibit the cell cycle progression. The INK4 group including p15, p16, p18 and p19 forms complexes with CDK4 and/or CDK6 and D-type cyclins and their functional activities depend on the presence of a normal retinoblastoma protein [5], [6]. A high frequency of gene deletions of p15 and p16 and mutations of p16 have been observed in human tumors and cell lines, thus suggesting that these genes may have a function as tumor suppressors [7], [8], [9].

Other group of CDK inhibitors includes p21/WAF1/CIP1, p27/kip1 and p57/kip2, which are proteins that inhibit the kinase activities of preactivated G1 cyclin E-CDK2, cyclin D-CDK4/6, and other cyclins [9], [10], [11], [12], [13], [14], [15]. The role of kip protein in regulating the cell cycle progression in normal and neoplastic cells has not yet been elucidated. The p27 protein levels are upregulated by growth inhibitory cytokines including transformation growth factor-β (TGF-β), Rapamycin, contact inhibition and serum starvation, which thus provide an important link between extracellular regulators and the cell cycle. The loss or decreased expression of p27, a negative cell-cycle regulator, may contribute to oncogenesis and tumor progression [16]. Recent studies with p27-deficient mice that developed multiple organ hyperplasia indicate this CDK inhibitor to have an anti-proliferative activity in vivo [17], [18], [19]. Although mutations in the p27 gene are rare in human tumors [20], [21], decreased p27 protein levels are found in aggressive cancer of the breast, colon, prostate gland, and gastric mucosa, thus suggesting that its loss may both reflect and participate in the process of tumor progression. In addition, p27 degradation is known to be due to proteasome-mediated proteolysis.

We previously reported that the mutation of the ras gene and p53 gene are unfavorable prognostic biological markers in non-small cell lung cancer (NSCLC) [21], [22], [23], [24]. The major purpose of research on biological or molecular factors is to identify of new biological prognostic markers that indicate the subsets of patients at high risk for recurrence or poor prognosis. We investigated the expression level of p27 by immunohistochemical studies of NSCLC and analyzed the clinical significance that the reduction in the expression of p27 may have.