Regular articleExpression of p27, p21, and p16 protein in early squamous cervical cancer and its relation to prognosis
Introduction
Stage I cervical carcinoma has a relatively favorable prognosis with a 5-year survival rate of 80% [1]. Adjuvant treatment, given to about one-third of patients primarily treated with surgery [1], is accompanied by considerable morbidity [2]. Since nearly half of patients present with stage I disease this represents a large group. Knowledge of prognostic factors could help tailor treatment so that overtreatment can be avoided in low risk groups and adjuvant treatment only be given to patients with a high risk of relapse. Pelvic lymph node metastasis, tumor diameter, deep stromal invasion, vascular invasion, and close resection margins have most frequently been identified as prognostic factors [3], [4], [5], [6]. Biomolecular factors could possibly improve this prediction of prognosis and because of their role in tumorigenesis lead to development of new treatment strategies.
The cell cycle is the coordinated sequence of events involved in cell replication. In general, progression through the cell cycle is governed by protein complexes composed of a cyclin and a cyclin-dependent kinase (cdk). The cyclin/cdk complexes drive the cell forward by phosphorylating the retinoblastoma protein (RB). In its active, hypophosphorylated form RB is bound to transcription factors of the E2F family. Upon phosphorylation, these factors are released, leading to transcription of genes responsible for S phase progression. Cdks are opposed by cdk inhibitors. These can be divided in two structurally related families: the Cip/Kip family (p21, p27, and p57), which preferentially inhibits cdk2-containing complexes (cyclin A/E–cdk2) and the INK4 family (p15, p16, p18, and p19), which inhibits cyclin D-containing complexes (cyclin D–cdk4/6). Through their role in the cell cycle, cdk inhibitors can be seen as tumor suppressors and loss of their function could lead to malignant transformation.
p27 is strongly expressed in nonproliferating cells and its primary function seems therefore to be control of proliferation as evidenced by the deregulated growth in p27 knock-out mice [7]. Expression is increased by growth inhibiting signals such as transforming growth factor β (TGF-β)[8] or contact inhibition [9], leading to cell cycle arrest, whereas it is decreased by growth stimulating signals leading to cell cycle progression [10]. p27 thus has a pivotal role in regulating the response of cells to extracellular signals, a rather unique feature among cdk inhibitors. p27 has also been implicated as a mediator in cell differentiation, cell–cell adhesion, and apoptosis (for review, see [11]). The main regulation of the p27 level is thought to be posttranslational through degradation by the ubiquitin–proteasome pathway [12], which was found increased in several human tumors [13]. The human papillomavirus (HPV)-16 E7 oncogene can bind and inactivate p27 without degrading it [14] but this inactivation may not be involved in oncogenic transformation [15].
p21 is often reduced or absent in quiescent cells and it may be primarily involved in controlling cdk activity in proliferating cells [16]. It also plays a role in the response of cells to DNA damage by arresting the cell cycle and enabling repair or, failing this, by triggering apoptosis. In this context p21 is regarded as a major effector of p53 as the p21 gene is under transcriptional control of p53 [17]. However, p21 can be induced in a p53-independent manner [18] and p21 is not required for p53-dependent apoptosis [19]. p21 also binds the proliferating cell nuclear antigen (PCNA), thereby inhibiting replicative DNA synthesis [20]. Through its role in apoptosis and DNA repair p21 could be an important modulator of resistance to chemotherapy and radiation and high levels seem to induce growth arrest and prevent apoptosis [21], [22]. p21 also mediates terminal differentiation in multiple cell types, including keratinocytes [23], and is involved in cell senescence [24]. The HPV-16 E7 oncogene can bind and inactivate p21 without degrading it. This results in high cdk2 activity despite high levels of p21, a situation analogous to that of p27 [25].
p16 is an element of the p16\ cyclinD–cdk4/6 \RB pathway [16]. Disruptions in this “RB pathway” are part of the life history of many human cancers (for review, see [26]), and alterations of the p16 gene have been found frequently in sporadic and in some familial cancers (for review, see [27]). p16 is probably important in cell senescence, and recent studies have identified a role for p16 in anoikis, cell spreading, and angiogenesis (for review, see [27]). The aim of this retrospective study was to investigate the extent of p27, p21, and p16 protein expression in early SCC, their relation to clinical parameters, and their prognostic significance.
Section snippets
Materials
From January 1987 to December 1993, 242 patients with International Federation of Gynecology and Obstetrics (FIGO) stage IB SCC were treated by radical hysterectomy and bilateral pelvic lymphadenectomy at the Norwegian Radium Hospital. All cervical cancers diagnosed within the hospitals’ catchement area were solely treated at this institution. All patients were preoperatively examined under general anesthesia and tumor diameter was assessed by inspection and palpation. No preoperative screening
Immunohistochemistry
In normal squamous epithelium no p21 or p16 expression was detected, whereas p27 expression was detected in >50% of cells in 9/10 normal epithelia and in 5–50% of cells in the remaining one. Staining was almost exclusively limited to the parabasal and intermediate layers. The staining results in SCC are summarized in Table 1, Table 2. Low (<5% positive nuclei) and high (≥5% positive nuclei) p21 expression were detected in 177/221 (80%) and 44/221 (20%) tumors, respectively (Fig. 1a). Nuclear
Discussion
This study was undertaken to evaluate the prognostic significance of p27, p21, and p16 expression in early stage SCC. p16 expression was correlated with disease-specific survival in univariate but not in multivariate analysis. p27 and p21 expression had no prognostic significance. The traditional clinicopathological variables were of prognostic significance in univariate analysis. In multivariate analysis, only depth of stromal invasion was statistically significant. Metastasis to pelvic lymph
Acknowledgements
We thank Mai Thi Phuong Nguyen, Liv Inger Håseth, Asle Bjåmer, and Ellen Hellesylt for excellent technical assistance.
Supported in part by grants from the Norwegian Cancer Society.
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