Expression of cell cycle control proteins in normal epithelium, premalignant and malignant lesions of oral cavity
Introduction
Abnormalities in various components of the cell cycle-regulatory machinery have been found in several types of human cancer including oral cancer. The cell cycle is governed by CDKs, the activities of which are regulated positively by the cyclins [1], [2] and negatively by CKIs [2]. The CDKs integrate mitogenic and growth-inhibitory signals and coordinate cell cycle transition [2]. Passage through G1 into S phase is regulated by the activities of G1 cyclins and CDK complexes [1], [2], [3]. Among those kinases that regulate G1 progression, CDK4 and CDK6 are activated by association with cyclin D in mid G1 [1], [3]. CDK2 is activated by binding to cyclin E, and its activity is essential for transition through the restriction point in late G1 [3]. INK4 family members p15INK4B, p16INK4A, p18INK4C and p19INK4D bind cyclin D-dependent kinases [2], [3], CDK4 and CDK6. Kinase inhibitor protein family members, including p21Cip1, p27KIP1, and p57Kip2, bind and inhibit their targets [2], [3]. P12DOC−1 is a growth suppressor gene isolated and identified from the hamster oral cancer model [4]. The human P12DOC−1 cDNA was recently cloned and the gene has been mapped to chromosome 12q24, and partially characterized [5]. Ectopic expression of doc-1 gene in keratinocytes is associated with growth suppression and increased doubling time [4]. Following the cloning of the hamster doc-1 cDNA, the human homologue was isolated, mapped to chromosome 12q24, and partially characterized [5]. The full-length human doc-1 cDNA is a 1634-bp in length and encodes for 115 amino acids (12.4 kDa polypeptide) [4], [5]. p12DOC−1 Is a specific CDK2-associating protein function to negatively regulate CDK2 activities by sequestering the monomeric pool of CDK2, effectively depleting the active pool of CDK2. While the p21WAF1/CIP1/CAP family of CDK inhibitors is universal for CDKs and the p16INK4A family is specific for CDK4 and CDK6, p12DOC−1 is a specific CDK2-associated protein that suppresses CDK2 activities [6].
The cyclins, CDKs, and CKIs are frequently altered in cancer or disrupted secondarily by oncogenic events [3]. For instance, several reports have demonstrated amplification and overexpression of the cyclin D1 gene in head and neck cancers [7]. There are also reports demonstrating alterations of specific CKIs, such as p21CIP1, p15INK4B, p16INK4A, in oral cancer [8], [9].
Proliferating cell nuclear antigen (PCNA) is an auxiliary factor essential for DNA polymerase δ activity, which is required for both replication and repair of DNA [10]. In addition, PCNA can exist in a quaternary complex with CDK/cyclin/p21CIP1 [11]. PCNA is expressed in cycling cells and is frequently used as a measure of the proliferating activity of tissues [12].
Although several studies have implicated factors involved in the cell cycle in the development of oral cancer, few have examined their expression in normal mucosa and preneoplasia. This study has evaluated the expression profile of key G1-S transition proteins (cyclins, CDKs, CDKIs and PCNA) in normal, premalignant and SCC to assess their role in oral cancer initiation and early progression. Because early preinvasive lesions have been shown to harbor genetic abnormalities and, specifically, loss of alleles (loss of heterozygosity) [13], we included premalignant lesions in this study.
Section snippets
Tissue samples
Tissue samples of 20 normal oral mucosa specimens, 42 epithelial dysplasia specimens, and 117 oral SCC, were obtained from previously untreated patients. The age and gender distributions of the patients are summarized in Table 1. The 117 cases of oral squamous cell carcinoma patients underwent surgery at Department of Oral and Maxillofacial surgery II, Okayama University Dental School (Okayama, Japan) from April 1989 to May 1999. The diagnoses of epithelial dysplasia were based on the criteria
Cyclin D1 and Cyclin E expression
In normal epithelia, the expression of cyclin D1 and cyclin E were either not detectable or weak in the basal cell layer (<5% positive cells; Fig. 1a). Epithelia with mild and moderate dysplasia showed infrequent expression of cyclin D1 and cyclin E. While epithelial with severe dysplasia also exhibited infrequent immunoreactivity for cyclin D1, eight of 14 cases (57.1%) were positive for cyclin E (Fig. 1c, Table 3). The expression of Cyclin E was observed sporadically in the basal and/or
Discussion
This study has examined the expression of various G1-S cell cycle regulatory proteins in a series of normal, premalignant, and malignant lesion of oral epithelium. This is one of the largest sample sizes used to compare the expression of a comprehensive range of the cell cycle regulators in oral tumorigenesis.
Progression through the G1 phase of the cell cycle is dependent upon the activity of G1 cyclins, which include the D-type cyclins and cyclin E. The D-type cyclins reach maximal levels of
Acknowledgements
This research was supported from grants PO1 DE12467 and RO1 DE08680 from the National Institute of Dental and Craniofacial Research to David T.W. Wong.
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