Down-regulation of Cdk inhibitor p27 in oral squamous cell carcinoma
Introduction
Squamous cell carcinoma is the most common malignant neoplasm of the oral cavity. Conversion of normal cells to cancer cells is achieved through a multi-step process that is closely associated with the accumulation of multiple gene changes including both oncogenes and tumour suppressor genes.1, 2 It has been known that the proliferation and progression of cancer cells relate closely to abnormalities of various positive and negative cell cycle regulators.3, 4 Control of eukaryotic cell cycle progression is maintained by the activity of Cdks.6, 7, 8 The activity of Cdks is regulated by phosphorylation and dephosphorylation of the catalytic subunit and by association with Cyclins.9 Cyclin/Cdk complexes are activated by phosphorylation by the Cdk-activating kinase (CAK), while Cyclin/Cdk complexes are negatively regulated by a number of Cdk inhibitors (Ckis).9 Ckis belong to two large families, Ink4 and Cip/Kip, based on their structural and functional properties. The Ink4 family includes p15, p16, p18 and p20 and consists of tandem repeats of an ankyrin-like sequence.10, 11, 12, 13 The Cip/Kip family includes p21, p27 and p57 and has a homologous amino-terminal domain that contains contiguous cyclin and Cdk-binding regions.14, 15, 16, 17 The Ink4 family members inhibit the activity of both cdk4 and cdk6, while the Cip/Kip family members specifically inhibit the activity of cdk2 by binding both to cyclin E/cdk2 and to cyclin A/cdk2 complexes.
p27 was first identified as a Cki due to its ability to block the activity of cyclin E/cdk2 and cyclin A/cdk2 in cells arrested in G1 by TGF-ß, lovastatin and contact inhibition.16, 17, 18 The crystal structure of p27 in complex with cyclin A/cdk2 has provided evidence into the mechanism by which p27 blocks the activity of Cyclin/Cdk complexes.19 Although p27 mRNA levels do not change during cell cycle progression, levels of p27 protein change during the cell cycle, with maximal levels occurring during G1 and quiescence (G0). The increase in the cellular abundance of p27 upon induction of cell quiescence is primarily due to a decrease in the rate of its degradation. Although p27 expression is also regulated at transcriptional20, 21, 22, 23, 24 and translational25, 26, 27 levels, p27 protein levels are mainly regulated by ubiquitin-dependent proteolysis.28 It has been revealed that down-regulation of p27 was frequently found and was well correlated with its malignancy including metastasis and poor prognosis in oral squamous cell carcinoma (OSCC) as well as in various cancers. Moreover, the lack of p27 is suggested to be due to an enhancement of its degradation.5 In the present review, we draw attention to down-regulation of p27 in OSCC and discuss the possibility of gene therapy aiming at inhibition of p27 degradation.
Section snippets
Ubiquitination and degradation of p27 protein
The ubiquitin–proteasome system marks proteins for destruction by attachment of a polyubiquitin chain and subsequently degrades these proteins through the activity of a multi-catalytic enzyme, the 26S proteasome.29 Ubiquitin in its monomeric form is a small protein that contains only 76 amino acids. Attachment of a chain of ubiquitin molecules to a substrate requires the concerted action of three enzymes, E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme) and E3 (ubiquitin
Down-regulation of p27 in OSCC
p27-deficient mice show gigantism, multiple organ hyperplasia and female infertility.50, 51, 52 Moreover, p27-/- mice frequently develop pituitary adenomas, implying that loss of p27 is involved in tumorigenesis. These results indicate that the negative regulation of positive cell cycle factors qualifies p27 as a tumour suppressor protein. In addition, p27 is haplo-insufficient for tumour suppression, because loss of just one allele of the [t6] p27 gene predisposes mice to tumours in multiple
p27 degradation in OSCC
As mentioned above, down-regulation of p27 protein in OSCC may be caused by increased ubiquitin–proteasome-mediated degradation. Skp2 is a specific factor for the ubiquitination and consequent degradation of p27. Moreover, Skp2 is frequently overexpressed in tumour cell lines, and forced expression of Skp2 in quiescent fibroblasts induces DNA synthesis.35, 38 These findings led us to hypothesize that enhanced p27 degradation, observed in various cancers including OSCC, might be due to increased
Cytoplasmic localization of p27
It is difficult to say that disorders of p27 in cancer are caused only by abnormalities of the SCFSkp2-dependent pathway, because some cancer cells without Skp2 expression show down-regulation of p27. As described before, two groups suggest that there are two pathways of p27 degradation, a Thr187 phosphorylation-dependent and a phosphorylation-independent manner.47, 48 Therefore, we think that another pathway may also be involved with the disorders of p27 in cancer (Fig. 3). Recently, three
Future perspectives
As mentioned above, down-regulation of p27 is a common event with the high frequency in various malignant tumours including OSCC and is frequently observed. As down-regulation of p27 is well known to correlate with malignant behavior in cancer, we believe that inhibition of p27 degradation can be a novel and powerful target of cancer therapy. So far, several groups have reported that overexpression of p27 protein induces cell cycle arrest and/or apoptosis in mammalian cancer cells.105, 106, 107
Conclusion
Reduced expression of p27 protein by ubiquitin-dependent degradation is involved in oral carcinogenesis through abnormal cell cycle regulation (Fig. 3). Although the inhibition of p27 degradation can be a novel and powerful target of cancer therapy as well as a diagnostic marker, the detailed mechanism of p27 degradation in cancer is still unclear. Therefore, further studies are required for the clarification of this important cancer-related process.
Acknowledgement
We thank Dr. Pagano (New York University School of Medicine) for critically reading the manuscript. This work was supported in part by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan (#15791043).
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