Elsevier

Oral Oncology

Volume 41, Issue 2, February 2005, Pages 105-116
Oral Oncology

Down-regulation of Cdk inhibitor p27 in oral squamous cell carcinoma

https://doi.org/10.1016/j.oraloncology.2004.05.003Get rights and content

Summary

Oral squamous cell carcinoma (OSCC) is the most frequent malignant neoplasm of the head and neck region. 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. The proliferation and progression of cancer may be caused by abnormalities of various positive and negative cell cycle regulators. Cell cycle progression is positively regulated by multiple cyclins and cyclin-dependent kinases (Cdks) and cyclin/Cdk complexes are negatively regulated by a number of Cdk inhibitors including p27. p27 is a Cdk inhibitor and plays an important role in negative regulation of the cell cycle during G0 and G1 phases. Degradation of p27 is a critical event for the G1/S transition and occurs through ubiquitination by SCFSkp2 and subsequent degradation by the 26S proteasome. It has been revealed that down-regulation of p27 is frequently found in various cancers, including OSCC, and is due to an enhancement of its degradation. Importantly, down-regulation of p27 is well associated with its malignancy including poor prognosis in various cancers. Moreover, aggressive human cancers express low levels of p27 because of its decreased stability. More recent evidence suggests that Skp2 and Cks1, the specific recognition factors for p27 ubiquitination, have oncogenic properties. This review will focus on down-regulation of p27 and mechanism of its down-regulation in OSCC.

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).

References (117)

  • J. Bloom et al.

    Deregulated degradation of the cdk inhibitor p27 and malignant transformation

    Semin. Cancer Biol.

    (2003)
  • H. Zhang et al.

    p19Skp-1 and p45Skp-2 are essential elements of the cyclin A–Cdk2 S phase kinase

    Cell

    (1995)
  • C. Spruck et al.

    A CDK-independent function of mammalian Cks1: targeting of SCFSkp2 to the CDK inhibitor p27

    Mol. Cell

    (2001)
  • Y. Bourne et al.

    Crystal structure and mutational analysis of the human CDK2 kinase complex with cell cycle-regulatory protein CksHs1

    Cell

    (1996)
  • D. Sitry et al.

    Three different binding sites of Cks1 are required for p27-ubiquitin ligation

    J. Biol. Chem.

    (2002)
  • T. Hara et al.

    Degradation of p27Kip1 at the G (0)–G (1) transition mediated by a Skp2-independent ubiquitination pathway

    J. Biol. Chem.

    (2001)
  • M. Fero et al.

    A syndrome of multi-organ hyperplasia with features of gigantism, tumorigenesis and female sterility in p27Kip1-deficient mice

    Cell

    (1996)
  • K. Nakayama et al.

    Mice lacking p27 disply inceased body size, multiple organ hyperplasia, retinal dysplasia, and pituitary tumors

    Cell

    (1996)
  • H. Kiyokawa et al.

    Enhanced growth of mice lacking the cyclin-dependent kinase inhibitor function of p27Kip1

    Cell

    (1996)
  • M.Y. Kuo et al.

    Prognostic role of p27Kip1 expression in oral squamous cell carcinoma in Taiwan

    Oral Oncol.

    (2002)
  • S. Shintani et al.

    Expression of cell cycle control proteins in normal epithelium, premalignant and malignant lesions of oral cavity

    Oral Oncol.

    (2002)
  • Y. Kudo et al.

    Reduced expression of p27Kip1 correlates with an early stage of cancer invasion in oral squamous cell carcinoma

    Cancer Lett.

    (2000)
  • M.L. Schoelch et al.

    Cell cycle proteins and the development of oral squamous cell carcinoma

    Oral Oncol.

    (1999)
  • R. Chiarle et al.

    Increased proteasome degradation of cyclin-dependent kinase inhibitor p27 is associated with a decreased overall survival in mantle cell lymphoma

    Blood

    (2000)
  • R. Chiarle et al.

    S-phase kinase-associated protein 2 expression in non-Hodgkin’s lymphoma inversely correlates with p27 expression and defines cells in S phase

    Am. J. Pathol.

    (2002)
  • S. Yokoi et al.

    A novel target gene, SKP2, within the 5p13 amplicon that is frequently detected in small cell lung cancers

    Am. J. Pathol.

    (2002)
  • N. Inui et al.

    High expression of Cks1 in human non-small cell lung carcinomas

    Biochem. Biophys. Res. Commun.

    (2003)
  • N. Fujita et al.

    Akt-dependent phosphorylation of p27Kip1 promotes binding to 14-3-3 and cytoplasmic localization

    J. Biol. Chem.

    (2002)
  • N. Fujita et al.

    Phosphorylation of p27Kip1 at threonine 198 by p90 ribosomal protein S6 kinases (RSKs) promotes its binding to 14-3-3 and cytoplasmic localization

    J. Biol. Chem.

    (2003)
  • N. Ishida et al.

    Phosphorylation of p27Kip1 on serine 10 is required for its binding to CRM1 and nuclear export

    J. Biol. Chem.

    (2002)
  • J. Marx

    Many gene changes found in cancer

    Science

    (1989)
  • J.M. Slingerland et al.

    Regulation of the cdk inhibitor p27 and its deregulation in cancer

    J. Cell Physiol.

    (2000)
  • M. Morgan

    Principles of cdk regulation

    Nature

    (1995)
  • C.J. Sherr et al.

    Inhibitors of mammalian G1 cyclin-dependent kinases

    Genes Dev.

    (1995)
  • C.J. Sherr et al.

    CDK inhibitors: positive and negative regulators of G1-phase progression

    Genes Dev.

    (1999)
  • M. Serrano et al.

    A new regulatory motif in cell-cycle control causing specific inhibition of cyclin D/CDK4

    Nature

    (1993)
  • G.J. Hannon et al.

    p15INK4B is a potent effector of TGF-beta-induced cell cycle arrest

    Nature

    (1994)
  • A. Kamb et al.

    A cell cycle regulator potentially involved in genesis of many tumor types

    Science

    (1994)
  • T. Nobori et al.

    Delations of the cyclin-dependent kinase-4 inhibitor gene in multiple human cancers

    Nature

    (1994)
  • J.W. Harper et al.

    The p21 Cdk-interacting protein Cip1 is a potent inhibitor of G1 cyclin-dependent kinases

    Cell

    (1993)
  • K. Polyak et al.

    p27Kip1, a cyclin-Cdk inhibitor, links transforming growth factor-beta and contact inhibition to cell cycle arrest

    Genes Dev.

    (1994)
  • A.A. Russo et al.

    Crystal structure of the p27Kip1 cyclin-dependent-kinase inhibitor bound to the cyclin A–Cdk2 complex

    Nature

    (1996)
  • M. Liu et al.

    Transcriptional activation of the Cdk inhibitor p21 by vitamin D3 leads to the induced differentiation of the myelomonocytic cell line U937

    Genes Dev.

    (1996)
  • S.K. Kolluri et al.

    p27Kip1 induction and inhibition of proliferation by the intracellular Ah receptor in developing thymus and hepatoma cells

    Genes Dev.

    (1999)
  • R.H. Medema et al.

    AFX-like Forkhead transcription factors mediate cell-cycle regulation by Ras and PKB through p27kip1

    Nature

    (2000)
  • D. Agrawal et al.

    Repression of p27kip1 synthesis by platelet-derived growth factor in BALB/c 3T3 cells

    Mol. Cell Biol.

    (1996)
  • L. Hengst et al.

    Translational control of p27kip1 accumulation during the cell cycle

    Science

    (1996)
  • M. Pagano et al.

    Role of the ubiquitin–proteasome pathway in regulating abundance of the cyclin-dependent kinase inhibitor p27

    Science

    (1995)
  • A. Hershko et al.

    The ubiquitin system

    Annu. Rev. Biochem.

    (1998)
  • R. Sheaff et al.

    Cyclin E-Cdk2 is a regulator of p27Kip1

    Genes Dev.

    (1997)
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