Cyclins and cyclin-dependent kinase (CDK) complexes have important regulatory roles during cell cycle progression. Cyclin-CDK complexes are in turn regulated by the cyclin-dependent kinase inhibitors (CDKIs), which generally inhibits cell cycle progression. These proteins fall into two families based on their structural and functional properties: the INK4 family and the Cip/Kip family.
CDKIs members of the INK4 group (p16/INK4A, or p16; p15/INK4B, or p15; p18/INK4C, or p18; p19/INK4D, or p19) have four ankyrin repeats and form complexes with CDK4 and/or CDK6 and the D-type cyclins (including
cyclin D1). They have functional activities that are dependent on the presence of a normal retinoblastoma protein. Maximal expression of the INK4 protein occurs during the middle of the S phase in proliferating cells. Both p15 and p16 shows a high frequency of gene deletions, and various human tumors and cell lines have mutations of the p16 gene, suggesting that these genes may function as tumor suppressors.
CDKIs members of the Cip/Kip group (p21/WAF1/CIP1, or p21; p27/kip1, or p27; p57/kip2, or p57), also designated as "universal CDKIs", inhibit kinase activity of pre-activated G
1 cyclin E-CDK2, cyclin D-CDK4/6, and other cyclins. Overexpression of the kip proteins lead to cell cycle arrest. Kip proteins all have a nuclear localization signal at their carboxyl-terminal domain.
Cell lines:
- In MDA-MB-231 breast cancer cells (BCC) overexpressing p27, apoptosis was found to be increased (Katayose Y. et al., 1997).
- In MDA-MB-157 BCC, p27 induction by cAMP activators such as forskolin or lovastatin was found to be independent of the protein kinase A pathway (Rao S. et al., 1999).
Tumors:
- 432 human cancers and 20 cancer cell lines were analyzed for alterations of the p27/Kip1 gene by Southern blot analysis and PCR/single-strand conformation polymorphism. In 140 tumors of various tissues and 18 transformed cell lines, no deletions or rearrangements of the gene were detected by Southern blot analysis using a part of the coding sequence as a probe. One polymorphism and one silent mutation were detected by PCR/single-strand conformation polymoprhism. The polymorphism was a nucleotide substitution of guanine for thymine (GTC-->GGC) at codon 109, resulting in an amino acid substitution of glycine for valine (Val-->Gly). In summary, no abnormalities of the p27/Kip1 gene were detected in human malignancies (Kawamata N. et al., 1995).
- The loss of p27 expression in tumor cells does not appear to result essentially from gene mutations. However 2 point mutations were found in an analysis of 36 primary breast carcinomas. One of the mutations was a polymorphous mutation at codon 142 and the other a nonsense mutation at codon 104 (Spirin K.S. et al., 1996). The rarity of mutations and other genetic alterations in the p27 gene during tumor development is not consistent with a role as a tumor suppressor gene.
- Several studies have shown that p27 protein expression is lower in more aggressive tumors. p27 appears to be a predictor of reduced disease-free survival by Kaplan-Meier analysis (Catzavelos C. et al., 1999); in a series of 202 patients with breast cancers less than 1 cm in size, low p27 expression and nodal status were found to be independent prognostic parameters by both univariate and multivariate analyses (Tan P. et al., 1997); an inverse correlation between the expression of p27(kip1) and the degree of tumor malignancy in human breast has been suggested (Fredersdorf S. et al., 1997).
- Apoptosis and expression of p27Kip1 and p53 were studied retrospectively in 181 human breast cancer specimens, and their relevance to the biological behaviour of breast cancer was examined. A significant association was found among high histological grade, high p53, low apoptosis and low p27. In addition, low levels of p27 and apoptotic index (AI) strongly correlated with the presence of lymph node metastasis and decreased patient survival. In node-negative patients, however, p27 also had prognostic value for relapse-free and overall survival in multivariate analysis. Furthermore p27 and AI had predictive value for the benefits of chemotherapy (Wu J. et al., 1999).
- In an immunohistochemical study of 102 patients with node-positive breast cancer, a significant correlation was found between high p27 and positive
estrogen receptor status, but there was no correlation between p27 staining and age, menopausal status, nodal status, or tumor size. Low expression of p27 was significantly associated with shorter survival (Tsuchya A. et al., 1999).
- In a series of 189 cases of primary breast carcinoma with long-term follow-up, there was a statistically significant association between the expression of p27 and both
cyclin D1 and the retinoblastoma gene product (pRb), corresponding to their close interactions in regulating the G1/S transition in the cell cycle. Low levels of p27 were seen in high-grade, rapidly proliferating,
oestrogen receptor-negative tumours. In univariate analysis, low p27 expression was associated with a reduced relapse-free and overall survival. In multivariate analysis, p27 was not an independent predictor of survival when either histological grade or proliferative activity (S-phase fraction) was included in the model (Gillett C.E. et al., 1999).
- p27 immunohistochemical expression was evaluated in 512 consecutive cases of breast carcinoma with 9 years of median-term follow-up. p27 expression was heterogeneous and frequently less intense than in normal cells. Low p27 expression (<50% of reacting cells) was associated with grade III tumors, N0 status,
estrogen receptor-negative status, and low
cyclin D1 expression. In the whole series of cases, p27 expression did not predict outcome. In node-negative cases (249 patients), high p27 expression indicated poor prognosis. p27 was not prognostically relevant in the group of 223 patients with pT1 disease or in the group of 154 patients <50 years of age. The prognostic value of the combined expression of p27 and
cyclin D1 was also investigated, but no differences in survival were seen in this bivariate analysis (Barbareschi M. et al., 2000).
