p16 Ink4a is a protein involved in regulation of the cell cycle. Currently, p16 Ink4a is considered a tumor suppressor protein because of its physiological role and downregulated expression in a large number of tumors. Intriguingly, overexpression of p16 Ink4a has also been described in several tumors. This review attempts to elucidate when and why p16 Ink4a overexpression occurs, and to suggest possible implications of p16 Ink4a in the diagnosis, prognosis and treatment of cancer.

Human epithelial tumors need to accumulate multiple genetic alterations to form invasive carcinomas. These genetic alterations are related with growth factor receptors, cell signalling, the cell cycle and cell invasiveness. Importantly, cells need to avoid senescence and become immortalized for this process. Recently, five genes: RPS6KA6, HDAC4, KIAA0828, TCP1 and Tip60, which modulate p53-dependent function and avoid senescence were identified in a large-scale RNA interference screen. Twenty colon, 20 prostate and 20 lung carcinomas were studied to investigate whether these genes might be related with human tumors. RNA was extracted from both normal and tumor tissue from each patient. Real-time RT-PCR was performed using TaqMan probes corresponding to the RPS6KA6, HDAC4, KIAA0828, TCP1, Tip60 and p53 genes. In colon carcinomas, the RPS6KA6, HDAC4, KIAA0828 and Tip60 genes were downregulated in tumor tissue as compared with normal tissue (P<0.001 for all genes). In lung carcinomas, HDAC4, KIAA0820 and Tip60 were downregulated (P<0.01, P<0.001 and P<0.001 respectively). Whereas no significant differences were observed in prostate carcinomas, striking downregulation of the RPS6KA6 and KIAA0828 genes was observed in colon carcinomas and KIAA0828 in a subset of lung carcinomas. mRNA expression of these genes may control p53 function as well as the ras-MAPK pathway, methylation and transcriptional cellular programs. These results could unravel a novel set of regulatory suppressor genes involved in human colon and lung tumors.

Purpose: The control of senescence and its biochemical pathways is a crucial factor for understanding cell transformation. In a large RNA interference screen, the RSK4 gene was found to be related to p53-dependent arrest. The purpose of the present study was to investigate the potential role of RSK4 as a tumor suppressor gene. Experimental Design: RSK4 expression was determined by quantitative real-time PCR and immunoblot in 30 colon and 20 renal carcinomas, and in 7 colon adenomas. Two HCT116 colon carcinoma cell lines (p53 wt and p53 null), IMR90 human fibroblasts, and E1A-expressing IMR90 cells were infected with RSK4 cDNA and/or shRNA. RSK4 expression levels were analyzed in HCT116 p53 wt or p53 null and IMR90 after senescence induction by quantitative real-time PCR and Western blot.Results: The RSK4 gene was down-regulated in 27 of 30 colon carcinomas (P < 0.001), 16 of 20 renal cell carcinomas (P < 0.01), and 6 of 7 colon adenomas (P < 0.01). In vitro overexpression of RSK4 induced cell arrest and senescence features in normal fibroblasts and malignant colon carcinoma cell lines. Interestingly, in these cell lines RSK4 mRNA levels were increased both in replicative and stress-induced senescence. Moreover, IMR90 partially immortalized by RSK4 shRNA and HCT116 with this short hairpin RNA were more resistant to cisplatin treatment. Finally, cells expressing E1A or Rb short interfering RNA were resistant to RSK4-mediated senescence. Conclusion: These results support the concept that RSK4 may be an important tumor suppressor gene by modulating senescence induction and contributing to cell proliferation control in colon carcinogenesis and renal cell carcinomas.Cellular senescence can be defined as an irreversible arrest of cell proliferation. It is activated in response to various types of stress, including oxidative stress (e.g. hydrogen peroxide treatment), oncogene activities, DNA damage, and treatment with DNA-damaging agents (e.g. doxorubicin or cisplatin), among others. This is called accelerated or stress-induced senescence. Moreover, senescence can occur as a result of telomere shortening after multiple cell divisions, a process known as replicative senescence (1, 2). Once cells enter senescence, they stop growing and develop dramatic morphologic changes, such as a flat, enlarged morphology, as well as metabolic changes (3 -7).Two main pathways are reported to be involved in senescence, p16INK4a /Rb and p19 ARF /p53, which are considered to be the main activators of senescence (7,8). P16 activates Rb by inhibiting CycD/Cdk4,6. P19 activates p53 by inhibiting MDM2; p53 can be also activated by phosphorylation done by the ATM/ATR and/or Chk1/Chk2 proteins. P53 and Rb can be connected through p21, activated by p53, which, in turn, can activate Rb by inhibiting CycE/Cdk2. Once Rb is activated, it is able to shut down transcription of the E2F target genes, inducing cell growth arrest. In addition, it seems that p53 can activate senescence in human cells independently of Rb (8,9). Whether both these s...