Regulation of Replicative and Stress-Induced Senescence by RSK4, which is Down-regulated in Human Tumors

Abstract

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...

RSK phosphorylates SOS1 creating 14-3-3-docking sites and negatively regulating MAPK activation

“…Very few studies have reported increased tumor-derived RSK expression. Rather, accumulating evidence support the idea that at least some RSK isoforms are downregulated in certain cancers (in particular RSK3 and RSK4 [44, 45]). While our data indicate a requirement for RSK1 and RSK2 (with RSK2 playing a more important role) in Ser1134 and Ser1161 phosphorylation of SOS1 in response to growth factor stimulation, further investigation will be necessary to determine the extent RSK3 or RSK4 participate in the regulation of SOS1 in other cellular contexts.…”

RSK3/4 mediate resistance to PI3K pathway inhibitors in breast cancer

“…RSKs are directly regulated by ERK signaling and are implicated in cell growth, survival, motility, and senescence (25)(26)(27)(28). Here, we present evidence that overexpression of RSK3 and RSK4 supports cellular proliferation under PI3K pathway blockade by inhibiting apoptosis and regulating cellular translation through phosphorylation of ribosomal proteins S6 and eIF4B.…”

RSK4 knockdown promotes proliferation, migration and metastasis of human breast adenocarcinoma cells

“…RSK4 is an outlier and functionally distinct, showing low expression compared with other RSKs, and overexpression of RSK4 may restrict cell growth (12). Several studies have reported that high expression of RSK4 in breast cancer cells is anti-oncogenic, anti-invasive, and anti-metastatic (13), and RSK4 also exhibits a tumor suppressor effect in colon and renal carcinomas (14).…”