Expression of p16INK4A variants in senescent human fibroblasts independent of protein phosphorylation

Snail suppresses cellular senescence and promotes fibroblast-led cancer cell invasion

Snail, a zinc finger transcription factor, induces an epithelial-mesenchymal transition (EMT) in various cancer and epithelial cells. We investigated the function of Snail (SNAI1) by downregulating its expression with short interfering RNA (siRNA). Suppression of Snail expression induced cellular senescence in several cancer cells and in normal fibroblast IMR90 cells. Cancer progression is facilitated by fibroblasts, so-called fibroblast-led cancer cell invasion. Snail-silenced cancer cells exhibited reduced motility, which was further decreased by cocultivation with Snail-silenced IMR90 cells. Our data suggest that cell motility and cellular senescence, which are regulated by Snail in cancer cells and fibroblasts, modulate fibroblast-led cancer cell invasion. Therefore, we propose that local suppression of Snail in cancer and the cancer microenvironment represents a potent therapeutic strategy.

Tiron protects against UVB-induced senescence-like characteristics in human dermal fibroblasts by the inhibition of superoxide anion production and glutathione depletion

BACKGROUND/OBJECTIVES

  Free radicals and reactive oxygen species (ROS), which are generated by UV irradiation, may induce an irreversible growth arrest similar to senescence. Tiron, 4,5-dihydroxy-1,3-benzene disulfonic acid, is a widely used antioxidant to rescue ROS-evoked cell death. The aim of the article was to explore the effects of tiron on skin photoaging and associated mechanisms.

METHODS

  The effects of tiron on cell proliferation were determined using 3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide. Senescent cells were determined by morphology and senescence-associated β-galactosidase activity analysis. Intracellular hydrogen peroxide, superoxide anion and glutathione concentration were analysed by a fluorescent probe. The concomitant changes of protein expression were analysed with Western blot.

RESULTS

  Human dermal fibroblasts were induced to premature senescence by sub-cytotoxic doses of irradiated UVB. Strong senescence-associated β-galactosidase activity and increased intracellular superoxide anion were observed in human dermal fibroblasts irradiated by UVB. Tiron blocks UVB-induced glutathione depletion and increase of superoxide anion and protects against UVB-induced senescence-like characteristics in human dermal fibroblasts. Compared with normal fibroblasts, UVB-irradiated human dermal fibroblasts showed a higher ratio of active (hypophosphorylated) to inactive (phosphorylated) forms of Rb and p38, upregulation of p53 or p16 and c-Myc and insulin-like growth factor 1 (IGF-1) downregulation. After treatment with tiron, p53, p16 c-Myc and IGF-1 as well as phosphorylation Rb and p38 could partially recover.

CONCLUSION

  These results indicate that tiron protects against UVB-induced senescence-like characteristics in human dermal fibroblasts via the inhibition of production of superoxide anion and glutathione depletion, and modulation of related senescence proteins.

Expression of regulatory proteins and proliferative activity in relation to phenotypic characteristics of upper urothelial carcinoma

BACKGROUND/AIM

Deregulation of the normal cell cycle is common in upper urothelial carcinoma (UUC). The aim of this study was to investigate the expression of regulatory proteins of the cell cycle (p53, p16, cyclin D1, HER-2) and proliferative Ki-67 activity in UUC, and to determine their interaction and influence on the phenotypic characteristics of UUC.

METHODS

In 44 patients with UUC, histopathological and immunohistochemical analyses (p53, p16, cyclin D1, HER-2, and Ki-67) of tumors were done.

RESULTS

Overexpression/altered expression of p53, p16, cyclin D1 or HER-2 was detected in 20%, 57%, 64%, and 57% of tumors, respectively. Eleven (25%) UUC had a high proliferative Ki-67 index. Forty patients (91%) had at least one marker altered, while four (9%) tumors had a wild-type status. Analysis of relationship between expressions of molecular markers showed that only high expression of p53 was significantly associated with altered p16 activity (p < 0.05). High Ki-67 index was associated with the high stage (p < 0.005), solid growth (p < 0.01), high grade (p < 0.05), and multifocality p < 0.05) of UUC, while high expression of p53 was associated with the solid growth (p < 0.05). In regression models that included all molecular markers and phenotypic characteristics, only Ki-67 correlated with the growth (p < 0.0001), stage (p < 0.01), grade (p < 0.05) and multifocality (p < 0.05) of UCC; (Ki-67 and HER-2 expression correlated with the lymphovascular invasion (p < 0.05).

CONCLUSIONS

This investigation showed that only negative regulatory proteins of the cell cycle, p53 and p16, were significantly associated in UUC, while proliferative marker Ki-67 was in relation to the key phenotypic characteristics of UUC in the best way.

The atr protein kinase controls UV-dependent upregulation of p16INK4A through inhibition of Skp2-related polyubiquitination/degradation

The tumor suppressor p16(INK4A), a phosphoprotein that exists in human cells under both phosphorylated and nonphosphorylated forms, plays crucial roles during the cellular response to UV light. However, it is still unclear how this protein is activated in response to this carcinogenic agent. We have shown here that UVC upregulates p16(INK4A) and the phosphorylated form of the protein at the 4 serine sites; Ser-7, Ser-8, Ser-140, and Ser-152. This accumulation of p16(INK4A) occurred through increasing the stability of both forms of the protein. Importantly, phospho-p16(INK4A) showed much higher stability, and UV treatment strongly increased its level in absence of de novo protein synthesis. Furthermore, we have shown that the UV-dependent upregulation of both forms of p16(INK4A) is under the control of the protein kinase Atr, which suppresses their UVC-dependent proteasomal degradation. Interestingly, although this degradation is ubiquitin-related for p16(INK4A) through the Skp2 ubiquitin ligase protein, it is ubiquitin-independent for the phosphorylated form. In addition, we present clear evidence that Skp2 is upregulated in ATR-deficient cells, leading to the downregulation of the p27(Kip1) protein in response to UVC light. Moreover, we have shown a preferential association of endogeneous phospho-p16(INK4A) with Cdk4. This association increased following UV-treatment mainly for p16(INK4A) phosphorylated at Ser-140 and Ser-152. Besides, we have shown that Atr regulates UV-related p16/Cdk4-dependent and -independent phosphorylation of pRB and G1 cell cycle delay. Together, these results indicate that p16(INK4A) and p27(Kip1) are key targets in the Atr-dependent signaling pathway in response to UV damage.

Subcellular localization, modification and protein complex formation of the cdk-inhibitor p16 in Rb-functional and Rb-inactivated tumor cells

The cdk-inhibitor p16 is a tumor suppressor gene that is inactivated in many forms of cancer. Despite numerous studies, the exact mechanism of regulation of p16 has not been clarified, although the status of retinoblastoma (Rb) seems to be one important factor that influences the p16 expression. The specificity and validity of cytoplasmic localization of p16 observed in some tumors has further been questioned. Here, by subcellular fractionation of Rb-functional and Rb-inactivated cell lines, we show that p16 indeed is expressed in the cytoplasm as well as in the nucleus. Post translational modifications of p16 in different subcellular compartments as well as its capacity to form protein complexes were further delineated. Two dimensional gel electrophoresis showed that two forms of p16 appeared in the cytoplasm, while only one form was detected in the nucleus. Samples of basal cell carcinoma and squamous cell carcinoma of the skin with either functional or non-functional Rb also exhibited at least two forms of p16. In addition, cytoplasmic p16 bound cyclin dependent kinase (cdk)4/6, potentially indicating that p16 could have a function in the cytoplasm.