Differential effect of ik3-1/cables on p53- and p73-induced cell death.

ik3-1/Cables is associated with cdk3 in self-replicating cells. In postmitotic neurons, it may serve as an adaptor molecule, functionally connecting c-abl and cdk5, and supporting neurite growth. Here we report that ik3-1 binds to p53 and p73 in vivo. Ectopically expressed ik3-1 potentiates p53-induced cell death but not p73-induced cell death in U2OS cells. On the contrary, coexpression of ik3-1-DeltaC, an ik3-1 deletion mutant lacking the C-terminal 139 [corrected] amino acids (corresponding to the cyclin box-homologous region), inhibits p73-induced cell death but not p53-induced cell death. ik3-1-DeltaC-mediated inhibition of p73-induced cell death are partially attenuated by overexpression of ik3-1. These data indicate that ik3-1 is not only a regulator for p53-induced cell death but also an essential regulator for p73-induced cell death, and ik3-1-DeltaC competes with ik3-1 only in p73-induced cell death. Furthermore, functional domains of p53 responsible for its interaction with ik3-1 are partially different from those of p73. In conclusion, we found that ik3-1, a putative component of cell cycle regulation, is functionally connected with p53 and p73, but in distinct fashions.     

Selenoprotein W depletion induces a p53- and p21-dependent delay in cell cycle progression in RWPE-1 prostate epithelial cells

The anticancer activity of selenium (Se) has been demonstrated in myriad animal and in vitro studies, yet the mechanisms remain obscure. The main form of Se in animal tissues is selenocysteine in selenoproteins, but the relative importance of selenoproteins versus smaller Se compounds in cancer protection is unresolved. Selenoprotein W (SEPW1) is a highly conserved protein ubiquitously expressed in animals, bacteria, and archaea. SEPW1 depletion causes a delay in cell cycle progression at the G1/S transition of the cell cycle in breast and prostate epithelial cells. Tumor suppressor protein p53 is a master regulator of cell cycle progression and is the most frequently mutated gene in human cancers. p53 was increased in SEPW1 silenced cells and was inversely correlated with SEPW1 mRNA in cell lines with altered SEPW1 expression. Silencing SEPW1 decreased ubiquitination of p53 and increased p53 half-life. SEPW1 silencing increased p21(Cip1/WAF1/CDKN1A), while p27 (Kip1/CDKN1B) levels were unaffected. G1-phase arrest from SEPW1 knockdown was abolished by silencing p53 or p21. Cell cycle arrest from SEPW1 silencing was not associated with activation of ATM or phosphorylation of Ser-15 in p53, suggesting the DNA damage response pathway was not involved. Silencing GPX1 had no effect on cell cycle, suggesting that G1-phase arrest from SEPW1 silencing was not due to loss of antioxidant protection. More research is required to identify the function of SEPW1 and how it affects stability of p53.     

Musashi2 promotes the progression of pancreatic cancer through a novel ISYNA1‐p21/ZEB‐1 pathway

Our previous studies found overexpression of Musashi2 (MSI2) conduced to the progression and chemoresistance of pancreatic cancer (PC) by negative regulation of Numb and wild type p53 (wtp53). Now, we further investigated the novel signalling involved with MSI2 in PC. We identified inositol‐3‐phosphate synthase 1 (ISYNA1) as a novel tumour suppressor regulated by MSI2. High MSI2 and low ISYNA1 expression were prevalently observed in 91 PC tissues. ISYNA1 expression was negatively correlated with MSI2 expression, T stage, vascular permeation and poor prognosis in PC patients. What's more, patients expressed high MSI2 and low ISYNA1 level had a significant worse prognosis. And in wtp53 Capan‐2 and SW1990 cells, ISYNA1 was downregulated by p53 silencing. ISYNA1 silencing promoted cell proliferation and cell cycle by inhibiting p21 and enhanced cell migration and invasion by upregulating ZEB‐1. However, MSI2 silencing upregulated ISYNA1 and p21 but downregulated ZEB‐1, which can be rescued by ISYNA1 silencing. Moreover, reduction of cell migration and invasion resulting from MSI2 silencing was significantly reversed by ISYNA1 silencing. In summary, MSI2 facilitates the development of PC through a novel ISYNA1‐p21/ZEB‐1 pathway, which provides new gene target therapy for PC.     

p73 regulates DRAM-independent autophagy that does not contribute to programmed cell death

Evading programmed cell death is a common event in tumour development. The p53 family member, p73, is a potent inducer of death and a determinant of chemotherapeutic response, but different to p53, is rarely mutated in cancer. Understanding cell death pathways downstream of p53 and p73 is therefore pivotal to understand both the development and treatment of malignant disease. Recently, p53 has been shown to modulate autophagy--a membrane trafficking process, which degrades long-lived proteins and organelles. This requires a p53 target gene, DRAM, and both DRAM and autophagy are critical for p53-mediated death. We report here that TA-p73 also regulates DRAM and autophagy, with different TA-p73 isoforms regulating DRAM and autophagy to varying extents. RNAi knockdown of DRAM, however, revealed that p73's modulation of autophagy is DRAM-independent. Also, p73's ability to induce death, again different to p53, is neither dependent on DRAM nor autophagy. In contrast to TA-p73, deltaN-p73 is a negative regulator of p53-induced and p73-induced autophagy, but does not affect autophagy induced by amino-acid starvation. These studies, therefore, represent not only the first report that p73 modulates autophagy but also highlight important differences in the mechanism by which starvation, p53 and p73 regulate autophagy and how this contributes to programmed cell death.     

Loss of Expression of Reprimo,a p53-induced Cell Cycle Arrest Gene,Correlates with Invasive Stage of Tumor Progression and p73 Expression in Gastric Cancer

Reprimo (RPRM), a downstream effector of p53-induced cell cycle arrest at G2/M, has been proposed as a putative tumor suppressor gene (TSG) and as a potential biomarker for non-invasive detection of gastric cancer (GC). The aim of this study was to evaluate the epigenetic silencing of RPRM gene by promoter methylation and its tumor suppressor function in GC cell lines. Furthermore, clinical significance of RPRM protein product and its association with p53/p73 tumor suppressor protein family was explored. Epigenetic silencing of RPRM gene by promoter methylation was evaluated in four GC cell lines. Protein expression of RPRM was evaluated in 20 tumor and non-tumor matched cases. The clinical significance of RPRM association with p53/p73 tumor suppressor protein family was assessed in 114 GC cases. Tumor suppressor function was examined through functional assays. RPRM gene expression was negatively correlated with promoter methylation (Spearman rank r = -1; p = 0.042). RPRM overexpression inhibited colony formation and anchorage-independent growth. In clinical samples, RPRM gene protein expression was detected in 75% (15/20) of non-tumor adjacent mucosa, but only in 25% (5/20) of gastric tumor tissues (p = 0.001). Clinicopathological correlations of loss of RPRM expression were significantly associated with invasive stage of GC (stage I to II-IV, p = 0.02) and a positive association between RPRM and p73 gene protein product expression was found (p<0.0001 and kappa value = 0.363). In conclusion, epigenetic silencing of RPRM gene by promoter methylation is associated with loss of RPRM expression. Functional assays suggest that RPRM behaves as a TSG. Loss of expression of RPRM gene protein product is associated with the invasive stage of GC. Positive association between RPRM and p73 expression suggest that other members of the p53 gene family may participate in the regulation of RPRM expression.     

Family Feud in Chemosensitvity: p73 and Mutant p53

The importance of p53 in chemotherapy-induced apoptosis of cancer cells is well established. p53 plays a critical role in the cellular response to DNA damage by regulating genes involved in cell cycle progression, apoptosis, and genomic stability. As a result, p53 tumor status is a critical determinant of both responses to anti-cancer treatment and clinical prognosis. Interestingly, tumors expressing certain mutant forms of p53 (“gain of function”) are particularly resistant to chemotherapy, even when compared to cells that lack any detectable p53. Until recently, the explanation for this enhanced chemoresistance was not clear. Recent studies have shown that the p53 homologues, p73 and p63, are also activated by chemotherapies, leading to tumor cell death. Now the discovery that mutant p53 interacts with p73, and that regulation of this interaction by a p53 polymorphism can modulate chemosensitvity provide a new model for how p53-family interactions can influence the response of tumors to anti-cancer therapies. Since p53 mutations are found in more than 50% of human tumors, strategies aimed at manipulating these interactions may prove useful in enhancing the chemotherapy response, and perhaps, overcoming chemoresistance.     

Family feud in chemosensitvity: p73 and mutant p53

The importance of p53 in chemotherapy-induced apoptosis of cancer cells is well established. p53 plays a critical role in the cellular response to DNA damage by regulating genes involved in cell cycle progression, apoptosis, and genomic stability. As a result, p53 tumor status is a critical determinant of both responses to anti-cancer treatment and clinical prognosis. Interestingly, tumors expressing certain mutant forms of p53 ("gain of function") are particularly resistant to chemotherapy, even when compared to cells that lack any detectable p53. Until recently, the explanation for this enhanced chemoresistance was not clear. Recent studies have shown that the p53 homologues, p73 and p63, are also activated by chemotherapies, leading to tumor cell death. Now the discovery that mutant p53 interacts with p73, and that regulation of this interaction by a p53 polymorphism can modulate chemosensitvity provide a new model for how p53-family interactions can influence the response of tumors to anti-cancer therapies. Since p53 mutations are found in more than 50% of human tumors, strategies aimed at manipulating these interactions may prove useful in enhancing the chemotherapy response, and perhaps, overcoming chemoresistance.     

The Fatty Acid Synthase Gene is a Conserved p53 Family Target Gene from Worm to Human

The discovery that the p53 family consists of three members (p53, p63 and p73) in vertebrates and of a single homolog in invertebrates has raised the challenge of understanding the functions of the ancestor and how they have evolved and differentiated within the duplicated genes in vertebrates. Here, we report that the fatty acid synthase (FAS) gene, encoding for a key enzyme involved in the biogenesis of membrane lipids in rapidly proliferating cells, is a conserved target of the p53 family throughout the evolution. We show that CEP-1, the C. elegans p53 homolog, is able to bind the two p53 family responsive elements (REs) identified in the worm fasn-1 gene. Moreover, we demonstrate that fasn-1 expression is modulated by CEP-1 in vivo, by comparing wild-type and CEP-1 knockout worms. In human, luciferase and chromatin immunoprecipitation assays demonstrate that TAp73α and ΔNp63α, but not p53, TAp73β and TAp63α bind the two p53 REs of the human FASN gene. We show that the ectopic expression of TAp73β and ΔNp63α leads to an increase of FASN mRNA levels, while their silencing produces a decrease of FASN expression. Furthermore, we present data showing a correlation between ΔNp63α and FASN expression in cellular proliferation. Of relevant importance is that fasn-1 is the first CEP-1 direct target gene identified so far in C. elegans and our results suggest a new CEP-1 role in cellular proliferation and development, besides the one already described in apoptosis of germ cells. These data confirm the hypothesis that the ancestral functions of the single invertebrate gene may have been spread out among the three vertebrate members, each of them have acquired specific role in cell cycle regulation.