Suppression of glucosylceramide synthase restores p53-dependent apoptosis in mutant p53 cancer cells

Tumor suppressor p53 plays an essential role in protecting cells from malignant transformation by inducing cell-cycle arrest and apoptosis. Mutant p53 that is detected in more than 50% of cases of cancers loses its role in suppression of tumors but gains in oncogenic function. Strategies to convert mutant p53 into wild-type p53 have been suggested for cancer prevention and treatment, but they face a variety of challenges. Here, we report an alternative approach that involves suppression of glucosylceramide synthase (GCS), an enzyme that glycosylates ceramide and blunts its proapoptotic activity in cancer cells. Human ovarian cancer cells expressing mutant p53 displayed resistance to apoptosis induced by DNA damage. We found that GCS silencing sensitized these mutant p53 cells to doxorubicin but did not affect the sensitivity of cells with wild-type p53. GCS silencing increased the levels of phosphorylated p53 and p53-responsive genes, including p21(Waf1/Cip1), Bax, and Puma, consistent with a redirection of the mutant p53 cells to apoptosis. Reactivated p53-dependent apoptosis was similarly verified in p53-mutant tumors where GCS was silenced. Inhibition of ceramide synthase with fumonisin B1 prevented p53 reactivation induced by GCS silencing, whereas addition of exogenous C6-ceramide reactivated p53 function in p53-mutant cells. Our findings indicate that restoring active ceramide to cells can resuscitate wild-type p53 function in p53-mutant cells, offering preclinical support for a novel type of mechanism-based therapy in the many human cancers harboring p53 mutations.     

The p53 pathway promotes efficient mitochondrial DNA base excision repair in colorectal cancer cells

The tumor suppressor p53 plays a central role in the DNA damage response. p53 enhances base excision repair (BER), in part, through direct interaction with the repair complex. Mitochondrial DNA (mtDNA) is repaired by a mtBER pathway. Many colorectal cancers harbor mtDNA mutations that are associated with poor prognosis. In addition to modulating the apoptotic response, mitochondria-localized p53 also stimulates mtBER. However, the mechanisms by which p53 enhances colorectal cancer mtBER after stress remain unclear. To explore this, we used colorectal cancer cells isogenic for p53 (HCT116p53+/+ and HCT116p53-/-). p53+/+ cells more efficiently repaired H(2)O(2) damaged DNA in vivo as measured by semiquantitative mtDNA displacement loop PCR. Mitochondrial extracts from p53+/+ cells more efficiently stimulated (32)P-dCTP incorporation into a uracil-oligonucleotide. Recombinant p53 complemented p53-/- mitochondrial extract repair of uracil or 8-oxo-G-containing oligonucleotides. As a measure of DNA glycosylase activity, p53+/+ mitochondrial extracts more efficiently incised uracil or 8-oxo-G oligonucleotides, although recombinant p53 could not stimulate oligonucleotide incision. p53 did not influence mitochondrial apurinic/apyrimidinic endonuclease activity measured by incision of a tetrahydrofuran-oligonucleotide. p53+/+ mitochondrial extracts had higher DNA polymerase-gamma activity measured by (32)P-dCTP incorporation into a single-nucleotide gap oligonucleotide, and recombinant p53 complemented p53-/- mitochondrial extract DNA polymerase-gamma activity. mtDNA ligase activity was not affected by p53 status. p53 protein was detected in an inner mitochondrial membrane subfraction containing components of the mtBER complex. Our data suggest that an intact p53 pathway stimulates specific mtBER steps and provides mechanistic insight into the development of mtDNA mutations in colorectal cancer.     

Quercetin enhances 5-fluorouracil-induced apoptosis in MSI colorectal cancer cells through p53 modulation

Purpose  

Colorectal tumors (CRC) with microsatellite instability (MSI) show resistance to chemotherapy with 5-fluorouracil (5-FU), the most widely used pharmacological drug for CRC treatment. The aims of this study were to test the ability of quercetin (Q) and luteolin (L) to increase the sensitivity of MSI CRC cells to 5-FU and characterize the dependence of the effects on cells’ p53 status.     

Eicosapentaenoic acid induces Fas-mediated apoptosis through a p53-dependent pathway in hepatoma cells

Eicosapentaenoic acid (EPA) has been demonstrated to induce apoptosis and cell cycle arrest in various cancer cell lines in vitro. In this study, we investigated the anti-tumor effects of EPA on hepatoma cell lines and the mechanisms responsible for induced cell death. Three hepatoma cell lines tested had different p53 status: HepG2 with a wild-type p53; Hep3B, of which the endogenous p53 was deleted; and Huh7 with its p53 mutated. MTT assay showed reduced viability of HepG2 cells after exposure to EPA, and the cytotoxicity of EPA was time and dose dependent. However, EPA had no effect on the viability and cell death in the two other hepatoma cell lines containing dysfunctional p53. DNA fragmentation analysis and TUNEL (terminal deoxynucleotidyl transferase [TdT]-mediated deoxyuridine diphosphate [dUTP] nick end labeling) staining showed a typical pattern of DNA laddering and DNA breaks staining, respectively, in wild-type p53-containing HepG2 cells after EPA treatment. We also observed that EPA induced transient nuclear accumulation of P53 protein that subsequently up-regulated the expression of Fas messenger RNA and protein in HepG2 cells. In contrast, these findings were not observed in Hep3B and Huh7 cells exposed to EPA. Most notably, EPA-induced apoptosis in HepG2 cells could be reduced almost completely by treatment with FasL antisense oligonucleotides. We conclude that EPA inhibits the growth of HepG2 cells and mediates its effect, at least in part, via the Fas-mediated apoptosis. It appears that the effects of EPA on hepatoma cells are determined by the status of p53 and that wild-type p53 is a prerequisite for the anticancer effect of EPA.     

Piperine induces apoptosis of lung cancer A549 cells via p53-dependent mitochondrial signaling pathway

The aim of this study was to evaluate the cytotoxic and apoptotic effects of piperine on human lung cancer A549 cells and to explore its mechanisms. Piperine was found to exert the greatest cytotoxic effect against A549 cells in a dose-dependent manner, whereas it showed no effect on WI38 human lung fibroblasts. This cell growth-inhibitory effect might be attributed to cell DNA damage and cytotoxic effects. Besides, piperine had the ability to cause cell cycle arrest in G2/M phase and to activate caspase-3 and caspase-9 cascades in A549 cells. Furthermore, piperine-induced apoptosis could be blocked by the broad caspase inhibitor z-VAD-fmk in majority. In addition, piperine treatment decreased Bcl-2 protein expression, but increased Bax protein expression in A549 cells, which were positively correlated with an elevated expression of p53 compared to control. Taken together, these results suggested that piperine could induce p53-mediated cell cycle arrest and apoptosis via activation of caspase-3 and caspase-9 cascades, as well as increasing the Bax/Bcl-2 ratio. Thus, piperine could be developed as an effective antitumor agent in the prevention and treatment of lung cancer without toxicity to the host.     

糖原合酶激酶-3β促进卵巢癌细胞增殖的实验研究

目的探讨糖原合酶激酶-3β（GSK-3β）对卵巢癌细胞增殖的影响及其意义。方法以蛋白印迹法检测GSK-3β和磷酸化GSK-3β（pGSK-3β）在卵巢癌细胞株SKOV3和ES-2中的表达水平;采用细胞计数法描记细胞生长曲线,以检测GSK-3β抑制剂LiCl和SB216763对SKOV3和ES-2细胞生长的影响;将SKOV3细胞分成4组,将增加GSK-3β活性的质粒GSK-3βS9A及其空载体对照质粒pCS2、降低GSK-3β活性的质粒GID5-6及其对照质粒GID5-6LP,分别用电转法与绿色荧光蛋白（GFP）质粒共同转染入细胞中,采用5-溴脱氧尿嘧啶（BrdU）掺入实验检测GSK-3β活性对卵巢癌细胞增殖的影响;利用G418筛选瞬时转染的SKOV3细胞得到稳定转染株后,采用克隆形成实验,观察改变GSK-3β的活性对卵巢癌细胞增殖的长期影响。结果SKOV3和ES-2细胞均表达GSK-3β和pGSK-3β,SKOV3细胞中pGSK-3β的表达水平比ES-2细胞低,而两者GSK-3β的表达水平相近。GSK-3β抑制剂LiCl和SB216763可抑制SKOV3和ES-2细胞的生长。与对照pCS2相比,转染GSK-3βS9A质粒可提高GSK-3β的活性,增加SKOV3细胞BrdU的掺入率;与对照GID5-6LP相比,转染GID5-6质粒可降低GSK-3β的活性,减少SKOV3细胞BrdU的掺入率。分别与各自的对照质粒相比,稳定转染GSK-3βS9A可形成较多的细胞克隆,而稳定转染GID5-6则形成的细胞克隆较少。结论GSK-3β具有促进卵巢癌细胞增殖的作用,抑制GSK-3β的活性可望成为卵巢癌治疗的新靶点。     

Rapamycin induces p53-independent apoptosis through the mitochondrial pathway in non-small cell lung cancer cells

The mammalian target of rapamycin (mTOR) is a key kinase acting downstream of growth factor receptor PI3K and AKT signaling, leading to processes resulting in increased cell size and proliferation through translation control. Rapamycin, a specific inhibitor of mTOR, results predominately in G1 cell cycle arrest through translation control and occasionally, cell type-dependent apoptosis by an unknown mechanism. In this study, we investigated the effect and mechanism of action of rapamycin on non-small cell lung cancer (NSCLC) cell lines with p53 mutations. Cell proliferation was evaluated by modified MTT assay. The apoptotic effect of rapamycin was measured by caspase-3 activation and flow cytometric analysis of Annexin V binding. The expression of Bcl-2 and the release of cytochrome?c from mitochondria were evaluated by western blotting. We found that rapamycin induced apoptosis in NSCLC cell lines with p53 mutations. Western blot analysis demonstrated that rapamycin downregulates the expression levels of Bcl-2, which leads to increased cytochrome c release from mitochondria and subsequent activation of caspase cascades. These findings suggest that rapamycin induces p53-independent apoptosis through downregulation of Bcl-2 and the mitochondrial pathway in NSCLC cell lines as a novel antitumor mechanism.     

低氧条件下食管癌细胞中PRRX1通过p53介导线粒体通路诱导细胞凋亡的研究

目的:探究低氧条件下配对相关同源框1（PRRX1）通过p53介导的线粒体通路诱导食管癌细胞凋亡。方法:GEPIA2数据库分析PRRX1基因在食管癌组织和正常食管组织中的表达变化。RT-qPCR和Western blotting分别检测HEEC、Eca-109、TE-1细胞中PRRX1的mRNA和蛋白表达。二氧化钴处理模拟低氧微环境,在常氧和低氧条件下检测Eca-109、TE-1细胞中PRRX1的mRNA和蛋白表达情况。采用小干扰RNA（Si-PRRX1）转染TE-1细胞,设置分组为Hypoxia-Si-NC、Hypoxia-Si-PRRX1。流式细胞术检测TE-1细胞凋亡,RT-qPCR检测p53 mRNA表达,Western blotting检测p53、BCL-2、BAX、Cleaved-Caspase3等蛋白表达,在TE-1-PRRX1 KO细胞系中过表达p53,检测 BCL-2、BAX、Cleaved-Caspase3等蛋白表达。结果:PRRX1在食管癌组织及细胞系中均高表达。在TE-1细胞中敲低PRRX1,抑制细胞凋亡,下调p53的mRNA及蛋白表达,抑制BAX、Cleaved-Caspase3蛋白表达,促进BCL-2蛋白表达;过表达PRRX1则上调p53蛋白表达。在TE-1-PRRX1 KO细胞系中过表达p53显著抑制BCL-2蛋白表达,促进BAX、Cleaved-Caspase3蛋白表达。结论:低氧条件下,PRRX1通过p53介导的线粒体通路诱导食管癌细胞凋亡。     

Mesothelin regulates growth and apoptosis in pancreatic cancer cells through p53-dependent and -independent signal pathway

Mesothelin, a secreted protein, is overexpressed in some cancers, including pancreatic cancer. Rescent studies have shown that overexpression of mesothelin significantly increased tumor cell proliferation, and downregulation of mesothelin inhibited cell proliferation in pancreatic cancer cells, but its exact function and mechanism remains unclear. The aim of the present study was to evaluate the effects of mesothelin on proliferation and apoptosis in pancreatic cancer cells with different p53 status and to explore its signal pathway. Mesothelin levels were detected by western blot and RT-PCR assay in human pancreatic cancer AsPC-1, HPAC and Capan-2, Capan-1 and MIA PaCa-2 cell lines. Mesothelin was slienced by shRNA in AsPC-1, Capan-2 and Capan-1 cells with rich mesothelin level, and mesothelin was overexpressed in the HPAC and Capan-2 cells with less mesothelin level. We observed that in the AsPC-1 and Capan-1cells with mt-p53, and Capan-2 cells with wt-p53, shRNA mediated sliencing of the mesothelin significantly increased PUMA and Bax expression and caspase-3 activity, and decreased bcl-2 expression, followed by the reduced proliferation and colony forming capability and increased cell apoptosis. When PUMA was slienced by siRNA in the stable mesothelin shRNA transfected cells, proliferative capability was significantly increased, and apoptosis was decreased. However, in the Capan-2 cells with wt-p53, suppression of the mesothelin significantly increased wt-p53 levels. When p53 was blocked by siRNA in the stable mesothelin shRNA transfected Capan-2 cells, PUMA was inhibited, followed by increased proliferative capability and decreased cell apoptosis. In the HPAC and Capan-2 cells with wt-p53 and in the MIA PaCa-2 cells with mt-p53, overexpression of the mesothelin significantly decreased bax levels and increased bcl-2 levels, followed by increased proliferative and colony forming capability. Furthermore, mesothelin-shRNA-transfected cells exhibited a reduced rate of tumor growth under in vivo conditions. However, mesothelin-transfected cells exhibited a increased rate of tumor growth under in vivo conditions. Our data demonstrated that mesothelin promotes proliferation and inhibited apoptosis through p53-dependent pathway in pancreatic cancer cells with wt-p53, and p53-independent pathway in pancreatic cancer cells with mt-p53. Targeting mesothelin by shRNA is the important method for pancreatic cancer therapy.     

PPAR gamma ligand-induced apoptosis through a p53-dependent mechanism in human gastric cancer cells

We have recently demonstrated that the PPARγ ligand troglitazone induced cell growth arrest and evoked apoptosis in a gastric cancer cell line, MKN–45. Since in general, p53 plays an important role in the induction of apoptosis and growth inhibition, we tried to clarify whether or not p53 mediates troglitazone-induced apoptosis and growth arrest in gastric cancer cells. Troglitazone increased the number of apoptoic cells in MKN-28, MKN-45 and MKN-74, but not in KATO-III cells. The troglitazone-induced apo-ptotic change was significantly reduced by coincubation with bisphenol A digycidyl ether (BADGE), a synthetic PPARy antagonist, in MKN-74 cells, suggesting that PPARγ mediates the apo-ptotic effect of troglitazone. Since KATO-III lacks the p53 gene, we speculated that p53 might be implicated in the PPARγ ligand-induced apoptosis. Western blot analysis revealed that p53 expression was increased by troglitazone in a time-dependent manner in MKN-74 cells, further suggesting that p53 may mediate the ap-optotic process induced by troglitazone. We next established a dominant-negative p53 mutant by stable transfection of p53 mutant into MKN-74 cells. In the dominant-negative p53 mutant cells, troglitazone failed to induce apoptosis, strongly supporting the hypothesis that p53 indeed mediates the process of the troglitazone-induced apoptosis. In the dominant-negative p53 mutant cells, troglitazone significantly induced cell growth arrest and increased expression of p27^Kip1 protein, which is thought to be the key molecule to evoke growth arrest, suggesting that p53 is not involved in the growth inhibition by troglitazone. All these results suggest that p53 mediates the PPARy ligand-induced apoptosis, but not the cell growth inhibition. (Cancer Sci 2003; 94: 338–343)     

Phosphorylation of glycogen synthase kinase-3 beta at serine 9 confers cisplatin resistance in ovarian cancer cells

Cisplatin is commonly used in the treatment of advanced ovarian carcinoma. A major limitation of the use of cisplatin is the development of resistance in tumors. Glycogen synthase kinase-3 beta (GSK-3beta) is a multi-functional serine/threonine kinase. Its activity is regulated negatively by the phosphorylation of serine 9 (pGSK-3beta-ser-9) and positively by the phosphorylation of tyrosine 216 (pGSK-3beta-tyr-216). We compared the expression/phosphorylation of GSK-3beta between the cisplatin-sensitive ovarian carcinoma cell line A2780 and its cisplatin-resistant derivative CP70. The expression levels of total GSK-3beta and pGSK-3beta-tyr-216 were similar in these cells; however, CP70 cells had a much higher expression of pGSK-3beta-ser-9 than A2780 cells. Lithium chloride, which is a GSK-3beta inhibitor and stimulates pGSK-3beta-ser-9, significantly increased the IC50 of cisplatin and counteracted cisplatin-induced apoptosis of A2780 and CP70 cells. In contrast, overexpression of a constitutively active S9A GSK-3beta mutant increased the sensitivity of CP70 cells to cisplatin and significantly enhanced cisplatin-mediated apoptosis. It is suggested that the cisplatin-resistance of CP70 cells is mediated by stabilizing p53. We demonstrated that GSK-3beta negatively regulated the expression of p53. Therefore, pGSK-3beta-ser-9 may confer the cisplatin resistance of ovarian carcinomas through the stabilization of p53 expression. Our study establishes a potential role of GSK-3beta in the development of cisplatin resistance in initially sensitive tumors.     

Resveratrol suppresses cell transformation and induces apoptosis through a p53-dependent pathway

Resveratrol, a plant constituent enriched in the skin of grapes, is one of the most promising agents for the prevention of cancer. However, the mechanism of the anti-carcinogenic activity of resveratrol is not well understood. Here we offer a possible explanation of its anti-cancer effect. Resveratrol suppresses tumor promoter-induced cell transformation and markedly induces apoptosis, transactivation of p53 activity and expression of p53 protein in the same cell line and at the same dosage. Also, resveratrol-induced apoptosis occurs only in cells expressing wild-type p53 (p53+/+), but not in p53-deficient (p53-/-) cells, while there is no difference in apoptosis induction between normal lymphoblasts and sphingomyelinase-deficient cell lines. These results demonstrate for the first time that resveratrol induces apoptosis through activation of p53 activity, suggesting that its anti-tumor activity may occur through the induction of apoptosis.     

Glycogen synthase kinase-3 (GSK-3) inhibition induces apoptosis in leukemic cells through mitochondria-dependent pathway

The roles of glycogen synthase kinase-3 (GSK-3) in cell survival and apoptosis are controversial. We examined the effect of a specific GSK-3 inhibitor (SB-415286) on the regulation of leukemic cells proliferation and apoptosis. SB-415286 (40 μM) induced cell growth inhibition, β-catenin stabilization, cell cycle arrest in G(2)/M phase, cyclin B1 downregulation, and apoptosis in leukemic cell lines KG1a, K562, and CMK. Blocking the death receptor pathway by using a specific inhibitor of caspase-8, did not inhibit SB-415286-induced apoptosis. This indicates that activation of caspase-8 is part of the intrinsic apoptotic pathway and occurs downstream of mitochondria membrane potential depolarization mediated by other caspases. Furthermore, we found that depolarization of mitochondria membrane caused by GSK-3 inhibition is regulated by dephosphorylation of anti-apoptotic protein Bcl-2 and downregulation of Bcl-xL. Thus, inhibition of GSK-3-induced apoptosis of leukemic cells could be an attractive target for treatment of leukemia.     

CDDP induces p53-dependent apoptosis in tongue cancer cells

We have investigated the CDDP sensitivities of two tongue cancer cell lines with differing p53 genetic status, one with wild-type p53 (SAS) and the other with mutant-type p53 (HSC-4). SAS was about 2 times more sensitive at the D10 dose and demonstrated increased p53 and Bax protein levels at 10 h after CDDP treatment on Western blot analysis. On the other hand, overexpression of p53 in HSC-4 was observed without CDDP treatment and no elevation of Bax could be detected. Apoptosis was observed after CDDP treatment in SAS but not in HSC-4 by Hoechst 33342-staining and electrophoresis methods. These findings indicate that p53 plays an important role in apoptosis as a positive regulator of Bax expression. It is suggested that p53 status may have predictive potential with regard to response to CDDP therapy.     

p53 polymorphism influences response in cancer chemotherapy via modulation of p73-dependent apoptosis

Intact p73 function is shown to be an important determinant of cellular sensitivity to anticancer agents. Inhibition of p73 function by dominant-negative proteins or by mutant p53 abrogates apoptosis and cytotoxicity induced by these agents. A polymorphism encoding either arginine (72R) or proline (72P) at codon 72 of p53 influences inhibition of p73 by a range of p53 mutants identified in squamous cancers. Clinical response following cisplatin-based chemo-radiotherapy for advanced head and neck cancer is influenced by this polymorphism, cancers expressing 72R mutants having lower response rates than those expressing 72P mutants. Polymorphism in p53 may influence individual responsiveness to cancer therapy.     

ShRNA silencing glycogen synthase kinase-3 beta inhibits tumor growth and angiogenesis in pancreatic cancer

Glycogen synthase kinase-3 beta (GSK-3β), a serine/threonine protein kinase, plays a vital role in the tumorigenesis of many cancers, but its role in pancreatic cancer remains unknown. In this study, we showed that GSK-3β was aberrantly activated in pancreatic cancer. GSK-3β knockdown resulted in arrested proliferation and increased apoptosis in pancreatic cancer cell lines. Expression of Bcl-2 and vascular endothelial growth factor (VEGF) decreased significantly in a GSK-3β knockdown group. In a xenograft tumor model, GSK-3β knockdown inhibited tumor growth and angiogenesis. Our study showed that GSK-3β may become a promising therapeutic target for human pancreatic cancer.     

过表达PRRX1 通过p53 介导的线粒体凋亡途径诱导肝癌SMMC7721 细胞凋亡

[摘要] 目的：探讨配对相关同源框1 蛋白（PRRX1）过表达对肝癌SMMC7721 细胞凋亡的影响及其分子机制。方法：分别用慢病毒介导PRRX1 过表达载体（pGMLV-PRRX1）、空载质粒（Vector）感染人肝癌SMMC7721 细胞,用qPCR和WB实验检测慢病毒感染后细胞中PRRX1 mRNA和蛋白的表达变化,用CCK-8 法、Annexin-V FITC/PI 染色流式细胞术分别检测PRRX1 过表达对SMMC7721 细胞增殖、凋亡的影响,用线粒体膜电位检测试剂盒（JC-10 染色法）检测细胞线粒体膜电位变化,用caspase 活性检测试剂盒（分光光度法）测定细胞中caspase-8 和caspase-9 酶活性,用WB实验检测细胞中p53、Bcl-2、Bax、Fas、Cleaved-caspase-3以及线粒体和细胞质中细胞色素C（Cty C）蛋白的表达。结果：成功构建PRRX1 过表达的SMMC7721 细胞株,感染细胞中PRRX1 mRNA和蛋白的表达水平显著升高（均P<0.01）。与对照组和空载组比较,PRRX1 过表达组SMMC7721 细胞的增殖能力显著下降、细胞凋亡率显著增高、Cleaved-caspase-3 剪切水平显著升高、线粒体膜电位显著下降、线粒体中Cty C蛋白表达下调、胞质中Cty C蛋白表达上调以及caspase-9 酶活性升高（P<0.05 或P<0.01）,同时p53 和Bax 蛋白表达增加而Bcl-2 蛋白表达降低（均P<0.05）,但Fas 蛋白表达及caspase-8 酶活性无显著变化（均P>0.05）。结论: PRRX1 过表达可诱导肝癌SMMC7721 细胞凋亡,其机制可能与p53介导的线粒体凋亡途径被激活有关。