The PTEN/MMAC1 tumor suppressor phosphatase functions as a negative regulator of the phosphoinositide 3-kinase/Akt pathway

The PTEN/MMAC1 phosphatase is a tumor suppressor gene implicated in a wide range of human cancers. Here we provide biochemical and functional evidence that PTEN/MMAC1 acts a negative regulator of the phosphoinositide 3-kinase (PI3-kinase)/Akt pathway. PTEN/MMAC1 impairs activation of endogenous Akt in cells and inhibits phosphorylation of 4E-BP1, a downstream target of the PI3-kinase/Akt pathway involved in protein translation, whereas a catalytically inactive, dominant negative PTEN/MMAC1 mutant enhances 4E-BP1 phosphorylation. In addition, PTEN/MMAC1 represses gene expression in a manner that is rescued by Akt but not PI3-kinase. Finally, higher levels of Akt activation are observed in human prostate cancer cell lines and xenografts lacking PTEN/MMAC1 expression when compared with PTEN/MMAC1-positive prostate tumors or normal prostate tissue. Because constitutive activation of either PI3-kinase or Akt is known to induce cellular transformation, an increase in the activation of this pathway caused by mutations in PTEN/MMAC1 provides a potential mechanism for its tumor suppressor function.

Replacement of Fhit in cancer cells suppresses tumorigenicity

The candidate tumor suppressor gene, FHIT, encompasses the common human chromosomal fragile site at 3p14.2, the hereditary renal cancer translocation breakpoint, and cancer cell homozygous deletions. Fhit hydrolyzes dinucleotide 5′,5‴-P1,P3-triphosphate in vitro and mutation of a central histidine abolishes hydrolase activity. To study Fhit function, wild-type and mutant FHIT genes were transfected into cancer cell lines that lacked endogenous Fhit. No consistent effect of exogenous Fhit on growth in culture was observed, but Fhit and hydrolase “dead” Fhit mutant proteins suppressed tumorigenicity in nude mice, indicating that 5′,5‴-P1,P3-triphosphate hydrolysis is not required for tumor suppression.

Role of transforming growth factor-α in von Hippel– Lindau (VHL)−/− clear cell renal carcinoma cell proliferation: A possible mechanism coupling VHL tumor suppressor inactivation and tumorigenesis

Mutations of the VHL tumor suppressor gene occur in patients with VHL disease and in the majority of sporadic clear cell renal carcinomas (VHL−/− RCC). Loss of VHL protein function is associated with constitutive expression of mRNAs encoding hypoxia-inducible proteins, such as vascular endothelial growth factor. Overproduction of angiogenic factors might explain why VHL−/− RCC tumors are so highly vascularized, but whether this overproduction is sufficient for oncogenesis still remains unknown. In this report, we examined the activity of transforming growth factor-α (TGF-α), another VHL-regulated growth factor. We show that TGF-α mRNA and protein are hypoxia-inducible in VHL−/− RCC cells expressing reintroduced VHL. In addition to its overexpression by VHL−/− RCC cells, TGF-α can also act as a specific growth-stimulatory factor for VHL−/− RCC cells expressing reintroduced wild-type VHL, as well as primary renal proximal tubule epithelial cells, the likely site of origin of RCC. This role is in contrast to those of other growth factors overexpressed by VHL−/− RCC cells, such as vascular endothelial growth factor and TGF-β1, which do not stimulate RCC cell proliferation. A TGF-α-specific antisense oligodeoxynucleotide blocked TGF-α production in VHL−/− RCC cells, which led to the dependence of those cells on exogenous growth factors to sustain growth in culture. Growth of VHL−/− RCC cells was also significantly reduced by a drug that specifically inhibits the epidermal growth factor receptor, the receptor through which TGF-α stimulates proliferation. These results suggest that the generation of a TGF-α autocrine loop as a consequence of VHL inactivation in renal proximal tubule epithelial cells may provide the uncontrolled growth stimulus necessary for the initiation of tumorigenesis.

Vascular tumors in livers with targeted inactivation of the von Hippel–Lindau tumor suppressor

von Hippel–Lindau (VHL) disease is a pleomorphic familial tumor syndrome that is characterized by the development of highly vascularized tumors. Homozygous disruption of the VHL gene in mice results in embryonic lethality. To investigate VHL function in the adult we have generated a conditional VHL null allele (2-lox allele) and null allele (1-lox allele) by Cre-mediated recombination in embryonic stem cells. We show here that mice heterozygous for the 1-lox allele develop cavernous hemangiomas of the liver, a rare manifestation of the human disease. Histologically these tumors were associated with hepatocellular steatosis and focal proliferations of small vessels. To study the cellular origin of these lesions we inactivated VHL tissue-specifically in hepatocytes. Deletion of VHL in the liver resulted in severe steatosis, many blood-filled vascular cavities, and foci of increased vascularization within the hepatic parenchyma. These histopathological changes were similar to those seen in livers from mice heterozygous for the 1-lox allele. Hypoxia-inducible mRNAs encoding vascular endothelial growth factor, glucose transporter 1, and erythropoietin were up-regulated. We thus provide evidence that targeted inactivation of mouse VHL can model clinical features of the human disease and underline the importance of the VHL gene product in the regulation of hypoxia-responsive genes in vivo.

Genetic disruption of PPARδ decreases the tumorigenicity of human colon cancer cells

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that have been implicated in a variety of biologic processes. The PPARδ isotype was recently proposed as a downstream target of the adenomatous polyposis coli (APC)/β-catenin pathway in colorectal carcinogenesis. To evaluate its role in tumorigenesis, a PPARδ null cell line was created by targeted homologous recombination. When inoculated as xenografts in nude mice, PPARδ −/− cells exhibited a decreased ability to form tumors compared with PPARδ +/− and wild-type controls. These data suggest that suppression of PPARδ expression contributes to the growth-inhibitory effects of the APC tumor suppressor.

PEA3 sites within the progression elevated gene-3 (PEG-3) promoter and mitogen-activated protein kinase contribute to differential PEG-3 expression in Ha-ras and v-raf oncogene transformed rat embryo cells

Transformation of normal cloned rat embryo fibroblast (CREF) cells with cellular oncogenes results in acquisition of anchorage-independent growth and oncogenic potential in nude mice. These cellular changes correlate with an induction in the expression of a cancer progression-promoting gene, progression elevated gene-3 (PEG-3). To define the mechanism of activation of PEG-3 as a function of transformation by the Ha-ras and v-raf oncogenes, evaluations of the signaling and transcriptional regulation of the ~2.0 kb promoter region of the PEG-3 gene, PEG-Prom, was undertaken. The full-length and various mutated regions of the PEG-Prom were linked to a luciferase reporter construct and tested for promoter activity in CREF and oncogene-transformed CREF cells. An analysis was also performed using CREF cells doubly transformed with Ha-ras and the Ha-ras specific suppressor gene Krev-1, which inhibits the transformed phenotype in vitro. These assays document an association between expression of the transcription regulator PEA3 and PEG-3. The levels of PEA3 and PEG-3 RNA and proteins are elevated in the oncogenically transformed CREF cells, and reduced in transformation and tumorigenic suppressed Ha-ras/Krev-1 doubly transformed CREF cells. Enhanced tumorigenic behavior, PEG-3 promoter function and PEG-3 expression in Ha-ras transformed cells were all dependent upon increased activity within the mitogen-activated protein kinase (MAPK) pathway. Electrophoretic mobility shift assays and DNase I footprinting experiments indicate that PEA3 binds to sites within the PEG-Prom in transformed rodent cells in an area adjacent to the TATA box in a MAPK-dependent fashion. These findings demonstrate an association between Ha-ras and v-raf transformation of CREF cells with elevated PEA3 and PEG-3 expression, and they implicate MAPK signaling via PEA3 as a signaling cascade involved in activation of the PEG-Prom.

Tumor metastasis suppressor nm23H1 regulates Rac1 GTPase by interaction with Tiam1

The putative tumor metastasis suppressor nm23H1 was originally identified in murine melanomas by subtraction cloning. It displays nucleoside diphosphate kinase activity and regulates cellular events, including growth and development. Recently nm23H1 has been reported to also act as a GTPase-activating protein of the Ras-related GTPase Rad. We attempted to determine whether nm23H1 also regulates Rho-family GTPases. Although we were unable to detect a direct association between nm23H1 and Rho-family GTPases, nm23H1 was shown to be associated with a Rac1-specific nucleotide exchange factor, Tiam1, by interaction with its amino-terminal region in extracts from the cells expressing exogenous Tiam1 and from native tissue. Overexpression of nm23H1 inhibited the Tiam1-induced production of GTP-bound Rac1 and activation of c-Jun kinase. On the other hand, forced overexpression of the wild type, but not the kinase-inactivated mutant of nm23H1, converted the GDP-bound forms of Rac1, Cdc42, and RhoA to their GTP-bound forms in vitro by its nucleoside diphosphate kinase activity, but nm23H1 alone apparently did not produce the GTP-bound form of these GTPases in vivo. These results suggest that nm23H1 negatively regulates Tiam1 and inhibits Rac1 activation in vivo. Moreover, adhesion-stimulated membrane ruffles of Rat1 fibroblasts were reduced by overexpression of nm23H1. Based on these observations, we concluded that we had identified a function of nm23H1 as a regulator of Rac1 and that it may be related to the effect of nm23H1 as a tumor metastasis suppressor.

Candidate tumor suppressor HYAL2 is a glycosylphosphatidylinositol (GPI)-anchored cell-surface receptor for jaagsiekte sheep retrovirus, the envelope protein of which mediates oncogenic transformation

Jaagsiekte sheep retrovirus (JSRV) can induce rapid, multifocal lung cancer, but JSRV is a simple retrovirus having no known oncogenes. Here we show that the envelope (env) gene of JSRV has the unusual property that it can induce transformation in rat fibroblasts, and thus is likely to be responsible for oncogenesis in animals. Retrovirus entry into cells is mediated by Env interaction with particular cell-surface receptors, and we have used phenotypic screening of radiation hybrid cell lines to identify the candidate lung cancer tumor suppressor HYAL2/LUCA2 as the receptor for JSRV. HYAL2 was previously described as a lysosomal hyaluronidase, but we show that HYAL2 is actually a glycosylphosphatidylinositol (GPI)-anchored cell-surface protein. Furthermore, we could not detect hyaluronidase activity associated with or secreted by cells expressing HYAL2, whereas we could easily detect such activity from cells expressing the related serum hyaluronidase HYAL1. Although the function of HYAL2 is currently unknown, other GPI-anchored proteins are involved in signal transduction, and some mediate mitogenic responses, suggesting a potential role of HYAL2 in JSRV Env-mediated oncogenesis. Lung cancer induced by JSRV closely resembles human bronchiolo-alveolar carcinoma, a disease that is increasing in frequency and now accounts for ≈25% of all lung cancer. The finding that JSRV env is oncogenic and the identification of HYAL2 as the JSRV receptor provide tools for further investigation of the mechanism of JSRV oncogenesis and its relationship to human bronchiolo-alveolar carcinoma.

ANX7, a candidate tumor suppressor gene for prostate cancer

The ANX7 gene is located on human chromosome 10q21, a site long hypothesized to harbor a tumor suppressor gene(s) (TSG) associated with prostate and other cancers. To test whether ANX7 might be a candidate TSG, we examined the ANX7-dependent suppression of human tumor cell growth, stage-specific ANX7 expression in 301 prostate specimens on a prostate tissue microarray, and loss of heterozygosity (LOH) of microsatellite markers at or near the ANX7 locus. Here we report that human tumor cell proliferation and colony formation are markedly reduced when the wild-type ANX7 gene is transfected into two prostate tumor cell lines, LNCaP and DU145. Consistently, analysis of ANX7 protein expression in human prostate tumor microarrays reveals a significantly higher rate of loss of ANX7 expression in metastatic and local recurrences of hormone refractory prostate cancer as compared with primary tumors (P = 0.0001). Using four microsatellite markers at or near the ANX7 locus, and laser capture microdissected tumor cells, 35% of the 20 primary prostate tumors show LOH. The microsatellite marker closest to the ANX7 locus showed the highest rate of LOH, including one homozygous deletion. We conclude that the ANX7 gene exhibits many biological and genetic properties expected of a TSG and may play a role in prostate cancer progression.

Human tumor suppressor EXT gene family members EXTL1 and EXTL3 encode α1,4- N-acetylglucosaminyltransferases that likely are involved in heparan sulfate/ heparin biosynthesis

The tumor suppressors EXT1 and EXT2 are associated with hereditary multiple exostoses and encode bifunctional glycosyltransferases essential for chain polymerization of heparan sulfate (HS) and its analog, heparin (Hep). Three highly homologous EXT-like genes, EXTL1–EXTL3, have been cloned, and EXTL2 is an α1,4-GlcNAc transferase I, the key enzyme that initiates the HS/Hep synthesis. In the present study, truncated forms of EXTL1 and EXTL3, lacking the putative NH2-terminal transmembrane and cytoplasmic domains, were transiently expressed in COS-1 cells and found to harbor α-GlcNAc transferase activity. EXTL3 used not only N-acetylheparosan oligosaccharides that represent growing HS chains but also GlcAβ1–3Galβ1-O-C2H4NH-benzyloxycarbonyl (Cbz), a synthetic substrate for α-GlcNAc transferase I that determines and initiates HS/Hep synthesis. In contrast, EXTL1 used only the former acceptor. Neither EXTL1 nor EXTL3 showed any glucuronyltransferase activity as examined with N-acetylheparosan oligosaccharides. Heparitinase I digestion of each transferase-reaction product showed that GlcNAc had been transferred exclusively through an α1,4-configuration. Hence, EXTL3 most likely is involved in both chain initiation and elongation, whereas EXTL1 possibly is involved only in the chain elongation of HS and, maybe, Hep as well. Thus, their acceptor specificities of the five family members are overlapping but distinct from each other, except for EXT1 and EXT2 with the same specificity. It now has been clarified that all of the five cloned human EXT gene family proteins harbor glycosyltransferase activities, which probably contribute to the synthesis of HS and Hep.

Enhanced antitumor efficacy of a herpes simplex virus mutant isolated by genetic selection in cancer cells

Replication-competent, attenuated herpes simplex virus-1 (HSV-1) derivatives that contain engineered mutations into the viral γ34.5 virulence gene have been used as oncolytic agents. However, as attenuated mutants often grow poorly, they may not completely destroy some tumors and surviving cancer cells simply regrow. Thus, although HSV-1 γ34.5 mutants can reduce the growth of human tumor xenografts in mice and have passed phase I safety studies, their efficacy is limited because they replicate poorly in many human tumor cells. Previously, we selected for a γ34.5 deletion mutant variant that regained the ability to replicate efficiently in tumor cells. Although this virus contains an extragenic suppressor mutation that confers enhanced growth in tumor cells, it remains attenuated. Here, we demonstrate that the suppressor virus replicates to greater levels in prostate carcinoma cells and, importantly, is a more potent inhibitor of tumor growth in an animal model of human prostate cancer than the γ34.5 parent virus. Thus, genetic selection in cancer cells can be used as a tool to enhance the antitumor activity of a replication-competent virus. The increased therapeutic potency of this oncolytic virus may be useful in the treatment of a wide variety of cancers.

Transcription factor EGR-1 suppresses the growth and transformation of human HT-1080 fibrosarcoma cells by induction of transforming growth factor beta 1.

The early growth response 1 (EGR-1) gene product is a transcription factor with role in differentiation and growth. We have previously shown that expression of exogenous EGR-1 in various human tumor cells unexpectedly and markedly reduces growth and tumorigenicity and, conversely, that suppression of endogenous Egr-1 expression by antisense RNA eliminates protein expression, enhances growth, and promotes phenotypic transformation. However, the mechanism of these effects remained unknown. The promoter of human transforming growth factor beta 1 (TGF-beta 1) contains two GC-rich EGR-1 binding sites. We show that expression of EGR-1 in human HT-1080 fibrosarcoma cells uses increased secretion of biologically active TGF-beta 1 in direct proportion (rPearson = 0.96) to the amount of EGR-1 expressed and addition of recombinant human TGF-beta 1 is strongly growth-suppressive for these cells. Addition of monoclonal anti-TGF-beta 1 antibodies to EGR-1-expressing HT-1080 cells completely reverses the growth inhibitory effects of EGR-1. Reporter constructs bearing the EGR-1 binding segment of the TGF-beta 1 promoter was activated 4- to 6-fold relative to a control reporter in either HT-1080 cells that stably expressed or parental cells cotransfected with an EGR-1 expression vector. Expression of delta EGR-1, a mutant that cannot interact with the corepressors, nerve growth factor-activated factor binding proteins NAB1 and NAB2, due to deletion of the repressor domain, exhibited enhanced transactivation of 2- to 3.5-fold over that of wild-type EGR-1 showing that the reporter construct reflected the appropriate in vivo regulatory context. The EGR-1-stimulated transactivation was inhibited by expression of the Wilms tumor suppressor, a known specific DNA-binding competitor. These results indicate that EGR-1 suppresses growth of human HT-1080 fibrosarcoma cells by induction of TGF-beta 1.

Post-transcriptional regulation of vascular endothelial growth factor mRNA by the product of the VHL tumor suppressor gene.

The VHL tumor suppressor gene is inactivated in patients with von Hippel-Lindau disease and in most sporadic clear cell renal carcinomas. Although VHL protein function remains unclear, VHL does interact with the elongin BC subunits in vivo and regulates RNA polymerase II elongation activity in vitro by inhibiting formation of the elongin ABC complex. Expression of wild-type VHL in renal carcinoma cells with inactivated endogenous VHL resulted in unaltered in vitro cell growth and decreased vascular endothelial growth factor (VEGF) mRNA expression and responsiveness to serum deprivation. VEGF is highly expressed in many tumors, including VHL-associated and sporadic renal carcinomas, and it stimulates neoangiogenesis in growing solid tumors. Despite 5-fold differences in VEGF mRNA levels, VHL overexpression did not affect VEGF transcription initiation or elongation as would have been suggested by VHL-elongin association. These results suggest that VHL regulates VEGF expression at a post-transcriptional level and that VHL inactivation in target cells causes a loss of VEGF suppression, leading to formation of a vascular stroma.

Use of genetic suppressor elements to dissect distinct biological effects of separate p53 domains.

p53 is a multifunctional tumor suppressor protein involved in the negative control of cell growth. Mutations in p53 cause alterations in cellular phenotype, including immortalization, neoplastic transformation, and resistance to DNA-damaging drugs. To help dissect distinct functions of p53, a set of genetic suppressor elements (GSEs) capable of inducing different p53-related phenotypes in rodent embryo fibroblasts was isolated from a retroviral library of random rat p53 cDNA fragments. All the GSEs were 100-300 nucleotides long and were in the sense orientation. They fell into four classes, corresponding to the transactivator (class I), DNA-binding (class II), and C-terminal (class III) domains of the protein and the 3'-untranslated region of the mRNA (class IV). GSEs in all four classes promoted immortalization of primary cells, but only members of classes I and III cooperated with activated ras to transform cells, and only members of class III conferred resistance to etoposide and strongly inhibited transcriptional transactivation by p53. These observations suggest that processes related to control of senescence, response to DNA damage, and transformation involve different functions of the p53 protein and furthermore indicate a regulatory role for the 3'-untranslated region of p53 mRNA.

Forced expression of the tumor suppressor adenomatosis polyposis coli protein induces disordered cell migration in the intestinal epithelium.

Mutations of the human adenomatosis polyposis coli (APC) gene are associated with the development of familial as well as sporadic intestinal neoplasia. To examine the in vivo function of APC, 129/Sv embryonic stem (ES) cells were transfected with DNA encoding the wild-type human protein under the control of a promoter that is active in all four of the small intestine's principal epithelial lineages during their migration-associated differentiation. ES-APC cells were then introduced into C57BL/6-ROSA26 blastocysts. Analyses of adult B6-ROSA26<-->129/Sv-APC chimeric mice revealed that forced expression of APC results in markedly disordered cell migration. When compared with the effects of forced expression of E-cadherin, the data suggest that APC-catenin and E-cadherin-catenin complexes have opposing effects on intestinal epithelial cell movement/adhesiveness; augmentation of E-cadherin-beta-catenin complexes produces a highly ordered, "adhesive" migration, whereas augmentation of APC-beta-catenin complexes produces a disordered, nonadhesive migratory phenotype. We propose that APC mutations may promote tumorigenesis by increasing the relative activity of cadherin-catenin complexes, resulting in enhanced adhesiveness and functional anchorage of initiated cells within the intestinal crypt. Our studies also indicate that chimeric mice generated from B6-ROSA26 blastocysts and genetically manipulated ES cells should be useful for auditing gene function in the gastrointestinal tract and in other tissues.