Activation of the mas oncogene involves coupling to human alphoid sequences

We have shown previously that mouse NIH3T3 cells transfected with DNA from a human ovarian carcinoma were rendered tumourigenic by an activated mas oncogene in four independent transfection experiments. In all cases the 5'-noncoding region was rearranged in comparison to the original ovarian tumour DNA. We now report that in all four transfectants the newly acquired sequences consist of human centromeric alpha satellite repeat DNA. In at least three transfectants the alphoid DNA originates from the centromere of chromosome three. Analysis of the sequences of the recombination site in one transfectant revealed that a homologous sequence of five base pairs (CAGCA) is present in both parental strands, and might thus have contributed to the recombinational event. To establish a conclusive role for alphoid DNA in the activation of mas, we performed a co-transfection experiment in NIH3T3 cells with cloned alphoid DNA and the mas coding sequence. We show that the transfectants expressing a transformed phenotype contain amplified mas linked to alphoid DNA. NIH3T3 cells transfected with plasmids that contained alphoid sequences cloned directly upstream of the mas coding sequence, and injected into nude mice, gave rise to tumours with amplified mas sequences (7/7). In six of these tumours the alphoid sequences were amplified as well. Our data suggest a novel mechanism of oncogene activation: recombination with normal alphoid repeat DNA resulting in amplification of the oncogene.     

Reversible tumorigenesis in mice by conditional expression of the HER2/c-erbB2 receptor tyrosine kinase

In the present study we describe the reversible transformation of NIH3T3 fibroblasts by overexpression of the HER2/c-erbB2 receptor tyrosine kinase. Cell lines expressing HER2 under control of a tetracycline-responsive promoter were isolated. Induction of HER2 expression resulted in cellular transformation in vitro as depicted by growth in soft agar and focus formation in tissue culture. Subsequent treatment of these cells with the effector anhydrotetracyline switched-off HER2 expression and induced morphological and functional changes characteristic for non-transformed cells. Subcutaneous transplantation of cells in nude mice resulted in the formation of solid tumors. Interestingly tumor formation was completely suppressed by treatment of the animals with anhydrotetracyline. Our findings indicate that overexpression of HER2 induces the transformed phenotype of NIH3T3 cells and is required for tumor formation and progression in nude mice. By linking the expression of the marker gene secreted placental alkaline phosphatase to the expression of HER2, a sensitive monitoring of tumor development in nude mice was feasible.     

Analysis of the Plasmodium vivax dihydrofolate reductase-thymidylate synthase gene sequence

Phosphoinositide-specific phospholipase Cgamma (PLCgamma) is a key regulatory enzyme that binds to the phosphoryl-tyrosine residues in the cytoplasmic domain of certain activated receptors and catalyses the hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] forming IP3 and diacylglycerol (DAG) in response to several mitogenic factors. Previously, we determined that microinjected PLCgamma induces DNA synthesis in G0-arrested NIH 3T3 cells, suggesting the possibility that PLCgamma may have an oncogenic potential. In this report, we demonstrate that overexpression of PLCgamma in NIH 3T3 cells results in altered growth properties and cellular transformation. The PLCgamma/3T3 transfectants do not require serum growth factors to proliferate, display anchorage-independent growth in soft agar and induce tumors when transplanted into nude mice. These findings suggest that overexpression of PLCgamma facilitates the transformation of NIH 3T3 cells. Furthermore, PLCgamma expression and activity have been shown to be elevated in many human tumors. Thus, PLCgamma signaling may contribute to the promotion and/or progression of human cancers.     

Mutations in the extracellular domain cause RET loss of function by a dominant negative mechanism

The RET proto-oncogene encodes a tyrosine kinase receptor expressed in neuroectoderm-derived cells. Mutations in specific regions of the gene are responsible for the tumor syndromes multiple endocrine neoplasia types 2A and 2B (MEN 2A and 2B), while mutations along the entire gene are involved in a developmental disorder of the gastrointestinal tract, Hirschsprung's disease (HSCR disease). Two mutants in the extracellular domain of RET, one associated with HSCR disease and one carrying a flag epitope, were analyzed to investigate the impact of the mutations on RET function. Both mutants were impeded in their maturation, resulting in the lack of the 170-kDa mature form and the accumulation of the 150-kDa immature form in the endoplasmic reticulum. Although not exposed on the cell surface, the 150-kDa species formed dimers and aggregates; this was more pronounced in a double mutant bearing a MEN 2A mutation. Tyrosine phosphorylation and the transactivation potential were drastically reduced in single and double mutants. Finally, in cotransfection experiments both mutants exerted a dominant negative effect over protoRET and RET2A through the formation of a heteromeric complex that prevents their maturation and function. These results suggest that HSCR mutations in the extracellular region cause RET loss of function through a dominant negative mechanism.     

Germline dinucleotide mutation in codon 883 of the RET proto-oncogene in multiple endocrine neoplasia type 2B without codon 918 mutation

The autosomal dominant multiple endocrine neoplasia type 2 syndromes (MEN 2) comprise three clinically distinct entities, MEN 2A, familial medullary thyroid carcinoma and MEN 2B, which share a common clinical feature: medullary thyroid carcinoma (MTC). MEN 2B is considered to have the most aggressive form of MTC. Therefore, early detection of MEN 2B in order to prevent potentially lethal MTC is important. More than 95% of all MEN 2B cases are caused by germline mutation at codon 918 (M918T) in exon 16 of the RET proto-oncogene. In this study, we demonstrate the presence of germline codon 883 mutation (A883F) in 2 of 3 unrelated MEN 2B cases without codon 918 mutation. Our data demonstrate a novel etiologic event which may have roles in predisposition to MEN 2B when present in the germline and in the pathogenesis of sporadic MTC when somatic.     

Ornithine decarboxylase transformation of NIH/3T3 cells is mediated by altered epidermal growth factor receptor activity

Ornithine decarboxylase (ODC) has been shown to be oncogenic in transfected NIH/3T3 cells overexpressing the enzyme from a heterologous promoter. These cells, designated as NODC-2 cells, acquire proliferative properties associated with tumorigenic transformation such as loss of contact inhibition, decreased population doubling time, anchorage-independent growth, and tumor production in nude mice. At least one of these parameters, loss of contact inhibition, remains dependent on elevated ODC levels. We have used these cells to investigate the molecular mechanisms by which ODC overexpression drives cell transformation and to examine the involvement of other proto-oncogene products in this process. An interaction between ODC overexpression and the epidermal growth factor receptor (EGF-R) was suggested initially by the elevation of both basal (300%) and ligand-induced (457%) EGF-R tyrosine kinase activities in NODC-2 cells compared to similarly treated control NLK cells. Disruption of EGF-R mediated signal transduction in NODC-2 cells both by treatment with tyrphostin-25 or by transfection with a vector expressing a dominant negative EGF-R mutant resulted in reacquisition of contact-inhibited growth and suppression of anchorage-independent, clonogenic growth in soft agar. We conclude that ODC-induced transformation of NIH/3T3 cells is mediated, at least partly, by alterations in EGF-R signal transduction activity.     

Mutation analysis of glial cell line-derived neurotrophic factor, a ligand for an RET/coreceptor complex, in multiple endocrine neoplasia type 2 and sporadic neuroendocrine tumors

Causative germline missense mutations in the RET proto-oncogene have been associated with over 92% of families with the inherited cancer syndrome multiple endocrine neoplasia type 2 (MEN 2). MEN 2A is characterized primarily by medullary thyroid carcinoma (MTC) and pheochromocytoma, both tumors of neural crest origin. Parathyroid hyperplasia or adenoma is also seen in MEN 2A, but rarely in MEN 2B, which has additional stigmata, including a marfanoid habitus, mucosal neuromas, and ganglioneuromatosis of the gastrointestinal tract. In familial MTC, MTC is the only lesion present. Somatic RET mutations have also been identified in a subset of sporadic MTCs, pheochromocytomas, and rarely, small cell lung cancer, but not in sporadic parathyroid hyperplasias/adenomas or other neuroendocrine tumors. Glial cell line-derived neurotrophic factor (GDNF) and its receptor molecule GDNFR-alpha, have recently been identified as members of the RET ligand binding complex. Therefore, the genes encoding both GDNF and GDNFR-alpha are excellent candidates for a role in the pathogenesis of those MEN 2 families and sporadic neuroendocrine tumors without RET mutations. No mutations were found in the coding region of GDNF in DNA samples from 9 RET mutation negative MEN 2 individuals (comprising 6 distinct families), 12 sporadic MTCs, 17 sporadic cases of parathyroid adenoma, and 10 small cell lung cancer cell lines. Therefore, we find no evidence that mutation within the coding regions of GDNF plays a role in the genesis of MEN 2 and sporadic neuroendocrine tumors.     

RET protooncogene mutational analysis in multiple endocrine neoplasia syndrome type 2B: case report and review of the literature

Multiple endocrine neoplasia, type 2B (MEN 2B), is a phenotypic variant of a group of autosomal-dominant neurocristopathies. MEN 2B is associated with medullary thyroid carcinoma and pheochromocytoma with oral, ocular, and alimentary submucosal ganglioneuromas and Marfanoid body features. Approximately 50% of cases are thought to be spontaneous mutations. The RET protooncogene (RET) is a 21-exon gene encoding a tyrosine kinase receptor. A codon 918 germ line mutation, which converts a highly conserved methionine to a threonine in the intracellular tyrosine kinase portion of this receptor of RET, has been identified in 95% of patients with MEN 2B. This mutation is easily detected by a direct deoxyribonucleic acid sequencing or restriction enzyme (Fok 1) analysis of amplified polymerase chain reaction products. The RET gene is normally expressed in the oral and gastrointestinal submucosal neural ganglia, and the codon 918 mutation is thought to cause neuromas by virtue of its transforming activity in these ganglia. Identifying clinical features of MEN 2B in an 11-year-old boy by an oral pathologist led to confirmation by mutational analysis. Before genetic testing was available, the patient, and at a later date his mother, underwent thyroidectomies based solely on biochemical testing. Results indicated the patient had the codon 918 mutation, whereas his phenotypically normal mother, father, and older brother had normal RET analyses. Studies in families have demonstrated that the mutant allele is derived from the father with possible acquisition during spermatogenesis. We believe the mother of our affected patient to be normal; the absence of phenotypic features of MEN 2B and a normal genotype suggest her calcitonin abnormalities and minimal evidence for C-cell hyperplasia were inconsequential. Molecular analysis for RET abnormalities will likely supplant biochemical methods of diagnosis in patients with MEN 2B.     

Invasion of fibrosarcoma cells transfected with PAI-2 gene

The effect of PAI-2 on the invasion of fibrosarcoma cells in vitro and in vivo was investigated. The control cells (C+, C+ pem) and PAI-2 transfectants (C+ exp) were used in the assay of the degradation of isotopically labelld 3H-ECM and were injected sc into athymic/nude mice. Recombinant PAI-2 could inhibit efficiently the degradation of 3H-ECM by tumor cells (86.5%). The PAI-2 transfectants remained tumorigenic in nude mice, but tumors originating from the PAI-2 transfectants showed histologically the presence of a thick capsule which was absent in tumors from control cells.     

The physical map of the human RET proto-oncogene

The RET proto-oncogene, a transmembrane tyrosine kinase receptor, is involved in the development of at least five different disease phenotypes. RET is activated through somatic rearrangements in a number of cases of papillary thyroid carcinoma while germ-line point mutations are associated with three inherited cancer syndromes MEN 2A, MEN 2B and FMTC. Moreover, point mutations or heterozygous deletions of RET are found in the dominant form of Hirschsprung disease or congenital colonic aganglionosis. We cloned the entire RET genomic sequence in a contig of cosmids encompassing 150 kb, from the CA repeat sTCL-2 to the region upstream the RET promoter, and established the position of the 20 exons of the RET gene with respect to a detailed restriction map based on eight endonucleases. A new highly polymorphic CA repeat sequence was identified within intron 5 of RET (RET-INT5). Finally the orientation of RET on chromosome 10q11.2 made it possible to orientate three other genes rearranged with RET in papillary thyroid carcinomas, namely H4/D10S170 on 10q21, R1 alpha on 17q23 and RFG2/Ele1 on 10q11.2.     

Malignant transformation by overproduction of translation initiation factor eIF4G

N4G3, a cell line that overexpresses translation initiation factor eIF4G, one of the components of eIF4F, was made by stable transfection of the human eIF4G cDNA into NIH3T3 cells. The cells expressed 80-100 times greater levels of eIF4G mRNA than did NIH3T3 cells. N4G3 cells formed transformed foci on a monolayer of cells, showed anchorage-independent growth, and formed tumors in nude mice. These results indicate that overexpression of eIF4G caused malignant transformation of NIH3T3 cells. It is also known that overexpression of eIF4E, another component of eIF4F, causes transformation of NIH3T3 cells. However, there was no difference in the amount of eIF4E protein between N4G3 and NIH3T3 cells, indicating that cell transformation does not involve a change in eIF4E levels. The results may be due to an effect of eIF4G on translational control of protein synthesis directed by mRNAs having long 5'-untranslated region.     

Stromelysin-3 expression promotes tumor take in nude mice

Stromelysin-3 (ST3) is a matrix metalloproteinase expressed in human carcinomas in ways suggesting that it may play a role in tumor progression. To test this possibility, we have performed gene transfer experiments using both anti-sense and sense ST3 expression vectors, and malignant cells either expressing (NIH 3T3 fibroblasts) or not (MCF7 epithelial cells) endogenous ST3. We have compared the ability of parental and transfected cells to cause subcutaneous tumor development in nude mice. 3T3 cells expressing anti-sense ST3 RNA showed reduced tumorigenicity, and MCF7 cells expressing mouse or human ST3 were associated with reduced tumor-free period leading to a significant increased tumor incidence(P<10(-4)). However, once established, the ST3 expressing tumors did not grow faster than those obtained with the parental MCF7 cell line. In addition, tumors obtained after sub-cutaneous injection of ST3-expressing or nonexpressing cells did not exhibit obvious histological differences, and careful examination did not reveal any local invasive tissue areas nor systemic metastases. These in vivo observations were in agreement with those obtained in vitro showing that ST3 expression did not modify proliferative nor invasive properties of transfected cells. Altogether, these results indicate that ST3 expression promotes tumor take in nude mice, presumably by favoring cancer cell survival in a tissue environment initially not permissive for tumor growth. These findings represent the first experimental evidence showing that ST3 can modulate cancer progression.     

Only the substitution of methionine 918 with a threonine and not with other residues activates RET transforming potential

Specific point-mutations of the RET receptor tyrosine kinase protooncogene are responsible for the inheritance of multiple endocrine neoplasia type 2A (MEN2A) and 2B (MEN2B), and familial medullary thyroid carcinoma (FMTC). MEN2B is caused by the substitution of methionine 918 by a threonine in the tyrosine kinase (TK) domain of RET. This mutation converts RET into a dominant transforming oncogene. We have substituted Met918 with four different residues and found that RET acquired transforming activity only when Met918 was substituted with a threonine. However, also when serine and valine, but not leucine or phenylalanine, were inserted in position 918, the RET TK function was activated and induced, especially in the case of the RET(918Ser), immmediate-early response genes. We conclude that the preservation of Met918 is critical for the control of RET kinase. However, only when a threonine residue is present in position 918, does RET efficiently couple with a transforming pathway.     

A complex nine base pair deletion in RET exon 11 common in sporadic medullary thyroid carcinoma

Genetic alteration of the RET proto-oncogene is associated with multiple endocrine neoplasia type 2A and 2B (MEN 2A and MEN 2B), familial medullary thyroid carcinoma (FMTC) and Hirschprung's disease. Oncogenically activated RET has also been demonstrated in sporadic medullary thyroid tumors, which in some cases show somatic missense mutations. We have recently described a complex 9 bp deletion in RET exon 11 in a single case of sporadic MTC. In order to determine the prevalence of this mutation among sporadic MTC tumors, we have now analysed 15 cases and five normal controls by PCR-based nonradioactive single-strand conformational polymorphism analysis (PCR-SSCP) and fragment size analysis of exon 11. DNA was extracted from microdissected tumor tissue or normal cells and subjected to nested PCR prior to analysis. A markedly divergent SSCP pattern and a PCR fragment 9 bp shorter than normal were demonstrated in 14 of the 15 MTC tumors. Sequencing revealed the deletion of nine bases encompassing a key cysteine at codon 634, often altered in MEN 2A. Four lymphocyte controls and normal thyroid tissue from one patient failed to show the deletion. Several factors in the DNA sequence environment immediately surrounding the deletions, including an extended inverted repeat, several direct repeats and a so-called symmetric element suggest that the deletional events may be non-random.     

HLA-DR4 (DRB1*0401) transgenic mice expressing an altered CD4-binding site: specificity and magnitude of DR4-restricted T cell response

Optimum function of HLA-DR molecules in transgenic mice requires efficient interaction between the class II molecules on APCs and CD4 on T cells. Residues 110 and 139 of the second domain of class II molecules are considered to be critical for recognition of CD4. We generated an HLA-DR4beta(NT) transgene construct in which positions 110 and 139 were altered to resemble endogenous mouse H2 Abeta molecules. This construct was introduced into (B10 x SWR) embryos, and DR4beta(NT) transgenic mice were produced. The transgene was transferred into B10.RFB3 (Ebeta0 EalphaP) mice. The transgene-encoded DR4beta molecules paired with endogenous Ealpha chains to form stable DR4beta/Ealpha dimers expressed on the cell surface. The hybrid dimers showed similar Ag-binding specificity to HLA-DR4 molecules and positively selected CD4+ T cells in vivo. Immunization of HLA-DR4beta(NT) transgenic mice with DR4-restricted peptides induced T cell proliferation in vitro. While the purified T cells from DR4beta(NT) transgenic mice responded strongly to the HA(307-319) presented by M12C3 transfectants expressing altered DR4beta/Ealpha heterodimers, the response to the same peptides presented by transfectants expressing wild-type DR4beta/Ealpha molecules was substantially reduced. Taken together, these data confirmed in vitro studies on the importance of these residues in CD4-MHC class II interaction. The altered HLA-DR4beta transgenic mice were able to overcome the species barrier and generate efficient HLA-DR4-restricted CD4-specific immune responses. Thus, residues 110 and 139 were critical for the interaction of class II with CD4 T cells during thymic selection as well as peripheral immune responses.     

Full activation of MEN2B mutant RET by an additional MEN2A mutation or by ligand GDNF stimulation

Germline mutations of RET gene, encoding a receptor tyrosine kinase, have been associated with the MEN2A and MEN2B inherited cancer syndromes. In MEN2A mutations affecting cysteine residues in the extracellular domain of the receptor cause constitutive activation of the tyrosine kinase by the formation of disulfide-bonded homodimers. In MEN2B a single mutation in the tyrosine kinase domain (Met918Thr) has been identified. This mutation does not lead to dimer formation, but has been shown (both biologically and biochemically) to cause ligand-independent activation of the Ret protein, but to a lesser extent than MEN2A mutations. Intramolecular activation by cis-autophosphorylation of RetMEN2B monomers has been proposed as a model for activation, although alternative mechanisms can be envisaged. Here we show that the activity of RetMEN2B can be increased by stable dimerization of the receptor. Dimerization was achieved experimentally by constructing a double mutant receptor with a MEN2A mutation (Cys634Arg) in addition to the MEN2B mutation, and by chronic exposure of RetMEN2B-expressing cells to the Ret ligand GDNF. In both cases full activation of RetMEN2B, measured by 'in vitro' transfection assays and biochemical parameters, was seen. These results indicate that the MEN2B phenotype could be influenced by the tissue distribution or concentration of Ret ligand(s).     

Overexpression of the Na+,K+-ATPase alpha1 subunit increases Na+,K+-ATPase function in A549 cells

Ron (the receptor for Macrophage Stimulating Protein) has never been implicated before in human malignancies or in cell transformation. In this report we show that Ron can acquire oncogenic potential by means of two amino acid substitutions-D1232V and M1254T-affecting highly conserved residues in the tyrosine kinase domain. The same mutations in Kit and Ret have been found associated with two human malignancies, mastocytosis and Multiple Endocrine Neoplasia type 2B (MEN2B), respectively. Both mutations caused Ron-mediated transformation of 3T3 fibroblasts and tumour formation in nude mice. Moreover, cells transformed by the oncogenic Ron mutants displayed high metastatic potential. The Ron mutant receptors were constitutively active and the catalytic efficiency of the mutated kinase was higher than that of wild-type Ron. Oncogenic Ron mutants enhanced activation of the Ras/MAPK cascade with respect to wild type Ron, without affecting the JNK/SAPK pathway. Expression of Ron mutants in 3T3 fibroblasts led to different patterns of tyrosine-phos-phorylated proteins. These data show that point mutations altering catalytic properties and possibly substrate specificity of the Ron kinase may force cells toward tumorigenesis and metastasis.     

Anti-cell death activity promotes pulmonary metastasis of melanoma cells

Bcl-2 inhibits apoptosis from a variety of stimuli, and a Bcl-2-binding protein BAG-1 also functions in protection from apoptosis in concert with Bcl-2. Here, we provide evidence that prolonged cell survival introduced by overexpression of Bcl-2 or BAG-1 proteins strongly promotes experimental pulmonary metastasis of melanoma B16-BL6 cells. In murine melanoma cell line B16-BL6, gene transfer-mediated expression of the Bcl-2 or BAG-1 led to prolonged cell survival against serum-starved apoptosis in vitro. The Bcl-2-expressing B16 cells, B16-Bcl-2 and the BAG-1-expressing B16 cells, B16-BAG-1 strongly enhanced pulmonary metastasis in allogenic BALB/c nude mice and whole lung weights were increased by 2.4-fold and 1.4-fold, respectively, compared with control transfectants, suggesting that Bcl-2 is a stronger positive modulator of metastasis. When the viable B16-Bcl-2 and control transfectants were injected subcutaneously into BALB/c nude mice, the colony numbers of pulmonary metastasis of the B16-Bcl-2 transfectant increased by 5.6-fold compared with the control transfectants. These enhanced metastatic potentials in the B16-Bcl-2 and the B16-BAG-1 transfectants were well correlated with anti-cell death activity against serum-starvation and enhanced cell viability on limiting dilution. Analysis of the transfectants however revealed that their growth rates, invasive ability and cell motility were not significantly altered by overexpression of either Bcl-2 or BAG-1 proteins. Taken together, these studies demonstrate that prolonged cell survival is a crucial factor to promote metastasis of melanoma, thereby contributing to tumor progression.     

Functional characterization in vivo of mutant p53 molecules derived from squamous cell carcinomas of the head and neck

Loss of wild-type p53, either through deletion or mutation, has been demonstrated in most squamous cell carcinomas of the head and neck (HNSCC). Whether these mutant molecules contribute to tumor progression purely through loss of wild-type functions or by growth-promoting mechanisms, however, remains unclear. To begin to address these issues, we isolated a series of p53 cDNAs from HNSCC cell lines that contain missense or nonsense point mutations, insertions, or deletions. The ability of each of these molecules to transform NIH/3T3 cells to a malignant phenotype was assessed by stable transfection and expression under the control of a strong heterologous promoter. NIH/3T3 cells transfected with pLTR6p53, which harbors an H179L missense mutation, formed large tumors rapidly (in less than 4 wk) when transplanted to athymic mice, as did cells expressing pLTR13p53, which had undergone a V173F missense mutation and an in-frame deletion of 48 bp between codons 208 and 223. Cells transfected with pLTR17p53, predicted from the nucleotide sequence to encode a severely truncated p53 corresponding to the N-terminal 56 amino acids, also formed tumors. Cells transfected with pLTR15p53, which was predicted to encode a less severely truncated molecule, formed much smaller tumors and at lower frequencies. NIH/3T3 cells transfected with pLTR12p53 (exon 7 splice donor mutant), pLTRwtp53 (wild-type p53), or vector alone failed to form tumors for up to 2 mo after transplantation. pLTR6p53-transfected cells exhibited a highly malignant phenotype with invasion of regional lymph nodes, mediastinal and lung metastases, invasion of the abdominal wall, and dissemination throughout the peritoneal cavity. Histological assessment of the tumors revealed intensely vascularized fibrosarcomas with numerous cellular atypia, including frequent and aberrant mitoses. Tumor explants were recultured, and northern blot analysis of cellular RNA confirmed that the expression of exogenous p53 was maintained in each case. These data indicate that different p53 mutants contribute to tumorigenesis by specific mechanisms. Furthermore, the results obtained by using the pLTR17p53 transfectants imply that some truncated molecules may overcome the effects of wild-type p53 to contribute to malignancy.