Genetic changes in the transforming growth factor beta (TGF-beta) type II receptor gene in human gastric cancer cells: correlation with sensitivity to growth inhibition by TGF-beta

Consistent loss of functional transforming growth factor beta receptor expression in murine plasmacytomas

Murine plasmacytomas are tumors of Ig-secreting plasma cells that can be induced in genetically susceptible BALB/c mice. The deregulation of the c-myc protooncogene is a critical oncogenic event in the development of plasmacytomas (PCTs) although it is not sufficient for their malignant transformation. We have demonstrated that PCTs produce active transforming growth factor beta (TGF-beta) in vitro. Because TGF-beta is a potent negative regulator of the proliferation and differentiation of B lymphocytes, we examined its role in plasmacytomagenesis by comparing responsiveness to TGF-beta of nonneoplastic plasma cells and PCTs. The nontransformed plasma cells that accumulate in interleukin 6 transgenic mice undergo accelerated apoptosis upon treatment with TGF-beta, but the 15 PCTs studied, including primary and transplanted tumors as well as established cell lines, were refractory to TGF-beta-mediated growth inhibition and apoptosis. Although PCTs lack functional TGF-beta receptors as demonstrated by chemical crosslinking to radiolabeled TGF-beta1, they nonetheless contain mRNA and protein for both type I and II TGF-beta receptors, suggesting a potential defect in receptor trafficking or processing. The results clearly show the consistent inactivation of TGF-beta receptors in plasmacytoma cells, demonstrating for the first time that interruption of a tumor suppressor pathway contributes to plasmacytomagenesis.

A novel ets-related transcription factor, ERT/ESX/ESE-1, regulates expression of the transforming growth factor-beta type II receptor

A 2.5-kilobase cDNA clone that encodes a 371-amino acid novel transcription factor was isolated from a human placenta cDNA library using a yeast one-hybrid system. The novel ets-related transcription factor (ERT) showed a homology with the ETS DNA-binding domain. Using constructs of the transforming growth factor-beta (TGF-beta) type II receptor (RII) promoter linked to the luciferase gene, we have demonstrated that ERT activates transcription of the TGF-beta RII gene through the 5'-TTTCCTGTTTCC-3' response element spanning nucleotides +13 to +24 and multiple additional ETS binding sites between -1816 and -82 of the TGF-beta RII promoter. A specific interaction between ERT and the ETS binding sites was also demonstrated using an electrophoretic mobility shift assay. Deletion mapping of ERT protein suggests that the transactivation domain resides in the amino terminus while the DNA-binding domain is localized to the carboxyl-terminal region. Our results suggest that ERT might be a major transcription factor involved in the transcriptional regulation of the TGF-beta RII gene.

Analysis of TGF-B and TGF-B-RII in Thyroid Neoplasms from the United States, Japan, and China

Transforming growth factor B (TGF-B) has an inhibitory effect on cell proliferation in various cells and tumors, so loss of TG-B-receptor (TGF-B-R) may lead to increase proliferative activity in these tumors. We compared the expression of TGF-B and TGF-B-Rll in a group of thyroid neoplasms from the United States, Japan, and China to determine if there were differences in the expression of this growth factor or its receptors in various tumor types from different countries. A total 108 neoplastic thyroids from the United States, 42 from Japan, and 46 from China were analyzed for TGF-B1, TGF-B3, and TGF-B-Rll by in situ hybridization with riboprobes. TGF-jB-RII expression was also examined by immunohistochemistry. TGF-B1 mRNA was expressed in all neoplastic thyroids from all three countries except for one anaplasti carcinoma (ACA). TGF-B3 expression was lowest in follicular carcinomas (FCA) from all three countries (30/42; 71%). TGF-B-RII was much lower in FCA from Japan (112; 50%) and China (6/11; 55%) compared to cases from the United States (26/29; 90%). TGF-B-RII expression in papillary carcinoma (PCA) was also lower in carcinomas from Japan (21/28; 75%) and China (23/30; 77%) compared to the United States (24/25; 96%). Most ACA from the United States (25/30; 83%) and from China (3/3; 100%) were positive for TGF-B-Rll. Immunohistochemical analysis for TGF-B-RII protein expression showed the highest levels in follicular adenomas (FA) (38/38; 100%) with decreased immunoreactivity in FCA (36142; 86%). PCA (66/83; 80%), and ACA (14/33; 42%). These findings suggest that loss of TGF-B--RII may be important in thyroid tumor progression and that environmental/geographic factors may play a role in the variable expression of TGF-B--RII in thyroid malignancy.

TGF-beta signal transduction

The transforming growth factor beta (TGF-beta) family of growth factors control the development and homeostasis of most tissues in metazoan organisms. Work over the past few years has led to the elucidation of a TGF-beta signal transduction network. This network involves receptor serine/threonine kinases at the cell surface and their substrates, the SMAD proteins, which move into the nucleus, where they activate target gene transcription in association with DNA-binding partners. Distinct repertoires of receptors, SMAD proteins, and DNA-binding partners seemingly underlie, in a cell-specific manner, the multifunctional nature of TGF-beta and related factors. Mutations in these pathways are the cause of various forms of human cancer and developmental disorders.

Cloning and genomic organization of the human transforming growth factor-beta type I receptor gene

Transforming growth factor-beta (TGF beta) regulates cell cycle progression by a unique signaling mechanism that involves its binding to the type II (T beta R-II) TGF beta receptor and activation of type I (T beta R-I). Both are transmembrane serine-threonine receptor kinases. As various types of human tumor cells are often refractory to TGF beta-mediated cell cycle arrest, it is likely that the T beta R-I receptor is inactivated in many of these cases. We determined the intron-exon organization of the TGFBR1 gene. We report here that this gene is approximately 31 kb in length and consists of nine exons. The organization of the segment of the TGFBR1 gene that encodes the C-terminal portion of the serine-threonine kinase domain appears to be highly conserved between members of the R-I gene family. This information should facilitate and expedite the structural analysis of TGFBR1 in human tumors and possibly other disease states.

Transforming growth factor-beta1 inhibits membrane association of protein kinase C alpha in a human prostate cancer cell line, PC3

The postreceptor signaling pathway(s) that mediates the effects of transforming growth factor-beta1 (TGF-beta1) is incompletely understood. The present study investigated the involvement of protein kinase C (PKC) in the growth-inhibitory action of TGF-beta1 in PC3, a human prostate cancer cell line. PKC alpha, the only conventional PKC isoform detected in PC3 cells, appeared to be constitutively active based on its presence in both Triton-soluble membrane fraction and cytosol. However, levels of membrane-associated PKC alpha were decreased by a growth-inhibitory dose of TGF-beta1. The response to TGF-beta1 was rapid (within 5 min), time dependent, isoform specific, and occurred without apparent changes in levels of total PKC alpha protein. TGF-beta1 also decreased the levels of membrane-associated PKC activity coincident with its inhibitory effect on PKC alpha's membrane association. Inhibition of PKC activity appeared to be associated with growth inhibition in PC3 cells, because chelerythrine (a specific PKC inhibitor) likewise decreased cell proliferation. Taken together, our data suggest that inhibition of PKC activity, at least in part due to inactivation of PKC alpha, is an early event associated with TGF-beta1 postreceptor signaling that might mediate suppression of cell proliferation.

Apoptosis in gastric epithelial cells is induced by Helicobacter pylori and accompanied by increased expression of BAK

Carriage of the bacterium H. pylori in the human stomach is associated with evidence of increased epithelial cell apoptosis. This may be of significance in the etiology of gastritis, peptic ulcers, and neoplasia. The ability of H. pylori to directly induce epithelial apoptosis was examined in vitro by fluorescence and electron microscopy, flow cytometry, and DNA fragmentation ELISA. The induction of apoptosis by H. pylori was time and concentration-dependent and inhibited by preventing direct bacterial-epithelial cell contact. Apoptosis was accompanied by increased expression of Bak, with little change in expression of other Bcl-2 family proteins. The expression of Bak was also increased in gastric biopsies from patients colonized by H. pylori. Thus, H. pylori induces gastric epithelial cell apoptosis, by a Bak-dependent pathway.

Expression of transforming growth factor beta type III receptor suppresses tumorigenicity of human breast cancer MDA-MB-231 cells

Transforming growth factor beta (TGF-beta) promotes tumor progression in some model systems including human breast cancer cells. In this study, we report that human breast cancer cell lines express reduced amounts of TGF-beta type III receptor (RIII) when compared with untransformed human mammary epithelial cells. Consequently, we examined whether expression of RIII in human breast cancer MDA-MB-231 cells could reduce TGF-beta's tumor promoting activity by sequestering active TGF-beta isoforms produced by the cells. A tetracycline-repressible human RIII expression vector was stably transfected into the cell line. RIII expression in a pool of transfected clones and a single clone was found to be reversibly repressed by tetracycline treatment. Expression of RIII reduced the amount of active TGF-beta1 and TGF-beta2 in the conditioned medium. The medium conditioned by control cells showed a significantly higher growth inhibitory effect than that conditioned by RIII-transfected cells on the growth of the mink lung epithelial CCL64 cells. A conditioned medium collected from RIII-transfected cells treated with tetracycline significantly increased its growth inhibitory activity to that of control cells. Expression of RIII also reduced tumor incidence and growth rate in two separate experiments when the cells were inoculated in athymic nude mice. Treatment of the mice with tetracycline repressed RIII expression in the tumors generated by RIII-transfected cells and increased tumor incidence and growth rate. These results suggest that TGF-beta RIII can reduce tumorigenicity of MDA-MB-231 cells apparently by sequestering TGF-beta isoforms produced by these cells.

Presence of two signaling TGF-beta receptors in human pancreatic cancer correlates with advanced tumor stage

Transforming growth factor-beta (TGF-beta) signal transduction is mediated via specific cell surface signaling TGF-beta receptors, most notably the type I ALK5 (TbetaR-I[ALK5]) and the type II (TbetaR-II). We evaluated TbetaR-I(ALK5) and TbetaR-II expression in 41 human pancreatic cancer tissue samples and correlated these findings with clinical data of the patients. Northern blot analysis indicated that, in comparison with the normal pancreas, pancreatic adenocarcinomas exhibited 8.0-fold and 4.5-fold increases (P < 0.01), respectively, in mRNA levels encoding TbetaR-I(ALK5) and TbetaR-II. In situ hybridization showed that both TbetaR-I(ALK5) and TbetaR-II mRNA were highly expressed in the majority of pancreatic cancer cells. Immunohistochemical analysis of TbetaR-I(ALK5) and TbetaR-II revealed positive immunostaining in 73% and 56% of the tumors, respectively. Both receptors were concomitantly present in 54% of the pancreatic cancer samples. The presence of TbetaR-I(ALK5) or TbetaR-II and the concomitant presence of TbetaR-I(ALK5) and TbetaR-II in the cancer cells was associated with advanced tumor stage (P < 0.01). These findings show that in many human pancreatic cancers, increased levels of the two signaling TbetaRs are present. The presence of the signaling TbetaRs in advanced tumor stages indicates a role in disease progression.

Transforming growth factor-beta, transforming growth factor-beta receptor II, and p27Kip1 expression in nontumorous and neoplastic human pituitaries

Transforming growth factor (TGF)-beta has been implicated in the regulation of normal and neoplastic anterior pituitary cell function. TGF-beta regulates the expression of various proteins, including p27Kip1 (p27), a cell cycle inhibitory protein. We examined TGF-beta, TGF-beta type II receptor (TGF-beta-RII), and p27 expression in normal pituitaries, pituitary adenomas, and carcinomas to analyze the possible roles of these proteins in pituitary tumorigenesis. Normal pituitary, pituitary adenomas, and pituitary carcinomas all expressed TGF-beta and TGF-beta-RII immunoreactivity. Reverse transcription polymerase chain reaction analysis showed TGF-beta 1, -beta 2, and -beta 3 isoforms and TGF-beta-RII in normal pituitaries and pituitary adenomas. Pituitary adenomas cells cultured for 7 days in defined media showed a biphasic response to TGF-beta with significant inhibition of follicle-stimulating hormone secretion at higher concentrations (10(-9) mol/L) and stimulation of follicle-stimulating hormone secretion at lower concentrations (10(-13) mol/L) of TGF-beta 1 in gonadotroph adenomas. Immunohistochemical analysis for p27 protein expression showed the highest levels in nontumorous pituitaries with decreased immunoreactivity in adenomas and carcinomas. When nontumorous pituitaries and various adenomas were analyzed for p27 and specific hormone production, growth hormone, luteinizing hormone, and thyroid-stimulating hormone cells and tumors had the highest percentages of cells expressing p27, whereas adrenocorticotrophic hormone cells and tumors had the lowest percentages. Immunoblotting analysis showed that adrenocorticotrophic hormone adenomas also had the lowest levels of p27 protein. Semiquantitative reverse transcription polymerase chain reaction and Northern hybridization analysis did not show significant differences in p27 mRNA expression in the various types of adenomas or in nontumorous pituitaries. In situ hybridization for p27 mRNA showed similar distributions of the gene product in nontumorous pituitaries, pituitary adenomas, and carcinomas. These results indicate that TGF-beta and TGF-beta-RII are widely expressed in nontumorous pituitaries and in pituitary neoplasms and that TGF-beta 1 regulates pituitary hormone secretion. The levels of the TGF-beta-regulated protein p27 decreases in the progression of normal to neoplastic pituitaries. In contrast, the mRNA levels of p27 remained relatively constant in nontumorous pituitaries, pituitary adenomas, and carcinomas, indicating that p27 protein levels in adenomas and carcinomas are regulated by translational and post-translational mechanisms.

Restored expression of transforming growth factor beta type II receptor in k-ras-transformed thyroid cells, TGF beta-resistant, reverts their malignant phenotype

Transforming growth factor beta 1 (TGF beta 1) inhibits the growth of normal rat epithelial thyroid cells (FRTL-5 strain) by counteracting thyrotropin (TSH)-stimulated DNA synthesis and by slowing the cells in the G1 phase of the cell cycle. Here, we have studied two clones of FRTL-5 thyroid cell line transformed by the wild type (wt) v-k-ras oncogene (K.M.A1, K.M.A2) and one clone (A6) transformed by a temperature-sensitive (ts) v-k-ras mutant. Anchorage-dependent as well as anchorage-independent growth of these k-ras-transformed cells was not inhibited by TGF beta 1. TGF beta 1 resistance appeared to be dependent by a functional p21 k-ras, because A6 cell growth was partially inhibited at the nonpermissive temperature (39 degrees C). To determine the basis for TGF beta 1 resistance in k-ras-transformed thyroid cells, we looked for possible defects in the expression of type I (T beta R-I/ALK5) and type II TGF beta receptors (T beta R-II). Lower levels of type II receptors were present in all of the k-ras-transformed clones, as revealed by both Northern blot and cross-linking experiments. A partial reversion of the malignant phenotype of the wt k-ras-transformed clone was obtained in two clones isolated after transfection of the malignant thyroid cells (K.M.A1) with a T beta R-II expression vector. These two clones also showed restored levels of exogenous T beta R-II mRNA and protein, and both clones showed a partially reacquired sensitivity to TGF beta 1. Similarly, the reversion of the malignant phenotype of the A6 clone grown at the nonpermissive temperature was accompanied by a restored expression of the T beta R-II receptors. These data indicate that active k-ras oncogene can induce TGF beta 1 resistance in rat thyroid cells and suggest that one of the possible mechanisms of escape from TGF beta 1 growth control in k-ras-induced thyroid carcinogenesis involves a reduced expression of T beta R-II receptors.

Loss of functional cell surface transforming growth factor beta (TGF-beta) type 1 receptor correlates with insensitivity to TGF-beta in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most common form of adult leukemia in Western countries, and there is significant variability in survival within CLL clinical stages. Earlier studies showed that CLL cells produce and are usually growth inhibited by transforming growth factor beta type 1 (TGF-beta1), suggesting a mechanism for the clinically indolent course of most CLL. Here we studied the mechanism by which CLL cells from about one-third of the patients are insensitive to TGF-beta1. Of the 13 patients studied, CLL cells isolated from the peripheral blood of 8 patients were sensitive to growth inhibition by TGF-beta1, as determined by incorporation of tritiated thymidine, whereas those from 5 patients were completely resistant to TGF-beta1. As judged by binding of radiolabeled TGF-beta1 followed by cross-linking and immunoprecipitation with anti-receptor antisera, CLL cells sensitive to TGF-beta1 exhibited normal cell surface expression of both types 1 and 2 TGF-beta receptors. In contrast, all CLL cells resistant to TGF-beta1 exhibited no detectable surface type I receptors able to bind TGF-beta1, but normal expression of type II receptors. Both TGF-beta1-sensitive and TGF-beta1-resistant CLL cells contained normal amounts of both type 1 and type 2 receptor mRNAs. Specific loss of type 1 receptor expression represents a new mechanism by which cells acquire resistance to TGF-beta1-mediated growth inhibition in the development and progression of human lymphoproliferative malignancies.

Down-regulation of protein and mRNA expression for transforming growth factor-beta (TGF-beta1) type I and type II receptors in human prostate cancer

Transforming growth factor-beta (TGF-beta1) is a potent negative regulator of cell growth that transduces signals through interactions with type I and II receptors. Abnormal expression and mutational alterations of these receptors have been observed in several human malignancies. In this study, we investigated the expression of the two types of TGF-beta1 receptors, R-I and R-II, in a normal human prostate, primary prostate adenocarcinoma and lymph nodes with metastatic deposits. Expression of receptor proteins was examined by immunohistochemical analysis in paraffin-embedded prostatic tissue sections, and mRNA expression was determined by Northern blot and RT-PCR analysis. Uniformly strong immunoreactivity for both TGF-beta receptor proteins, R-I and R-II, was exclusively localized to the prostatic glandular epithelium of normal prostates. In contrast, tumor epithelial cells in primary and metastatic prostatic cancer specimens exhibited a weak heterogeneous immunoreactivity for both R-I and R-II receptors; 25% of primary prostatic tumors and 45% of the lymph nodes with metastases were totally negative for R-I and R-II expression, while the rest exhibited a significantly reduced immunoreactivity for both types of receptors compared to the normal prostate (p < 0.05). Moreover, there was a significant decrease in the expression of R-I and R-II mRNA, in all 20 primary prostatic tumors and 4 lymph nodes positive for metastases, indicating that the decreased protein expression was due to down-regulation of gene expression for the two receptors. Our findings imply that decreased expression of TGF-beta1 type I and type II receptors might be involved in prostate tumorigenesis.

Expression of transforming growth factor beta (TGFbeta) type III receptor restores autocrine TGFbeta1 activity in human breast cancer MCF-7 cells

While transforming growth factor beta (TGFbeta) type III receptor (RIII) is known to increase TGFbeta1 binding to its type II receptor (RII), the significance of this phenomenon is not known. We used human breast cancer MCF-7 cells to study the role of RIII in regulating autocrine TGFbeta1 activity because they express very little RIII and no detectable autocrine TGFbeta activity. A tetracycline-repressible RIII expression vector was stably transfected into this cell line. Expression of RIII increased TGFbeta1 binding to TGFbeta type I receptor (RI) as well as RII. Treatment with tetracycline suppressed RIII expression and abolished TGFbeta1 binding to RI and RII. Growth of RIII-transfected cells was reduced by 40% when plated at low density on plastic. This reduction was reversed by tetracycline treatment and was partially reversed by treatment with a TGFbeta1 neutralizing antibody. The activity of a TGFbeta-responsive promoter construct when transiently transfected was more than 3-fold higher in the RIII-transfected cells than in the control cells. Treating the cells with tetracycline or the TGFbeta1 neutralizing antibody also significantly attenuated the increased promoter activity. These results suggest that expression of RIII restored autocrine TGFbeta1 activity in MCF-7 cells. The RIII-transfected cells were also much less clonogenic in soft agarose than the control cells indicating a reversion of progression. Thus, RIII may be essential for an optimal level of the autocrine TGFbeta activity in some cells, especially in the transformed cells with reduced RII expression.

Analysis of a polyadenine tract of the transforming growth factor-β type II receptor gene in colorectal cancers by non-gel-sieving capillary electrophoresis

We developed a method to analyze a polyadenine tract, the (A)10 repeat, within the cysteine-rich domain of the transforming growth factor-β (TGF-β) type II receptor gene using a non-gel-sieving capillary electrophoresis technique and applied it to the DNA diagnosis of colorectal cancers. This method consists of single-strand DNA amplification of the (A)10 repeat by an asymmetric PCR technique and capillary electrophoresis. A higher concentration of dATP in the PCR reaction mixture led to more specific amplification of the (A)10 repeat. Under the optimal electrophoretic conditions, one nucleotide difference could be determined in 8 to 32 nucleotides. One or two base deletions of the (A)10 repeat in colorectal cancers could be detected under these conditions within 30 min, and the results coincided with those obtained on DNA sequencing analyses. According to a sensitivity study, we could detect the deleted sequence if it was present in 12.5% or more of the wild-type allele. The reproducibility of this technique was satisfactory because the intraassay imprecision (CV) (n = 10) was 1.4%. These results indicate that capillary electrophoretic analysis of small repeated sequences results in easier handling and more feasible automation, compared with conventional gel electrophoretic analysis.

Positive and negative regulation of type II TGF-beta receptor signal transduction by autophosphorylation on multiple serine residues

The type II transforming growth factor-beta (TGF-beta) receptor Ser/Thr kinase (TbetaRII) is responsible for the initiation of multiple TGF-beta signaling pathways, and loss of its function is associated with many types of human cancer. Here we show that TbetaRII kinase is regulated intricately by autophosphorylation on at least three serine residues. Ser213, in the membrane-proximal segment outside the kinase domain, undergoes intra-molecular autophosphorylation which is essential for the activation of TbetaRII kinase activity, activation of TbetaRI and TGF-beta-induced growth inhibition. In contrast, phosphorylation of Ser409 and Ser416, located in a segment corresponding to the substrate recognition T-loop region in a three-dimensional structural model of protein kinases, is enhanced by receptor dimerization and can occur via an intermolecular mechanism. Phosphorylation of Ser409 is essential for TbetaRII kinase signaling, while phosphorylation of Ser416 inhibits receptor function. Mutation of Ser416 to alanine results in a hyperactive receptor that is better able than wild-type to induce TbetaRI activation and subsequent cell cycle arrest. Since on a single receptor either Ser409 or Ser416, but not both simultaneously, can become autophosphorylated, our results show that TbetaRII phosphorylation is regulated intricately and affects TGF-beta receptor signal transduction both positively and negatively.

Differential localization of transforming growth factor-beta isoforms in human gastric mucosa and overexpression in gastric carcinoma

Transforming growth factor beta (TGF-beta) isoforms comprise a family of multifunctional polypeptide growth factors that either inhibit or stimulate cell proliferation. We examined TGF-beta expression in normal human gastric mucosa and carcinoma. The distribution and expression of TGF-beta isoforms in 4 normal mucosa samples from organ donors, in 12 normal mucosa samples adjacent to gastric cancer and in 12 gastric carcinomas were examined using immunohistochemistry and Northern blot analysis. Because TGF-beta s regulate collagen expression, collagen type I alpha1 mRNA amounts were also examined. Immunohistochemical analysis of normal human gastric tissue samples indicated that TGF-beta1 localized principally in parietal cells but also in some surface mucus cells, TGF-beta2 was present exclusively in chief cells and TGF-beta3 was present in parietal, chief and mucus cells. In the gastric cancers, strong colocalization of TGF-beta1, -beta2 and -beta3 was evident in the cancer cells. Northern blot analysis indicated that, compared to normal gastric tissue, gastric cancers showed a 4.8- and 6-fold increase in mRNA amounts encoding TGF-beta1 and TGF-beta3, respectively. In contrast, TGF-beta2 mRNA amounts were comparable in both groups. Northern blot analysis showed a 10-fold increase in human collagen type I alpha1 mRNA amounts compared to normal gastric tissue. These findings imply a role forTGF-beta s in normal human gastric mucosa function, and raise the possibility that the aberrant colocalization and overexpression of all 3 TGF-beta isoforms in human gastric cancer cells in vivo may contribute to the pathobiology of gastric carcinoma.

TGF beta in prostate cancer: a growth inhibitor that can enhance tumorigenicity

A common feature of cancer cells is the autocrine production of growth promoters and the loss of function of tumor suppressors. In our search for such features of prostate cancer, we discovered that transforming growth factor beta 1 (TGF beta 1) levels are higher in prostate cancer than in normal prostate, and prostate cancer cells can activate endogenously-produced latent TGF beta to a bioactive form. Because TGF beta 1 is a potent growth inhibitor of epithelial cells, it seems paradoxical that malignant epithelial cells make high levels of a growth inhibitor. Even prostate cancer cells can be growth-inhibited by TGF beta 1, but only under specific conditions in vitro (plating at low cell density in serum-free medium), and this response is readily disrupted by growth factors, serum, and extracellular matrix, to all of which the cells are exposed in vivo. This explains why prostate cancer cells are resistant to the growth-inhibitory effect of TGF beta in vivo. In vivo, TGF beta 1 actually enhances prostate tumor growth and metastasis, but not by affecting tumor cell proliferation directly. One possibility is that TGF beta affects the host to allow increased numbers of tumor cells to survive and produce progeny. In addition, since prostate cancer cells can still respond to TGF beta, e.g., by increased cell motility, even under conditions that prevent growth inhibition, the ability of TGF beta to enhance tumorigenicity in vivo might also occur via direct effects on the tumor cells themselves. I will discuss new developments in our understanding of TGF beta action, which provide a framework for elucidating the mechanism by which prostate cancer cells have devised a way to protect themselves from being growth-inhibited by TGF beta 1 in vivo. Since the cells retain the ability to be growth-inhibited by TGF beta, indicating that the TGF beta receptors and signaling pathways for growth inhibition are intact, albeit inactive, it might be possible to reactivate this pathway to achieve a therapeutic benefit in vivo.

Processing of the transforming growth factor beta type I and II receptors. Biosynthesis and ligand-induced regulation

Three cell surface transforming growth factor beta (TGFbeta) receptor (R) proteins regulate the effects of TGFbeta isoforms on growth and differentiation. TGFbeta-IR and -IIR are transmembrane serine/threonine kinases directly mediating the signaling across the plasma membrane. Both TGFbeta and its receptors are ubiquitously expressed, hence the fine regulation of the multiplicity of responses most likely involves several levels of control including the regulation of expression, complex formation, and down-regulation of the receptor proteins. In mink lung epithelial cells, TGFbeta-IIR was first synthesized as a approximately 60-kDa endoglycosidase H-sensitive precursor protein, which was converted to a mature approximately 70-kDa protein. The half-life of metabolically labeled mature TGFbeta-IIR was estimated to be 60 min and was further reduced to approximately 45 min in the presence of exogenous TGFbeta1. Minimal internalization of 125I-TGFbeta1 at 37 degrees C was detected suggesting that the rapid turnover was not due to endocytosis and degradation of the ligand-receptor complexes. TGFbeta-IR was synthesized as a approximately 53-kDa precursor protein, which was processed to a mature approximately 55-kDa receptor protein. The half-life of TGFbeta-IR was >12 h. A fraction of tunicamycin-treated type I and II receptors that reach the cell surface was able to associate in the presence of ligand suggesting that heteromeric complexes can form in a post-endoplasmic reticulum compartment before full glycosylation is achieved. These results show differential processing and turnover of TGFbeta-IR and TGFbeta-IIR providing a potential additional mechanism for modulation of cellular responses to TGFbetas.

Establishment and characterization of human gastric carcinoma cell lines

We report 8 newly established gastric-carcinoma cell lines (SNU-216, 484, 520, 601, 620, 638, 668, 719) from Korean patients. Morphologic study was carried out using light and electron microscopes. CEA, alpha FP, and CA 19-9 and TPA in supernatant and in cell lysate were measured by radioimmunoassay. p53 and c-Ki-ras gene mutations were screened and confirmed by sequencing. The cell lines, derived from tumors with moderate differentiation, grew as a diffuse monolayer, and those from tumors with poor differentiation and minimal desmoplasia grew exclusively as non-adherent. Out of the 8 gastric-cancer cell lines, 5 had detectable levels of CEA both in supernatant and in cell lysate; there was no expression or secretion of alpha FP in these cells; 4 cell lines showed high levels of CA 19-9 in cell pellets. All cell lines except SNU-484 had high concentrations of TPA both in cell lysate and in supernatants. p53 mutation was found in 6 cell lines (75%): 2 (SNU-216 and SNU-668) had mutations in exon 6, and other 3 in exon 8. The c-Ki-ras mutation was found in 2 cell lines (25%), SNU-601 and SNU-668. The former showed GGT-to-GAT transition mutation at codon 12, while the latter showed CAA-to-AAA transversion mutation at codon 61. DNA profiles using restriction endonuclease HinfI and polymorphic DNA probes ChdTC-15 and ChdTC-114 showed different unique patterns; which suggests that these cell lines are unique and not cross-contaminated. We believe that the newly characterized gastric-cancer cell lines presented in this paper will provide a useful in vitro model for studies related to human gastric cancer.

Uncoordinated regulation of mRNA expression of the three isoforms of transforming growth factor-beta in the mouse skin carcinogenesis model

The mRNA expression and autoregulation of expression of the three isoforms of transforming growth factor-beta (TGFbeta) were examined in the mouse skin carcinogenesis model by northern analyses. We found that TGFbeta3 mRNA levels followed a pattern similar to those of TGFbeta1 during carcinogenesis: the levels were somewhat low in normal skin but became highly overexpressed in late-stage papillomas and squamous cell carcinomas (15- to 20-fold higher than in normal skin). On the other hand, the TGFbeta2 mRNA levels remained relatively low in all benign and malignant tumors, even though the levels were higher than the nearly undetectable levels in normal skin. In a squamous cell carcinoma cell line (CH72), stable transfection and expression of a mutated simian TGFbeta1 cDNA producing bioactive TGFbeta1 significantly downregulated (mean greater than ten-fold) TGFbeta2 mRNA levels and modestly downregulated (about twofold) murine TGFbeta1 expression but had no effect on TGFbeta3 mRNA. In contrast, treatment of all CH72 clones with exogenous TGFbeta1, TGFbeta2, or TGFbeta3 either had no effect or slightly downregulated TGFbeta1 mRNA, upregulated TGFbeta2 mRNA expression an average of twofold to threefold, and strongly upregulated (mean 13- to 27-fold) TGFbeta3 mRNA levels. TGFbeta treatment of primary cultures of mouse skin keratinocytes upregulated all three TGFbeta mRNA levels slightly to moderately (1.3- to 5-fold). Thus, although TGFbeta1 and TGFbeta3 mRNA expressions were apparently coordinately upregulated during mouse skin carcinogenesis, the three TGFbeta mRNAs were differentially regulated by stable transfection of active TGFbeta1 versus exogenous TGFbeta treatment in CH72 cells and by TGFbeta treatments of normal keratinocytes versus carcinoma CH72 cells.

Mechanism of E1A-induced transforming growth factor-beta (TGF-beta) resistance in mouse keratinocytes involves repression of TGF-beta type II receptor transcription

Cellular transformation driven by the E1A oncogene is associated with the development of cellular resistance to the growth inhibitory effects of transforming growth factor-beta (TGF-beta). We demonstrate that development of resistance occurs simultaneously with decreased expression of TGF-beta type II receptor (TGF-beta RII) mRNA and protein. To determine whether changes in transcriptional regulation are responsible for the decreased receptor expression in E1A-transformed cells, a series of mobility shift assays was performed utilizing nuclear extracts from E1A-transformed and untransformed murine keratinocytes using radiolabeled positive regulatory elements (PRE1 and PRE2) of the TGF-beta RII promoter. The results from these assays suggest that E1A-transformed cells express markedly lower levels of nuclear proteins that bind specifically to PRE1 and PRE2. Transfection of both E1A-transformed and untransformed cell lines with a series of mutant promoter constructs confirmed that both PREs contribute significantly to basal expression of TGF-beta RII and that inactivation of either element leads to markedly reduced promoter activity. We conclude that development of TGF-beta resistance in E1A-transformed cells is achieved in part through transcriptional down-regulation of the TGF-beta RII gene and that this down-regulation is the result of decreased expression of unidentified transcription factor complexes that interact with PRE1 and PRE2.

Proliferation of mutators in A cell population

A Lac- strain of Escherichia coli that reverts by the addition of a G to a G-G-G-G-G-G sequence was used to study the proliferation of mutators in a bacterial culture. Selection for the Lac+ phenotype, which is greatly stimulated in mismatch repair-deficient strains, results in an increase in the percentage of mutators in the selected population from less than 1 per 100,000 cells to 1 per 200 cells. All the mutators detected were deficient in the mismatch repair system. Mutagenesis results in a similar increase in the percentage of mutators. Mutagenesis combined with a single selection can result in a population of more than 50% mutators when a sample of several thousand cells is grown out and selected. Mutagenesis combined with two or more successive selections can generate a population that is 100% mutator. These experiments are discussed in relation to ideas that an early step in carcinogenesis is the creation of a mutator phenotype.

Deletions of the short arm of chromosome 3 in solid tumors and the search for suppressor genes

The concept that cells can become malignant upon the elimination of parts of chromosomes inhibiting cell division dates back to Boveri in 1914. Deletions occurring in tumor cells are therefore considered a first indication of possible locations of tumor suppressor gene. Approaches used to localize and identify the paradigm of tumor suppressors, RB1, have also been applied to localize tumor suppressor genes on 3p, the short arm of chromosome 3. This review discusses the methodological advantages and limitations of the various approaches. From a review of the literature on losses of 3p in different types of solid tumors it appears that some tumor types show involvement of the same region, while between others the regions involved clearly differ. Also discussed are results of functional assays of tumor suppression by transfer of part of chromosome 3 into tumor cell lines. The likelihood that a common region of deletions would contain a tumor suppressor is strongly enhanced by coincidence of that region with a chromosome fragment suppressing tumorigenicity upon introduction in tumor cells. Such a situation exists for a region in 3p21.3 as well as for one or more in 3p12-p14. The former region is considered the location of a lung cancer suppressor. The same gene or a different one in the same region may also play a role in the development of other cancers including renal cell cancer. In the latter cancer, there may be additional roles of the VHL region and/or a 3p12-p14 region. The breakpoint region of a t(3;8) originally found to be constitutively present in a family with hereditary renal cell cancer now seems to be excluded from such a role. Specific genes on 3p have been suggested to act as suppressor genes based on either their location in a common deletion region, a markedly reduced expression or presence of aberrant transcripts, their capacity to suppress tumorigenicity upon transfection in to tumor cells, the presumed function of the gene product, or a combination of several of these criteria. A number of genes are evaluated for their possible role as a tumor suppressor according to these criteria. General agreement on such a role seems to exist only for VHL. Though hMLH1 plays an obvious role in the development of specific mismatch repair-deficient cancers, it cannot revert the tumor phenotype and therefore cannot be considered a proper tumor suppressor. The involvement of VHL and MLH1 also in some specific hereditary cancers allowed to successfully apply linkage analysis for their localization. TGFBR2 might well have a tumor suppressor function. It does reduce tumorigenicity upon transfection. Other 3p genes coding for receptor proteins THRB and RARB, are unlikely candidates for tumor suppression. Present observations on a possible association of FHIT with tumor development leave a number of questions unanswered, so that provisionally it cannot be considered a tumor suppressor. Regions that have been identified as crucial in solid tumor development appear to be at the edge of synteny blocks that have been rearranged through the chromosome evolution which led to the formation of human chromosome 3. Although this may merely represent a chance occurrence, it might also reflect areas of genomic instability.

Growth Factors

Growth factors are polypeptides that interact with specific cellular receptors leading to many different biological responses. There are various families of growth factors that have similar biochemical structures. Although many growth factors stimulate cell proliferation, a few have primarily inhibitory functions, such as transforming growth factor-p (TGFB). Growth factors regulate various modes of action of endocrine tissues, including autocrine, paracrine, and endocrine functions. Recent studies have shown that growth factors also regulate various proteins in the cell cycle, and may have a direct or indirect effect on cell proliferation. For example, TGFB regulates various inhibitory cell-cycle proteins, including p27(kip1) and p15(INK4B). Molecular analyses of growth factors, including the cloning and sequencing of specific growth factor receptors, have contributed greatly to our understanding of the role these factors play in cellular homeostasis and neoplastic development.

Expression of transforming growth factor-beta receptor type I and type II in rat ventral prostate and Dunning R3327 PAP adenocarcinoma in response to castration and oestrogen treatment

In the normal prostate, transforming growth factor-beta 1 (TGF-beta 1) inhibits epithelial cell growth and is associated with apoptosis. The role of TGF-beta 1 in prostate cancer remains, however, unclear. In this work, the expression of TGF-beta receptor type I and II (TGF beta-RI and TGF beta-RII) in the Dunning R3327 PAP adenocarcinoma was studied, after castration and oestrogen treatment. Since castration induces apoptosis in the rat ventral prostate (VP) [21], but not in the Dunning R3327 PAP tumour [46], the TGF-beta receptor levels in the tumour were compared to the receptor levels in the VP. Methods used were competitive reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. In the VP, TGF beta-RI and TGF beta-RII expressions were increased after castration, indicating a negative regulation of TGF beta receptors by androgens. In the Dunning tumour, TGF beta-RI and TGF beta-RII levels were elevated and only TGF beta-RI showed a clear-cut increase after castration. The receptors were located in epithelial and smooth muscle cells in the VP and mainly in epithelial cells in the Dunning tumour. In conclusion, the elevated TGF beta receptor levels and the diminished androgen regulation of TGF beta-RII in the tumour distinguish the Dunning R3327 PAP tumour from the normal prostate and need to be further elucidated.

TGFB, TGFB Receptors, Ki-67, and p27(Kip)l Expression in Papillary Thyroid Carcinomas

Although most papillary thyroid carcinomas behave as low-grade neoplasms and are generally associated with a good prognosis, some subgroups of these neoplasms represent more aggressive variants. In order to determine if differences in the behavior of these papillary carcinomas were related to expression of growth factors or cell-cycle proteins, we analyzed a series of papillary carcinomas including the conventional or usual type (n = 27), tall cell (n = 27), diffuse sclerosing (n = 5), and columnar cell (n = 2) variants for expression of transforming growth factor beta (TGB), TGB receptors (TGB-RI and II, the proliferation marker Ki-67, and for the cell-cycle inhibitory protein p27(Kip)1 (p27). All groups of thyroid tumors expressed TGFB and TGFB-RI and RlI by immunohistochemical staining. There was a marked increase in the Ki-67 labeling index after staining with antibody MIB-1 in the columnar cell tumors compared to the other groups, but this difference was not significant because of the small number of tumors in this group. The cell-cycle inhibitory protein p27 was expressed in all groups and was not significantly different between groups. Normal thyroid cells had a higher labeling index for p27 compared to papillary carcinomas. These results indicate that TGFB and TGFB receptors I and II are commonly expressed in the usual and in variant forms of papillary thyroid carcinomas, and that there is decreased expression of p27 protein in all of these neoplasms compared to normal thyroid. The biological basis for the more aggressive behavior of these variants of papillary thyroid carcinoma remains uncertain.

Cyclin-dependent kinase regulation during G1 phase and cell cycle regulation by TGF-beta

The aim of this review is to provide insight into the molecular mechanisms by which transforming growth factor-beta (TGF-beta) modulates cell cycle progression in different cell types. Particular attention is focused on the differences between these mechanisms in cells of epithelial origin and in mesenchymally derived cells. This is important because many transformed epithelial cells lose responsiveness to the growth-inhibitory effects of TGF-beta, thus generating a more fibroblast-like phenotype. Loss of negative growth control, including a lack of response to growth-inhibitory factors, is a common feature of many tumor cells. G1 phase cyclin-dependent kinases (cdks) and their inhibitors (ckis) are central to the pathways that regulate commitment to cellular division in response to positive as well as negative growth effectors. Many checkpoints are deregulated in oncogenesis, and this is often due to alterations in cyclin-cdk complexes. The loss of R-point regulation, in particular, can allow cell growth and division to proceed autonomously of external signals. This may occur due to either the aberrant expression of positive regulators, such as the cyclins and cdks, or the loss of negative regulators, such as the ckis. Beginning with a survey of the role of the cdks in the mammalian cell cycle, the review examines how cdk activity is modulated by cyclin binding, phosphorylation, and ckis, including the Ink4 proteins and the closely related inhibitors p21Cip1 and p27Kip1. Particular attention is paid to the role of p27Kip1 and p21Cip1 in the mechanisms of TGF-beta-induced suppression or stimulation of the cell cycle and how these mechanisms contrast between epithelial cells and cells of mesenchymal origin. Other aspects of TGF-beta signal transduction are discussed, including its effects on cyclin and cdk expression in various cell types, and the downstream targets of cdks and their modulation by TGF-beta and other growth factors are also discussed. These include proteins of the retinoblastoma family, and the related modulation of the transcriptional activity of the E2F family members. Finally, the role of cell cycle regulatory proteins in oncogenesis is review in view of the findings described here.

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.

Mutations of the transforming growth factor beta type II receptor gene and microsatellite instability in gastric cancer

Forty-three sporadic gastric cancers were analyzed with regard to whether mutations of simple repeated sequences in the transforming growth factor beta type II receptor (TbetaR-II) gene are associated with microsatellite instability (MSI) and gastric carcinogenesis. In 12 of the 43 cancers (28%), MSI was observed at least at 1 of the 2 microsatellite loci. Frameshift mutations of the TbetaR-II gene, all of which were 1 base deletion of 10 adenine repeats, were detected in 3 of 6 cancers, with MSI at 2 loci. However, mutations were not detected in 6 cancers, with MSI only at 1 locus and 31 cancers without MSI. Moreover, microanalysis in these cases revealed that the mutant-type alleles of TbetaR-II were invariably common in different areas within the tumor, in contrast to the markedly variable alleles of microsatellite loci. Our results suggest that frameshift mutation of the TbetaR-II gene may be a critical event associated with MSI and may contribute to carcinogenesis of the stomach. One of the possible mechanisms of escape from growth control by TGFbeta during gastric carcinogenesis could involve frameshift mutations of the TbetaR-II gene caused by DNA replication errors.

Defects of TGF-beta receptor signaling in mammary cell tumorigenesis

Transforming growth factor beta (TGF-beta) receptor expression and signal transduction in human breast cancer are reviewed as a function of estrogen receptor (ER) expression. ER+ breast cancer cells are generally resistant to the inhibitory effects of TGF-beta. The only known exception appears to be MCF-7 early passage cells which are initially sensitive to TGF-beta, but gain resistance after long-term passage in tissue culture. A number of studies have shown that loss of sensitivity is due to inadequate TGF-beta type II (TGFRII) receptor expression. Stable transfection of TGFRII into ER+ breast cancer cell lines results in the acquisition of TGF-beta sensitivity and reversion of malignancy. Although there are exceptions, ER- breast cancer cells usually express TGFRII, but nevertheless show a low level of sensitivity to TGF-beta. Thus resistance in these cells implies a postreceptor mechanism. Given the frequency with which loss of TGF-beta sensitivity has been associated with loss of TGFRII, the ER- breast cancer cell lines may represent valuable models for identifying postreceptor mechanisms of resistance.

TGF beta-induced growth inhibition in primary fibroblasts requires the retinoblastoma protein

Transforming growth factor beta (TGF beta) inhibits cell proliferation by inducing a G1 cell-cycle arrest. Cyclin/CDK complexes have been implicated in this arrest, because TGF beta treatment leads to inhibition of cyclin/CDK activity. We have investigated the role of the retinoblastoma protein (pRb) in TGF beta-induced growth arrest by using RB+/+ and RB-/- primary mouse embryo fibroblasts. In both of these cell types, TGF beta inhibits CDK4-associated kinase activity. However, whereas CDK2-associated kinase activity was completely inhibited by TGF beta in the wild-type cells, it was reduced only slightly in the RB mutant cells. In addition, at high-cell density the growth-inhibitory effects of TGF beta are no longer observed in the RB-/- cells; on the contrary, TGF beta treatment promotes the growth of these mutant fibroblasts. Thus, under certain cellular growth conditions, elimination of pRb transforms the growth-inhibitory effects of TGF beta into growth-stimulatory effects. These observations could help to explain why TGF beta is often found to enhance tumorigenicity in vivo and why inactivation of the RB gene leads to tumorigenesis.

Differential control of expression of type I and type II receptors of transforming growth factor-beta in colon carcinoma cells

We investigated TGF-beta response and the expression of TGF-beta receptors in clones of MOSER colon carcinoma cells (designated MOSER II and MOSER III-10) as a function of their growth state. TGF-beta 1 response as assessed by induction of fibronectin expression was higher (3.0-fold) in exponentially growing cells than in quiescent cells. The expression of type I receptor (RI) mRNA was greater (2.5 to 3.0-fold) in exponentially growing cells than in quiescent cells. In contrast, the expression of type II receptor (RII) mRNA was marginally increased in quiescent cells relative to exponential cells. Nuclear run-off assays, and actinomycin-D treatment indicated that the increased expression of RI mRNA in exponentially growing cells was primarily due to an increase in transcription, while a marginal increase in mRNA level for RII in quiescent cells was primarily due to an increase in mRNA stability. Affinity cross-linking with 125I-labeled TGF-beta 1, showed that the exponentially growing cells displayed greater amounts of 125I TGF-beta 1 binding to RI and RII than quiescent cells, indicating that increased cell surface expression of receptors was correlated with increased response in the exponential growth state. Immunoblot analysis also indicated greater amounts of RI protein in exponential compared to quiescent cells; however, no difference in RII protein was observed in the two growth states. These data indicate that expression of the receptors responsible for TGF-beta signal transduction are differentially controlled.

MADR2 maps to 18q21 and encodes a TGFbeta-regulated MAD-related protein that is functionally mutated in colorectal carcinoma

The MAD-related (MADR) family of proteins are essential components in the signaling pathways of serine/threonine kinase receptors for the transforming growth factor beta (TGFbeta) superfamily. We demonstrate that MADR2 is specifically regulated by TGFbeta and not bone morphogenetic proteins. The gene for MADR2 was found to reside on chromosome 18q21, near DPC4, another MADR protein implicated in pancreatic cancer. Mutational analysis of MADR2 in sporadic tumors identified four missense mutations in colorectal carcinomas, two of which display a loss of heterozygosity. Biochemical and functional analysis of three of these demonstrates that the mutations are inactivating. These findings suggest that MADR2 is a tumor suppressor and that mutations acquired in colorectal carcinomas may function to disrupt TGFbeta signaling.

Serine phosphorylation, chromosomal localization, and transforming growth factor-beta signal transduction by human bsp-1

The transforming growth factor-beta (TGF-beta) superfamily regulates a multitude of cellular and developmental events. TGF-beta family ligands signal through transmembrane serine/threonine kinase receptors whose downstream effectors are largely unknown. Using genetic data from the fruit fly, we have identified a downstream effector of TGF-beta-induced signaling. TGF-beta signaling protein-1 (BSP-1) is rapidly phosphorylated in response to TGF-beta. Localization of bsp-1 to chromosome 4q28 suggests a role in carcinogenesis. These data suggest that BSP-1 is the prototype of a new class of signaling molecules.

Reduced expression of transforming growth factor beta type I receptor contributes to the malignancy of human colon carcinoma cells

Transforming growth factor beta (TGFbeta) type I (RI) and type II (RII) receptors are essential for TGFbeta signal transduction. A human colon carcinoma cell line, designated GEO, is marginally responsive to TGFbeta and expresses a low level of RI mRNA relative to colon carcinoma cells, which are highly responsive to TGFbeta. Hence, the role of RI as a limiting factor for TGFbeta sensitivity and the contribution of low RI levels to the malignant phenotype of GEO cells were examined. Stable transfection of a tetracycline-regulatable rat RI cDNA increased TGFbeta1 binding to RI and resulted in increased growth inhibition by exogenous TGFbeta1. In contrast, although stable transfection of an RII expression vector into the same GEO cells increased TGFbeta1 binding to RII, growth inhibition by exogenous TGFbeta1 was not altered. This indicated that the low level of RI is a limiting factor for the growth-inhibitory effects of TGFbeta in GEO cells. RI-transfected cells were growth-arrested at a lower saturation density than GEO control cells. They also showed reduced growth and clonogenicity in plating efficiency and soft agarose assays, whereas RII-transfected cells did not show any differences from the NEO control cells in these assays. Tetracycline repressed RI expression in transfected cells and reversed the reduction in plating efficiency of RI-transfected clones, confirming that growth effects were due to increased RI expression in transfected cells. TGFbeta1 neutralizing antibody stimulated the proliferation of RI-transfected cells but had little effect on GEO control cells, indicating that increased autocrine-negative TGFbeta activity also resulted from increased RI expression. Tumorigenicity in athymic nude mice was significantly delayed in RI-transfected cells. These results indicate that low RI expression can be a limiting factor for response to exogenous TGFbeta, as well as TGFbeta autocrine-negative activity, and that reduction of RI expression can contribute to malignant progression.

Attenuated ALK5 receptor expression in human pancreatic cancer: correlation with resistance to growth inhibition

Transforming growth factor-beta (TGF-beta) receptors constitute a family of transmembrane proteins that bind TGF-beta ligands. In this study we assessed the growth responsiveness to TGF-beta 1 in pancreatic cancer cell lines and characterized the levels of expression of TGF-beta receptors in these cell lines and in human pancreatic cancer tissues. COLO 357 cells were most sensitive to the growth inhibitory actions of TGF-beta 1, PANC-1 cells exhibited moderate sensitivity, Hs766T cells exhibited slight sensitivity and MIA PaCa-2 and T3M4 cells were resistant to TGF-beta 1. Only COLO 357 cells expressed high levels of ALK5, the major type I TGF-beta receptor (T beta RI). Hs766T and PANC-1 cells expressed high levels of SKR1, another T beta RI subtype. Only MIA PaCa-2 cells did not exhibit the type II TGF-beta receptor (T beta-RII) transcript, whereas type III TGF-beta receptor (T beta-RIII) mRNA levels were elevated in this cell line and in HS766T cells. All the cell lines expressed TGF-beta 1, but TGF-beta 2 and TGF-beta 3 mRNA levels were variable. ALK5 and SKR1 mRNA levels were 6.8- and 9-fold greater in the pancreatic tumors in comparison with the corresponding levels in the normal pancreas. However, in the cancer cells, ALK5 immunoreactivity was faint, whereas T beta RII immunoreactivity was focal and intense. Conversely, in ductal cells adjacent to cancer cells ALK5 immunoreactivity was strong, whereas T beta RII immunoreactivity was weak. Since ALK5 heterodimerization with T beta RII is crucial for TGF-beta-mediated signaling, our findings suggest that low levels of ALK5 in pancreatic cancer cells within a tumor may protect against growth inhibition.

A dominant inhibitory mutant of the type II transforming growth factor beta receptor in the malignant progression of a cutaneous T-cell lymphoma

In many cancers, inactivating mutations in both alleles of the transforming growth factor beta (TGF-beta) type 11 receptor (TbetaRII) gene occur and correlate with loss of sensitivity to TGF-beta. Here we describe a novel mechanism for loss of sensitivity to growth inhibition by TGF-beta in tumor development. Mac-1 cells, isolated from the blood of a patient with an indolent form of cutaneous T-cell lymphoma, express wild-type TbetaRII and are sensitive to TGF-beta. Mac-2A cells, clonally related to Mac-1 and isolated from a skin nodule of the same patient at a later, clinically aggressive stage of lymphoma, are resistant to TGF-beta. They express both the wild-type TbetaRII and a receptor with a single point mutation (Asp-404-Gly [D404G]) in the kinase domain (D404G-->TbetaRII); no TbetaRI or TbetaRII is found on the plasma membrane, suggesting that D404G-TbetaRII dominantly inhibits the function of the wild-type receptor by inhibiting its appearance on the plasma membrane. Indeed, inducible expression, under control of a tetracycline-regulated promoter, of D404G-TbetaRII in TGF-beta- sensitive Mac-1 cells as well as in Hep3B hepatoma cells results in resistance to TGF-beta and disappearance of cell surface TbetaRI and TbetaRII. Overexpression of wild-type TbetaRII in Mac-2A cells restores cell surface TbetaRI and TbetaRH and sensitivity to TGF-beta. The ability of the D404G-TbetaRH to dominantly inhibit function of wild-type TGF-beta receptors represents a new mechanism for loss of sensitivity to the growth-inhibitory functions of TGF-beta in tumor development.

Lack of expression of transforming growth factor-beta type II receptor associated with malignant progression in human salivary gland cell clones

To understand the molecular mechanisms whereby normal human salivary gland cells become malignant and escape growth-inhibitory control by transforming growth factor (TGF)-betaI, we examined the effect of TGF-betaI on the proliferation and expression of TGF-beta receptors in cells and the expression of TGF-beta type II receptor (TbetaR-II) mRNA. An SV40-immortalized normal human salivary gland duct cell clone (NS-SV-DC) with no tumorigenic ability, originally obtained via s.c. implantation into nude mice, was partially resistant to the growth-inhibitory effect of TGF-betaI, while a neoplastic human salivary gland duct cell clone (HSGc) with tumorigenic, but not metastatic, potential in nude mice was more resistant to the growth-suppressive effect of TGF-betaI than NS-SV-DC. Metastatic cell clones derived from carcinogen-treated HSGc were completely refractory to the anti-proliferative effect of TGF-betaI. Affinity cross-linking revealed that NS-SV-DC possesses the types I, II (TbetaR-II) and III receptors. However, HSGc and metastatic cell clones lacked expression of detectable levels of the TbetaR-II protein. Moreover, we evaluated TbetaR-II mRNA expression in these cell clones by Northern blot analysis and observed that, although NS-SV-DC expressed a large amount of TbetaR-II mRNA, a small amount of TbetaR-II mRNA was detectable in HSGc. In contrast, no significant bands were detected in metastatic cell clones. Our results, therefore, suggest that one of the possible mechanisms of escape from autocrine or paracrine growth inhibition by TGF-betaI during human salivary gland carcinogenesis involves reduced expression or lack of TbetaR-II.

Transforming Growth Factor-beta Receptors: Role in Physiology and Disease

Transforming growth factor-beta (TGF-beta) plays a pivotal role in numerous vital cellular activities, most significantly the regulation of cellular proliferation and differentiation and synthesis of extracellular matrix components. Its ubiquitous presence in different tissues and strict conservation of nucleotide sequence down through the most primitive vertebrate organism underscore the essential nature of this family of molecules. The effects of TGF-beta are mediated by a family of dedicated receptors, the TGF-beta types I, II, and III receptors. It is now known that a wide variety of human pathology can be caused by aberrant expression and function of these receptors or their cognate ligands. The coding sequence of the human type II receptor appears to render it uniquely susceptible to DNA replication errors in the course of normal cell division. There are now substantial data suggesting that TGF-beta type II receptor should be considered a tumor suppressor gene. High levels of mutation in the TGF-beta type II receptor gene have been observed in a wide variety of primarily epithelial malignancies, including colon, gastric, and hepatic cancer. It appears likely that mutation of the TGF-beta type II receptor gene represents a very critical step in the pathway of carcinogenesis. Copyright 1996 S. Karger AG, Basel

Expression and autoregulation of transforming growth factor beta receptor mRNA in small-cell lung cancer cell lines

In small-cell lung cancer cell lines resistance to growth inhibition by transforming growth factor (TGF)-beta 1, was previously shown to correlate with lack of TGF-beta receptor I (RI) and II (RII) proteins. To further investigate the role of these receptors, the expression of mRNA for RI, RII and beta-glycan (RIII) was examined. The results showed that loss of RII mRNA correlated with TGF-beta 1 resistance. In contrast, RI-and beta-glycan mRNA was expressed by all cell lines, including those lacking expression of these proteins. According to Southern blot analysis, the loss of type II mRNA was not due to gross structural changes in the gene. The effect of TGF-beta 1 on expression of TGF-beta receptor mRNA (receptor autoregulation) was examined by quantitative Northern blotting in four cell lines with different expression of TGF-beta receptor proteins. In two cell lines expressing all three TGF-beta receptor proteins beta-glycan mRNA was rapidly down-regulated and this effect was sustained throughout the 24 h observation period. RI and RII mRNAs were slightly increased 24 h after treatment. In one cell line sensitive to growth inhibition by TGF-beta, 1 but lacking beta-glycan expression, and one cell line expressing only beta-glycan and thus TGF-beta 1 -resistant, no autoregulation of mRNA of either TGF-beta receptor was demonstrated. The results suggest that TGF-beta 1 regulates the expression of its receptors, in particular beta-glycan, and that this effect is dependent on co-expression of beta-glycan, RI and RII.

Distribution of transforming growth factor-beta and its receptors in gastric carcinoma tissue

The distribution of the three mammalian isoforms of transforming growth factor (TGF)-beta (TGF-beta 1, -beta 2, and -beta 3) as well as their signaling receptors, TGF-beta type I and type II receptors (T beta R-I and T beta R-II, respectively), in gastric carcinoma tissue was examined by immunohistochemistry using specific antibodies. Tissue specimens were obtained from 25 cases of gastric carcinoma, which were classified into two groups according to Lauren's classification, i.e. 15 cases of diffuse carcinoma and 10 cases of intestinal carcinoma. In normal gastric mucosa apart from carcinoma nests, all of TGF-beta 1, -beta 2, -beta 3, T beta R-I and T beta R-II were clearly demonstrated in fundic glands. In sharp contrast, none of them was detectable in surface mucous cells. In carcinoma cells, strong staining for TGF-beta 1, -beta 2 and -beta 3 was obtained only in diffuse-type carcinoma. In particular, carcinoma cells scattered as single cells or small nests had a tendency to show strong staining for TGF-betas. The receptors tended to be distributed concomitantly with the ligands, and diffuse-type carcinoma showed stronger receptor staining than intestinal-type carcinoma. In cancer stroma, TGF-betas and receptors were detected in both diffuse and intestinal types, but the area with positive staining was wider and more dispersed in diffuse-type carcinoma than in intestinal carcinoma. These results suggest that TGF-beta may contribute in part to the variety of histogenesis and mode of progression of gastric carcinoma.

Loss of responsiveness to transforming growth factor beta (TGFbeta) is tightly linked to tumorigenicity in a model of thyroid tumour progression

It has been suggested that an important step in the progression of some epithelial tumours is the loss of responsiveness to the growth-inhibitory effects of transforming growth factor beta (TGFbeta). Here we describe the use of a model of thyroid tumorigenesis to investigate this question. Seven genetically closely related epithelial cell lines were derived following infection of primary cultures of rat thyroid epithelium with retroviral vectors encoding mutant ras. A strong negative correlation (p < 0.001) was found between the responsiveness of the lines to TGFbeta growth inhibition in vitro and their tumorigenicity in nude mice. Whereas TGFbeta-unresponsive and TGFbeta-stimulated lines formed rapidly growing, poorly differentiated tumours at all injection sites, cells that retained a partial inhibitory response formed much more slowly growing tumours, which showed a high degree of glandular differentiation. A line which retained full inhibition by TGFbeta formed slowly growing tumours at only 30% of injection sites, and cells explanted from these tumours subsequently showed a much reduced TGFbeta response in vitro. Our data using thyroid cells thus greatly strengthen the suggestion from previous studies that loss of growth inhibition by TGFbeta is associated with malignant progression of epithelial tumours. We also present an experimental model of papillary thyroid cancer which may prove useful in identifying the molecular changes involved in progression to the anaplastic form of the disease.