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

Expression and Regulation of Transforming Growth Factor B1 in Cultured Normal and Neoplastic Rat Pituitary Cells

Pituitary prolactin (PRL) cell gene expression and proliferation are regulated by hormones and growth factors. Transforming growth factor beta (TGFB) and blast growth factor (bFGF) have been implicated in the regulation of antenor pituitary function. To study the roles of TGFB and bFGF in anterior pituitary cell function, we analyzed normal and neoplastic pituitary cells in serum-free media. The various isoforms of TGFB and TGFB receptor types I, II, and III were also analyzed by reverse transcription-polymerase chain reaction (RT-PCR) in pituitary cells. Transforming growth factor beta 1 (TGFB1) stimulated PRL expression and PRL cell proliferation in normal pituitary. TGFB1 stimulated PRL expression, but inhibited proliferation in the growth hormone (GH) and PRL-producing GH(3) cells. Estradiol 17 B (E(2)) and bFGE stimulated PRL gene expression in normal pituitary and GH(3) cells, whereas E(2) inhibited and bFGF stimulated TGFB1 mRNA levels in normal pituitary PRL cells, but not in GH(3) cells. Both normal pituitary and GH(3) cells expressed the mRNAs for TGFB1, TGFB2, and TGFB3 isoforms and for TGFB receptors I, II, and III. These results indicate that there is a relative loss of regulatory control by growth factors in neoplastic GH(3) cells compared to normal pituitary PRL cells.

Characterization of the promoter region of the human transforming growth factor-beta type II receptor gene

Diminished cellular responsiveness to transforming growth factor-beta (TGF-beta) is frequently correlated with decreased transcription of the type II receptor for TGF-beta (TGF-beta RII). We have cloned and characterized the human TGF-beta RII promoter and, using S1 nuclease mapping and 5' rapid amplification of cDNA ends polymerase chain reaction, have identified five alternative transcription start sites within the region -33 to +57. DNA transfection experiments and electrophoretic mobility shift assays have revealed the existence of five distinct regulatory regions including two positive regulatory elements and two negative regulatory elements in addition to the core promoter region. The first positive regulatory element (-219 to -172) interacts with two distinct nuclear protein complexes, at least one of which appears to be a previously unidentified transcription factor. The second positive regulatory element (+1 to +35) also interacts with two separate protein complexes, both of which appear to be novel transcription factors. Deletion of either positive regulatory element markedly decreased expression of the target gene, suggesting that both positive regulatory elements are necessary for basal expression levels of TGF-beta RII.

Microsatellite instability and a TGF beta receptor: clues to a growth control pathway

Defects of growth inhibitory pathways have an important role in disorders of cell growth and differentiation. The discovery of a mutation in one of the principle components of the transforming growth factor beta (TGF beta) receptor system which is linked to a DNA repair defect(1) represents one possible mechanism of escape from negative regulatory influences acting upon cells. TGF beta is a pre-eminent negative growth factor and this article discusses (1) the role of TGF beta in maintaining epithelial homeostasis; (2) how breakdown of inhibitory pathways can promote neoplastic development; (3) the significance of a receptor defect in a negative signalling pathway; and (4) the potential therapeutic sequelae resulting from restoration of cellular responsiveness to TGF beta.

Increased tumorigenicity in the human pancreatic cell line MIA PaCa-2 is associated with an aberrant regulation of an IGF-1 autocrine loop and lack of expression of the TGF-beta type RII receptor

The growth characteristics associated with tumorigenicity were determined in clones of MIA PaCa-2 and PANC-1 pancreatic carcinoma cells. MIA PaCa-2 cells differed from PANC-1 cells in that they rapidly formed tumors in nude mice, formed colonies more rapidly and formed larger colonies in soft agar, and were cloned more efficiently when seeded at low density. MIA PaCa-2 cells but not PANC-1 cells were stimulated to escape quiescence and undergo DNA synthesis with nutrient media lacking growth factors. Both cell lines were stimulated to proliferate with serum-free media containing EGF, transferrin, and insulin. Antibody neutralization assays indicated that an IGF-1 autocrine loop was required for the nutrient stimulation of growth in MIA PaCa-2 cells and for the growth-factor stimulation in both MIA PaCa-2 and PANC-1 cells. Both cell lines were stimulated to proliferate with exogenous IGF-1 in basal media; this stimulation was specifically blocked by antibodies to IGF-1 or its receptor. MIA PaCa-2 and PANC-1 cells expressed similar levels of IGF-1 receptor mRNA and showed similar binding kinetics in receptor binding assays. In contrast to PANC-1 cells, MIA PaCa-2 cells were insensitive to TGF-beta 1 and did not express TGF-beta receptor type II. The results suggest that the growth-factor independence is representative of a more tumorigenic phenotype. We hypothesize that growth-factor independence of MIA PaCa-2 cells is mediated by an aberrant regulation of an IGF-1 autocrine loop. A decreased regulation of this IGF-1 loop may be potentiated by loss of response to TGF-beta.

Demonstration that mutation of the type II transforming growth factor beta receptor inactivates its tumor suppressor activity in replication error-positive colon carcinoma cells

Escape from negative growth regulation by transforming growth factor beta (TGF-beta) as a result of the loss of TGF-beta type II receptor (RII) expression has been found to be associated with the replication error (RER) colorectal cancer genotype, which is characteristic of hereditary nonpolyposis colorectal cancers. The RER-positive HCT 116 colon carcinoma cell line was examined for RII mutations. A 1-base deletion was found within a sequence of 10 repeating adenines (nucleotides 709-718), which resulted in a frameshift mutation. Although it is reasonable to predict that the loss of RII function would be an important determinant of malignancy, the large number of potential mutations in cells of this phenotype raises the possibility that an RII mutation may not be a key event in the tumorigenic phenotype of these cells. One way to test directly the importance of RII mutations in determining the malignant phenotype would be to restore its expression. If restoration of expression leads to diminished tumorigenicity, it would indicate that RII mutation is an important determinant of malignancy in the RER phenotype. To determine whether restoration of RII would lead to reversal of malignancy in RER colon cancers, an RII expression vector was transfected into the HCT 116 cell line. RII stable clones showed mRNA and protein expression of transfected RII. The fibronectin mRNA level was increased by exogenous TGF-beta 1 treatment in a dose-dependent manner in RII-positive clones, whereas the control cells remained insensitive. The RII transfectants showed reduced clonogenicity in both monolayer culture and soft agarose. They were growth arrested at a lower saturation density than control cells. TGF-beta 1-neutralizing antibody stimulated the proliferation of RII-transfected but not control cells, indicating that the alterations in the growth parameters of the transfected cells were due to the acquisition of autocrine-negative activity. Tumorigenicity in athymic mice was reduced and delayed in RII transfectants. These results indicate that reconstitution of TGF-beta autocrine activity by reexpression of RII can reverse malignancy in RER colon cancers, thus verifying that the malignancy of hereditary nonpolyposis colorectal cancer can be directly associated with the loss of RII expression.

Inactivation of the type II TGF-beta receptor in colon cancer cells with microsatellite instability

Transforming growth factor-beta (TGF-beta) is a potent inhibitor of epithelial cell growth. Human colon cancer cell lines with high rates of microsatellite instability were found to harbor mutations in the type II TGF-beta receptor (RII) gene. Eight such examples, due to three different mutations, were identified. The mutations were clustered within small repeated sequences in the RII gene, were accompanied by the absence of cell surface RII receptors, and were usually associated with small amounts of RII transcript. RII mutation, by inducing the escape of cells from TGF-beta-mediated growth control, links DNA repair defects with a specific pathway of tumor progression.

Colon cancer cells that are not growth inhibited by TGF-beta lack functional type I and type II TGF-beta receptors

OBJECTIVE

The authors determined the molecular mechanisms for the failure of transforming growth factor-beta (TGF-beta) to inhibit the growth of SW1116 and SW48 colon cancer cell lines.

BACKGROUND

Transforming growth factor-beta is a bifunctional regulator of cell growth that typically stimulates proliferation of mesenchymal cells, but inhibits proliferation of normal epithelial cells. In the colon, TGF-beta appears to arrest proliferation of enterocytes as they leave the intestinal crypt and move to the villus tip. Transforming growth factor-beta actions are mediated by binding to heteromeric complexes of type I and type II TGF-beta receptors. Loss of TGF-beta responsiveness may contribute to uncontrolled cell growth and cancer.

METHODS

The effects of TGF-beta 1 on DNA synthesis were measured by incorporation of tritiated thymidine into DNA of cultures of moderately differentiated adenocarcinoma (SW48) and poorly differentiated adenocarcinoma (SW1116) colon cell lines and a mink lung epithelial cell line (CCL-64). The effects of TGF-beta on the expression of c-myc, TGF-alpha, and TGF-beta in SW48 cells, SW1116 cells, and CCL-64 cells (c-myc only) were measured by Northern blot analysis. Expression of TGF-beta receptors in the cell lines was measured using competitive binding assays, receptor affinity labelling techniques, and reverse transcriptase-polymerase chain reaction.

RESULTS

Incubation with TGF-beta 1 (50 ng/mL) did not decrease serum-stimulated uptake of [3H]-thymidine into actively growing cultures of SW48 or SW1116 cells, but suppressed DNA synthesis of actively growing CCL-64 cells by 90%. Similarly, incubation with TGF-beta 1 (12 ng/mL) for 4 hours did not substantially alter the mRNA levels of c-myc, TGF-alpha, and TGF-beta 1 in either colon tumor cell line, although levels of c-myc mRNA in CCL-64 cells were reduced by TGF-beta 1 treatment. Competitive displacement of [125I]-TGF-beta 1 binding detected high levels (16,500 TGF-beta receptors per cell) of specific, high-affinity (200 pmol/L half-displacement) TGF-beta receptors on CCL-64 cells. In marked contrast, very low levels of TGF-beta 1 binding to SW1116 cells (250 receptors per cell) and SW48 cells (260 receptors per cell) were detected. Autoradiograms of CCL-64 cells affinity labelled with [125I]TGF-beta 1 revealed the presence of type I, type II, and type III TGF-beta receptors. No TGF-beta receptors were identified on SW1116 cells, and only very low levels of the nonsignaling type III TGF-beta receptors were detected on SW48 cells. Reverse transcriptase-polymerase chain reaction amplification detected mRNAs for type I, type II, and type III TGF-beta receptors in CCL-64 cells. SW48 cells, and SW1116 cells.

CONCLUSIONS

These results suggest that the lack of growth inhibition by TGF-beta in SW48 and SW1116 colon cancer cells may be caused by a lack of expression of functional TGF-beta receptors.