Malignant transformation by G protein-coupled hormone receptors

STAT-3 activity in chemically-induced hepatocellular carcinoma

The signal transducer and activator of transcription (STAT)-3 regulates basic biological processes and it has been reported to be constitutively active in different types of malignant tumours. STAT-3 is active during the regenerative growth of the liver, but there are hardly any data about its presence in liver tumours. We investigated and found a high activity of STAT-3 using an electrophoretic mobility shift assay (EMSA) in chemically-induced rat hepatocellular carcinomas (HCCs). Dexamethasone treatment downregulated both STAT-3 activity and cell proliferation in the tumours. Therefore, the activity of the STAT-3 signal transduction pathway seems to be required for the growth of HCCs and could be a potential new target for therapeutic trials of this tumour type.

Antiproliferative signaling of luteinizing hormone-releasing hormone in human endometrial and ovarian cancer cells through G protein alpha(I)-mediated activation of phosphotyrosine phosphatase

The signaling pathway through which LHRH acts in endometrial and ovarian cancers is distinct from that in the anterior pituitary. The LHRH receptor interacts with the mitogenic signal transduction of growth factor receptors, resulting in down-regulation of expression of c-fos and proliferation. Only limited data are available on the cross-talk between LHRH receptor signaling and inhibition of mitogenic signal transduction. The present experiments were performed to analyze in endometrial and ovarian cancer cells: 1) whether mutations or splice variants of the LHRH receptor are responsible for differences in LHRH signaling, 2) the coupling of G protein subtypes to LHRH receptor, 3) the phosphotyrosine phosphatase (PTP) activation counteracting growth factor receptor tyrosine kinase activity. For these studies, the well characterized human Ishikawa and Hec-1A endometrial cancer cell lines and human EFO-21 and EFO-27 ovarian cancer cell lines were used, which express LHRH and its receptor. 1) Sequencing of the complementary DNA of the LHRH receptor from position 31 to position 1204, covering the complete coding region (position 56 to position 1042) showed that there are neither mutations nor splice variants of the LHRH receptor transcript in Ishikawa and Hec-1A endometrial cancer cells or in EFO-21 and EFO-27 ovarian cancer cells. 2) All analyzed cell lines except for the ovarian cancer cell line EFO-27 expressed both G proteins, alpha(i) and alpha(q), as shown by RT-PCR and Western blotting. In the EFO-27 cell line only G protein alpha(i), not G protein alpha(q), expression was found. Cross-linking experiments using disuccinimidyl suberate revealed that in the cell lines expressing G protein alpha(i) and G protein alpha(q), both G proteins coupled to the LHRH receptor. Inhibition of epidermal growth factor (EGF)-induced c-fos expression by LHRH, however, was mediated through pertussis toxin (PTX)-sensitive G protein alpha(i). Moreover, LHRH substantially antagonized the PTX-catalyzed ADP-ribosylation of G protein alpha(i). 3) Using a phosphotyrosine phosphatase assay based on molybdate-malachite green, treatment of quiescent EFO-21 and EFO-27 ovarian cancer cells and quiescent Ishikawa and Hec-1A endometrial cancer cells with 100 nM of the LHRH agonist triptorelin resulted in a 4-fold increase in PTP activity (P < 0.001). This effect was completely blocked by simultaneous treatment with PTX, supporting the concept of mediation through G protein alpha(i). As shown by quantitative Western blotting, EGF-induced tyrosine autophosphorylation of EGF receptors was reduced 45-63% after LHRH (100 nM) treatment (P < 0.001). This effect was completely blocked using the PTP inhibitor vanadate (P < 0.001). These results demonstrate that mutations or splice variants of the LHRH receptor in human endometrial and ovarian cancer cells are not responsible for the different signal transduction compared with that in pituitary gonadotrophs. We provide evidence that the tumor LHRH receptor couples to multiple G proteins, but the antiproliferative signal transduction is mediated through the PTX-sensitive G protein alpha(i). The tumor LHRH receptor activates a PTP counteracting EGF-induced tyrosine autophosphorylation of EGF receptor, resulting in down-regulation of mitogenic signal transduction and cell proliferation.

Role of the cAMP and MAPK pathways in the transformation of mouse 3T3 fibroblasts by a TSHR gene constitutively activated by point mutation

Constitutive activating mutations of the TSHR gene, have been detected in about 30 per cent of hyperfunctioning human thyroid adenomas and in a minority of differentiated thyroid carcinomas. The mutations activating the TSHR gene(s) in the thyroid carcinomas, were located at the codon 623 changing an Ala to a Ser (GCC-->TCC) or in codon 632 changing a Thr to Ala or Ile (ACC-->GCC or ACC-->ATC). In order to study if the constitutively activated TSHR gene(s) has played a role in the determination of the malignant phenotype presented by these tumors, we investigated: (1) the transforming capacity after transfection of mouse 3T3 cells, of a TSHR cDNA activated by an Ala-->Ser mutation in codon 623 or an Thr-->Ile mutation in codon 632 and (2) the pathway(s) eventually responsible(s) for the malignant phenotype of the cells transformed by these constitutively activated TSHR cDNAs. Our results show that (1) the TSHR(M623) or (M632) cDNAs give rise to 3T3 clones presenting a fully neoplastic phenotype (growth in agar and nude mouse tumorigenesis); this phenotype was weaker in the cells transformed by the 632 cDNA; (2) suggest that the fully transformed phenotype of our 3T3 cells, may be the consequence of the additive effect of the activation of at least two different pathways: the cAMP pathway through G(alpha)s and the Ras dependent MAPK pathway through G(beta)gamma and PI3K and (3) show that the PI3K isoform playing a key role as an effector in the MAPK pathway activation in our 3T3-transformed cells is PI3Kgamma. Signaling from PI3Kgamma to MAPK appears to require in our murine cellular system a tyrosine kinase (still not characterized), Shc, Grb2, Sos, Ras and Raf. It is proposed that the constitutively activated TSHR genes detected in the thyroid carcinomas, may have played an oncogenic role, participating in their development through these two pathways.

Epidermal growth factor receptor transactivation mediates substance P-induced mitogenic responses in U-373 MG cells

Ligand-induced activation of G protein-coupled receptors is emerging as an important pathway leading to the activation of certain receptors with intrinsic tyrosine kinase activity, such as the epidermal growth factor receptor (EGFR). Substance P (SP) exerts many effects via activation of its G protein-coupled receptor (neurokinin-1, NK-1). SP participates in acute inflammation and activates key proteins involved in mitogenic pathways, such mitogen-activated protein kinases (MAPKs), stimulating DNA synthesis. We tested the hypothesis that SP-induced MAPK activation and DNA synthesis require activation of the EGFR. In U-373 MG cells, which express functional NK-1, SP induced tyrosine phosphorylation of several proteins including EGFR. SP induced formation of an activated EGFR complex containing the adapter proteins SHC and Grb2, but not c-Src. SP activated the MAPK pathway as shown by increased Erk2 kinase activity. SP induced Erk2 activation, and DNA synthesis was inhibited in cells transfected with a dominant negative EGFR plasmid lacking kinase activity, as well as in cells treated with a specific EGFR inhibitor. In addition, pertussis toxin, an inhibitor of Galpha(iota) protein subunits, prevented SP-induced EGFR transactivation and subsequent DNA synthesis. Our results implicate EGFR as an essential regulator in SP/NK-1-induced activation of the MAPK pathway and cell proliferation in U-373 MG cells, and these events are mediated by a pertussis toxin-sensitive Galpha protein. We suggest that this mechanism by which SP controls cell proliferation is an important pathway in tissue restoration and healing.

STAT signaling in head and neck cancer

The upper aerodigestive tract is predisposed to the formation of multiple primary tumors due to field cancerization. TGF-alpha/EGFR autocrine signaling appears to play an important role in squamous cell carcinoma of the head and neck (SCCHN) and upregulation of TGF-alpha and EGFR is an early event in SCCHN carcinogenesis. STAT proteins, including Stat3, are activated by TGF-alpha and EGFR and strategies that downmodulate TGF-alpha or EGFR inhibit SCCHN cell proliferation and abrogate Stat3 activation. Targeting Stat3 leads to SCCHN growth inhibition, increases apoptosis and a downmodulation of Bcl-xL expression in head and neck tumors. These studies support the role of Stat3 as an oncogene, which is activated early in SCCHN carcinogenesis, and efforts to understand EGFR-mediated Stat3 signaling could facilitate novel strategies that will interfere with this growth promoting pathway. Oncogene (2000).

The influence of the cellular context on receptor function: a necessary consideration for physiologic interpretations of receptor expression studies

The cell model studied has a fundamental influence on the function and regulation of G protein linked receptors. These cell-dependent effects are illustrated in the current communication focusing on M3 muscarinic, CCK and GRP receptors. Receptors interact with multiple cellular mechanisms. The most obvious are those involved in coupling to signaling mechanisms such as G proteins. Receptors are themselves phosphorylated and dephosphorylated by cellular kinases and phosphatases. Receptors may sequester, internalize, down-regulate and recycle via interactions with a number of separate cellular mechanisms. When the number and complexity of interactions between the cell and the receptor are taken into account it is not surprising that the cell model has a primary influence on receptor function and regulation. The implications of the importance of the cell model in receptor function for studies aimed at answering physiologic questions are discussed.

Molecular characterization of CXCR-4: a potential brain tumor-associated gene

BACKGROUND AND OBJECTIVES

We have previously reported the isolation of a G protein-coupled receptor, CXCR-4, that is overexpressed in glioblastoma multiforme tumor tissue (GMTT), as compared to normal brain tissue (NBT).

METHODS

Gene-specific RT-PCR, Northern blotting, and in situ hybridization techniques were used to study its expression in a variety of normal tissues, tumor tissues, and cell lines, as well as during development. Antisense CXCR-4 was overexpressed in glioblastoma cells to study its effect on cell proliferation.

RESULTS

Gene-specific RT-PCR analysis indicated that the CXCR-4 gene is overexpressed in several malignant glioma tissues, breast tumor tissues and cell lines. Northern blot analysis indicated that CXCR-4 is expressed at high levels in certain leukemias, uterine cancer, and Burkitt's lymphoma cell line. The occipital and temporal lobe showed high levels of CXCR4 in normal human brain. The CXCR-4 gene was expressed in all organs in the early stages of development (days 8-10). In adult mouse, CXCR-4 is expressed only in brain, spinal cord, bone marrow, and pituitary gland. Antisense CXCR-4 overexpression in glioblastoma cells caused inhibition of cell proliferation and induction of cellular differentiation in vitro. This suggests that CXCR-4 expression may play an important role during embryonic development and also in the genesis of human gliomas.

CONCLUSIONS

On the basis of CXCR4 expression data and antisense overexpression data, we conclude that CXCR-4 plays an important role in the tumorigenic properties of brain, breast, and other tumor types. On the basis of its unique expression during mouse development, we conclude that it may play an important role in the normal functioning of brain, spinal cord, and bone marrow during development.

Altered expression of inhibitory guanine nucleotide regulatory proteins (Gi-proteins) in experimental hepatocellular carcinoma

Guanine nucleotide regulatory proteins (G-proteins) play an important role in the onset and progression of malignancy. We hypothesized that alterations in inhibitory G-protein (Gi) expression and/or function may contribute to cellular invasion and formation of hepatocellular carcinoma (HCC). H4IIE hepatoma cells were inoculated directly into the liver parenchyma of ACI strain rats, and membranes were prepared from HCC livers and adjacent nonneoplastic livers 12 days following the initial inoculation. Expression of inhibitory Gialpha proteins was determined by Western blot analysis and changes in the functional activity of these proteins confirmed by pertussis toxin catalyzed ADP ribosylation and adenylyl cyclase activity. Inhibitory Gialpha1, Gialpha1/2, and Gialpha3 protein expression was significantly elevated in HCC when compared to adjacent nonneoplastic liver and sham-operated hepatic tissue. Pertussis toxin catalyzed ADP ribosylation of Gialpha substrates was significantly enhanced in HCC concomitant with increased basal and stimulated adenylyl cyclase activity following uncoupling of Gi-proteins with manganese ions. The role of Gi-proteins in cellular proliferation was confirmed using cultured H4IIE cells and normal hepatocytes. In quiescent H4IIE cells, mastoparan (Gialpha activator) increased [3H] thymidine incorporation and cell growth in a dose-dependent manner, whereas both pertussis toxin (a Gi-protein inhibitor) and 8-bromo-cAMP inhibited mitogenesis. In contrast, in isolated cultured hepatocytes, mastoparan inhibited [3H] thymidine incorporation, while pertussis toxin and 8-bromo-cAMP were mitogenic. We conclude that HCC is associated with marked changes in Gialpha-protein expression in vivo and in vitro, direct activation of which leads to increased mitogenesis in H4IIE cells in vitro.

Functional expression of bombesin receptor in most adult and pediatric human glioblastoma cell lines; role in mitogenesis and in stimulating the mitogen-activated protein kinase pathway

Functional bombesin receptors were identified in most human glioblastoma cell lines examined (approximately 85% of lines). Bombesin stimulated the release of intracellular Ca2+ in human adult (U-373MG, D-247MG, U-118MG, U-251MG, D-245MG, U-105MG, D-54MG, A-172MG, and D-270MG lines) and pediatric (SJ-S6 and SJ-G2 lines) glioblastoma cell lines. Stimulation of the glioblastoma cell line U-373MG with bombesin or gastrin-releasing peptide (GRP) induced mitogenesis, measured by [3H]thymidine incorporation into DNA, and stimulated the tyrosine phosphorylation of the mitogen-activated protein (MAP) kinases (Erk1 and Erk2). The stimulation of the MAP kinase phosphorylation in U-373MG cells was time- and peptide concentration-dependent. Both bombesin and GRP showed similar potencies in stimulation of intracellular Ca2+ release and activation of the MAP kinase pathway in U-373MG cells, whereas neuromedin B (NMB) peptide was less potent. Bombesin and GRP induced the release of cytosolic Ca2+ in a concentration-dependent manner. Because bombesin and GRP were more potent than NMB peptide in increasing the cytosolic Ca2+ levels in U-373MG cells, we concluded that the BB2 subtype (also known as GRP-preferring receptor subtype) of the bombesin receptor is expressed in this cell line. The bombesin receptor antagonist ([Leu13-psi(CH2NH)Leu14]bombesin) blocked bombesin induced Ca2+ release and attenuated MAP kinase activation in U-373MG cells demonstrating that bombesin is acting through a receptor-dependent mechanism. This study indicates that functional bombesin receptors are widely expressed in human glioblastoma cell lines.

EGF increases retinoid X receptor-alpha expression in human trophoblastic cells in culture: relationship with retinoic acid induced human chorionic gonadotropin secretion

In the present study, the effect of retinoic acid (RA) and epidermal growth factor (EGF) on the functions of human trophoblastic cells in culture were analysed. In these cells, RA potentiated the hCG secretion increase induced by EGF. To gain a better understanding of such a synergistic effect, the expression of retinoic acid receptors (RAR alpha and beta) and retinoid X receptor (RXR alpha) was studied by immunoblotting in RA- and EGF-treated cells. EGF treatment specifically increased the level of RXR alpha protein and RXR alpha transcripts. In parallel, we demonstrated that the choriocarcinoma cells JEG 3, which respond to RA by an increase in hCG secretion, express constitutively high levels of RXR alpha protein. Furthermore, RXR alpha-transfected trophoblastic cells also become RA responsive for hCG secretion. All these data suggest that RXR alpha expression is modulated by EGF, and may be involved in the effect of RA on hCG secretion.