No abstract

Multiple endocrine neoplasia type 1 (MEN1) is an autosomal dominant disorder characterized by endocrine tumors of parathyroids, pancreatic islets, and anterior pituitary. The MEN1 gene encodes a nuclear protein called menin. In MEN1 carriers inactivating mutations give rise to a truncated product consistent with menin acting as a tumor suppressor gene. However, the role of menin in tumorigenesis and its physiological functions are not known. Here, we show that menin inactivation by antisense RNA antagonizes transforming growth factor type ␤-mediated cell growth inhibition. Menin interacts with Smad3, and antisense menin suppresses transforming growth factor type ␤-induced and Smad3-induced transcriptional activity by inhibiting Smad3͞4-DNA binding at specific transcriptional regulatory sites. These results implicate a mechanism of tumorigenesis by menin inactivation.

The transforming growth factor-beta (TGFβ) superfamily encompasses widespread and evolutionarily conserved polypeptide growth factors that regulate and orchestrate growth and differentiation in all cell types and tissues. While they regulate asymmetric cell division and cell fate determination during early development and embryogenesis, TGFβ family members play a major regulatory role in hormonal and immune responses, cell growth, cell death and cell immortalization, bone formation, tissue remodeling and repair, and erythropoiesis throughout adult life. The biological and physiological functions of TGFβ, the founding member of this family, and its receptors are of central importance to human diseases, particularly cancer. By regulating cell growth, death, and immortalization, TGFβ signaling pathways exert tumor suppressor effects in normal cells and early carcinomas. Thus, it is not surprising that a high number of human tumors arise due to mutations or deletions in the genes coding for the various TGFβ signaling components. As tumors develop and progress, these protective and cytostatic effects of TGFβ are often lost. TGFβ signaling then switches to promote cancer progression, invasion, and tumor metastasis. The molecular mechanisms underlying this dual role of TGFβ in human cancer will be discussed in depth in this paper, and it will highlight the challenge and importance of developing novel therapeutic strategies specifically aimed at blocking the prometastatic arm of the TGFβ signaling pathway without affecting its tumor suppressive effects.

The prolactin receptor (PRLR) belongs to the superfamily of cytokine/growth hormone/prolactin receptors. Members of this family do not contain a tyrosine kinase domain but are associated with cytoplasmic kinases of the Jak family. Here, we examine different mutants of the PRLR with respect to their ability to associate and activate the kinase Jak2 and the transcription factor Stat1. Moreover, using a biological assay system we are able to correlate these activities with activation of prolactin-responsive gene transcription. Our results indicate that interaction between Jak2 and PRLR requires a proline-rich sequence in the membrane proximal region of the receptor, which is conserved among the different members of the cytokine receptor superfamily. We also show that association of Jak2 with the receptor is sufficient for activation of the kinase as well as the transcription factor Stat1. Moreover, our findings indicate that association of PRLR with Jak2 is necessary but not sufficient for the transmission of a lactogenic signal. We have identified two other cytoplasmic regions of the PRLR that are required for activation of transcription. These two regions are located between boxes 1 and 2 and are in the carboxyl-terminal tail of the receptor. These sites probably involve specific interactions with other effector molecules.

Members of the transforming growth factor beta (TGF-beta) family regulate fundamental physiological processes, such as cell growth, differentiation and apoptosis, in almost all cell types. As a result, defects in TGF-beta signalling pathways have been linked to uncontrolled cellular proliferation and carcinogenesis. Here, we explored the signal transduction mechanisms downstream of the activin/TGF-beta receptors that result in cell growth arrest and apoptosis. We show that in haematopoietic cells, TGF-beta family members regulate apoptosis through expression of the inositol phosphatase SHIP (Src homology 2 (SH2) domain-containing 5' inositol phosphatase), a central regulator of phospholipid metabolism. We also demonstrated that the Smad pathway is required in the transcriptional regulation of the SHIP gene. Activin/TGF-beta-induced expression of SHIP results in intracellular changes in the pool of phospholipids, as well as in inhibition of both Akt/PKB (protein kinase B) phosphorylation and cell survival. Our results link phospholipid metabolism to activin/TGF-beta-mediated apoptosis and define TGF-beta family members as potent inducers of SHIP expression.

We have investigated whether human lymphoid cells are able to synthesize and secrete human PRL (hPRL) and to express PRL receptors. Metabolic labeling with [35S]methionine and immunoprecipitation of cell extracts from human mononuclear cells (MNC) and a human T lymphocyte cell line with an antiserum against hPRL revealed protein of M(r) 23,000, identical in size to pituitary hPRL. Dilution curves of lymphocyte immunoreactive hPRL were parallel to those obtained with pituitary hPRL in an immunoradiometric assay using two monoclonal antibodies against hPRL. Polymerase chain reaction experiments with primers located in the coding sequence of hPRL showed that the hPRL gene was expressed in MNC. Furthermore, cDNA cloning and sequence analysis indicated the presence of an extra 5' noncoding exon previously described for decidual hPRL. When MNCs were further separated into B cells, T cells, and monocytes, the expression of hPRL appeared to be mainly associated with the T lymphocyte fraction. The hPRL transcript was also detected in thymocytes and in a set of human lymphoid cell lines. Finally, polymerase chain reaction experiments revealed a ubiquitous distribution of PRL receptor gene expression in B cells, T cells, and monocytes. The presence of the receptor for PRL and production of PRL by T lymphocytes suggest a possible autocrine or paracrine effect of PRL in immune cell function.

ABSTRACT

Activin, a member of the TGF␤ family inhibits cell growth in various target tissues. Activin interacts with a complex of two receptors that upon activation phosphorylate specific intracellular mediators, the Smad proteins. The activated Smads interact with diverse DNA binding proteins and co-activators of transcription in a cell-specific manner, thus leading to various activin biological effects. In this study, we investigated the role and mechanism of action of activin in the human breast cancer T47D cells. We found that activin treatment of T47D cells leads to a dramatic decrease in cell growth. Thus activin appears as a potent cell growth inhibitor of these breast cancer cells. We show that activin induces the Smad pathway in these cells but also activates the p38-mitogen-activated protein kinase pathway, further leading to phosphorylation of the transcription factor ATF2. Finally, specific inhibitors of the p38 kinase (SB202190, SB203580, and PD169316) but not an inactive analogue (SB202474) or the MEK-1 inhibitor PD98059 completely abolish the activin-mediated cell growth inhibition of T47D cells. Together, these results define a new role for activin in human breast cancer T47D cells and highlight a new pathway utilized by this growth factor in the mediation of its biological effects in cell growth arrest.Activin, a member of the TGF␤ 1 family, regulates cell growth of various cell types. Activin interacts with a complex of two receptors (types I and II), both containing an extracellular domain, a single transmembrane region, and a large intracellular domain that contain a serine/threonine kinase domain. The type II receptor, which is constitutively phosphorylated (1) transphosphorylates the type I receptor (ALK4) upon ligand stimulation, on serine and threonine residues (2-4). The activated receptor complex then recruits the two receptor-regulated Smad2 and Smad3 (5-8). Following binding and phosphorylation by the activin type I receptor, Smad2 and Smad3 are released to the cytoplasm where they associate with the common-partner Smad4 before being translocated to the nucleus (8 -11).Both Smad3 and Smad4 but not Smad2 can directly bind DNA elements (Smad binding element) and activate the transcription of the target genes (12). However, the DNA binding affinity of the Smads is low (13), and they usually require the presence of other DNA binding proteins to efficiently interact with the promoters of their responsive target genes. As a result, the Smad binding elements are often found close to the DNA binding element of other transcription factors. Among those are the FAST family members, FAST1 (14) and FAST2 (15), TFE3 (16), Fos and Jun (17), Sp1 (18), CBP/p300 (19), Evi-1 (20), and ATF2 (21).The Smad proteins are central elements in the activin receptor signaling pathway but are not the sole pathway activated by this receptor complex. Other members of the TGF␤ superfamily have been shown to activate different signaling pathways, in addition to the Smads. TGF␤ itself can activate a member of the MAPKKK family ...