Lalage M. Wakefield

Evidence that transforming growth factor-β is a hormonally regulated negative growth factor in human breast cancer cells

The hormone-dependent human breast cancer cell line MCF-7 secretes transforming growth factor-beta (TGF-beta), which can be detected in the culture medium in a biologically active form. These polypeptides compete with human platelet-derived TGF-beta for binding to its receptor, are biologically active in TGF-beta-specific growth assays, and are recognized and inactivated by TGF-beta-specific antibodies. Secretion of active TGF-beta is induced 8 to 27-fold under treatment of MCF-7 cells with growth inhibitory concentrations of antiestrogens. Antiestrogen-induced TGF-beta from MCF-7 cells inhibits the growth of an estrogen receptor-negative human breast cancer cell line in coculture experiments; growth inhibition is reversed with anti-TGF-beta antibodies. We conclude that in MCF-7 cells, TGF-beta is a hormonally regulated growth inhibitor with possible autocrine and paracrine functions in breast cancer cells.

TGF-β signaling: positive and negative effects on tumorigenesis

TGF-β binding to the cell surface triggers activation of multiple signal transduction pathways that are connected in intricate ways with each other, and with other response networks involved in sensing cellular information input. Recent data indicate that changes in signal intensity and connectivity of these pathways may underlie the complex transition of the TGF-β pathway from tumor suppressor to oncogene during tumorigenesis.

The two faces of transforming growth factor β in carcinogenesis

Hero or villain? Trustworthy guardian of normal homeostasis or double agent? Over the past two decades, the perceived role of transforming growth factor β (TGF-β) in carcinogenesis has undergone more plot twists than an Agatha Christie mystery. The initial experiments leading to the discovery of TGF-β and its naming as a “transforming” growth factor were based on its ability to induce malignant behavior of normal fibroblasts, leading to the notion that TGF-β might be a key factor in uncoupling a cell from normal growth control in such a way that it could become tumorigenic (1). However, at the time, this presumed function of the protein was difficult to reconcile with its ubiquitous pattern of expression in normal tissues, including its prevalence in human platelets. The next twist in the story came several years later, when it emerged that TGF-β has profound growth-suppressive effects on many cells, including epithelial cells and lymphoid cells, which form the basis of the majority of human cancers. At this point TGF-β began to be given serious consideration as a candidate tumor suppressor gene (2). Indeed, data from both experimental model systems and studies of human cancers clearly show that not only the ligand itself but also its downstream elements, including its receptors, and its primary cytoplasmic signal transducers, the Smad proteins, are important for suppressing primary tumorigenesis in many organs (3, 4). While solid credentials were being established for the role of TGF-β as a good citizen in the battle to maintain cellular order, a darker side was emerging. It is now appreciated that metastasis of many different types of tumor cells actually requires TGF-β activity and that, in the context of advanced disease, it actually has prooncogenic effects (3, 5). To date, understanding of this complex, dual role of TGF-β in …

The mammary pathology of genetically engineered mice: the consensus report and recommendations from the Annapolis meeting{

NIH sponsored a meeting of medical and veterinary pathologists with mammary gland expertise in Annapolis in March 1999. Rapid development of mouse mammary models has accentuated the need for definitions of the mammary lesions in genetically engineered mice (GEM) and to assess their usefulness as models of human breast disease. The panel of nine pathologists independently reviewed material representing over 90% of the published systems. The GEM tumors were found to have: (1) phenotypes similar to those of non-GEM; (2) signature phenotypes specific to the transgene; and (3) some morphological similarities to the human disease. The current mouse mammary and human breast tumor classifications describe the majority of GEM lesions but unique morphologic lesions are found in many GEM. Since little information is available on the natural history of GEM lesions, a simple morphologic nomenclature is proposed that allows direct comparisons between models. Future progress requires rigorous application of guidelines covering pathologic examination of the mammary gland and the whole animal. Since the phenotype of the lesions is an essential component of their molecular pathology, funding agencies should adopt policies ensuring careful morphological evaluation of any funded research involving animal models. A pathologist should be part of each research team.

Lifetime exposure to a soluble TGF-β antagonist protects mice against metastasis without adverse side effects

TGF-betas play diverse and complex roles in many biological processes. In tumorigenesis, they can function either as tumor suppressors or as pro-oncogenic factors, depending on the stage of the disease. We have developed transgenic mice expressing a TGF-beta antagonist of the soluble type II TGF-beta receptor:Fc fusion protein class, under the regulation of the mammary-selective MMTV-LTR promoter/enhancer. Biologically significant levels of antagonist were detectable in the serum and most tissues of this mouse line. The mice were resistant to the development of metastases at multiple organ sites when compared with wild-type controls, both in a tail vein metastasis assay using isogenic melanoma cells and in crosses with the MMTV-neu transgenic mouse model of metastatic breast cancer. Importantly, metastasis from endogenous mammary tumors was suppressed without any enhancement of primary tumorigenesis. Furthermore, aged transgenic mice did not exhibit the severe pathology characteristic of TGF-beta null mice, despite lifetime exposure to the antagonist. The data suggest that in vivo the antagonist may selectively neutralize the undesirable TGF-beta associated with metastasis, while sparing the regulatory roles of TGF-betas in normal tissues. Thus this soluble TGF-beta antagonist has potential for long-term clinical use in the prevention of metastasis.

Physicochemical activation of recombinant latent transforming growth factor-beta's 1, 2, and 3.

Native and recombinant forms of transforming growth factor-beta 1 (TGF-beta 1) are synthesized predominantly as biologically latent complexes. Physicochemical analysis demonstrates that the more recently described TGF-beta 2 and TGF-beta 3 are also latent, and reveals a common series of sharply defined parameters for activation. Human recombinant latent TGF-betas 1 and 2 show identical profiles of activation by acid and base; the transition from latency occurs between pH 4.1 and 3.1, and between pH 11.0 and 11.9. The profile for chicken recombinant latent TGF-beta 3 is slightly shifted with activation between pH 3.1 and 2.5, and between pH 10.0 and 12.3. Thermal activation of native and recombinant latent TGF-beta 1 occurs over the temperature ranges of 75-100 degrees C and 65-100 degrees C, respectively, with complete activation after 5 min at 80 degrees C. Temperatures above 90 degrees C result in thermal denaturation of TGF-beta 1 itself. Recombinant latent TGF-betas 2 and 3 are also activated over this temperature range; however, maximum activation occurs at 100 degrees C. These results suggest common elements in latent complex structure despite differences between the TGF-beta subtypes in pro-region primary sequence.

TGF-β switches from tumor suppressor to prometastatic factor in a model of breast cancer progression

The TGF-β signaling network plays a complex role in carcinogenesis because it has the potential to act as either a tumor suppressor or a pro-oncogenic pathway. Currently, it is not known whether TGF-β can switch from tumor suppressor to pro-oncogenic factor during the course of carcinogenic progression in a single cell lineage with a defined initiating oncogenic event or whether the specific nature of the response is determined by cell type and molecular etiology. To address this question, we have introduced a dominant negative type II TGF-β receptor into a series of genetically related human breast–derived cell lines representing different stages in the progression process. We show that decreased TGF-β responsiveness alone cannot initiate tumorigenesis but that it can cooperate with an initiating oncogenic lesion to make a premalignant breast cell tumorigenic and a low-grade tumorigenic cell line histologically and proliferatively more aggressive. In a high-grade tumorigenic cell line, however, reduced TGF-β responsiveness has no effect on primary tumorigenesis but significantly decreases metastasis. Our results demonstrate a causal role for loss of TGF-β responsiveness in promoting breast cancer progression up to the stage of advanced, histologically aggressive, but nonmetastatic disease and suggest that at that point TGF-β switches from tumor suppressor to prometastatic factor.

IL-13 Activates a Mechanism of Tissue Fibrosis That Is Completely TGF-β Independent

Fibrosis is a characteristic feature in the pathogenesis of a wide spectrum of diseases. Recently, it was suggested that IL-13-dependent fibrosis develops through a TGF-β1 and matrix metalloproteinase-9-dependent (MMP-9) mechanism. However, the significance of this pathway in a natural disorder of fibrosis was not investigated. In this study, we examined the role of TGF-β in IL-13-dependent liver fibrosis caused by Schistosoma mansoni infection. Infected IL-13−/− mice showed an almost complete abrogation of fibrosis despite continued and undiminished production of TGF-β1. Although MMP-9 activity was implicated in the IL-13 pathway, MMP-9−/− mice displayed no reduction in fibrosis, even when chronically infected. To directly test the requirement for TGF-β, studies were also performed with neutralizing anti-TGF-β Abs, soluble antagonists (soluble TGF-βR-Fc), and Tg mice (Smad3−/− and TGF-βRII-Fc Tg) that have disruptions in all or part of the TGF-β signaling cascade. In all cases, fibrosis developed normally and with kinetics similar to wild-type mice. Production of IL-13 was also unaffected. Finally, several genes, including interstitial collagens, several MMPs, and tissue inhibitors of metalloprotease-1 were up-regulated in TGF-β1−/− mice by IL-13, demonstrating that IL-13 activates the fibrogenic machinery directly. Together, these studies provide unequivocal evidence of a pathway of fibrogenesis that is IL-13 dependent but TGF-β1 independent, illustrating the importance of targeting IL-13 directly in the treatment of infection-induced fibrosis.

Expression of a dominant-negative mutant TGF-β type II receptor in transgenic mice reveals essential roles for TGF-β in regulation of growth and differentiation in the exocrine pancreas

Using a dominant‐negative mutant receptor (DNR) approach in transgenic mice, we have functionally inactivated transforming growth factor‐β (TGF‐β) signaling in select epithelial cells. The dominant‐negative mutant type II TGF‐β receptor blocked signaling by all three TGF‐β isoforms in primary hepatocyte and pancreatic acinar cell cultures generated from transgenic mice, as demonstrated by the loss of growth inhibitory and gene induction responses. However, it had no effect on signaling by activin, the closest TGF‐β family member. DNR transgenic mice showed increased proliferation of pancreatic acinar cells and severely perturbed acinar differentiation. These results indicate that TGF‐β negatively controls growth of acinar cells and is essential for the maintenance of a differentiated acinar phenotype in the exocrine pancreas in vivo. In contrast, such abnormalities were not observed in the liver. Additional abnormalities in the pancreas included fibrosis, neoangiogenesis and mild macrophage infiltration, and these were associated with a marked up‐regulation of TGF‐β expression in transgenic acinar cells. This transgenic model of targeted functional inactivation of TGF‐β signaling provides insights into mechanisms whereby loss of TGF‐β responsiveness might promote the carcinogenic process, both through direct effects on cell proliferation, and indirectly through up‐regulation of TGF‐βs with associated paracrine effects on stromal compartments.

Transforming growth factor beta: biochemistry and roles in embryogenesis, tissue repair and remodeling, and carcinogenesis.

Transforming growth factor (TGF)- β plays essential roles in embryogenesis, particularly during periods of morphogenesis. Some of the same embryological mechanisms are reiterated in the adult during the normal processes of tissue remodeling and repair and aberrantly in various pathological processes, including carcinogenesis. This chapter highlights the new advances in the understanding of the complex biology of TGF- β and discusses the chemistry of TGF- β . The broad range of biological activities of TGF- β makes it highly likely that other peptide activities—purified by presumably novel and specific assays—will result from TGF- β once their amino acid sequence is determined. TGF- β 1 and 2 are two homologous forms of a homodimeric peptide with molecular weight of 25,000. Every chain of the peptide contains 112 amino acids of which nine are cysteine residues. The chapter reviews the structure of TGF- β 1 and 2 and TGF- β gene family. The biological activities of the members of the TGF- β family are described in the chapter. The chapter further reviews the regulation of gene activity by TGF- β, antibodies to TGF- β , and role of TGF- β in embryogenesis, tissue repair and remodeling, and carcinogenesis and other proliferative diseases.