Acidic cellular environments: activation of latent TGF-beta and sensitization of cellular responses to TGF-beta and EGF

TGF-β signaling in health, disease, and therapeutics

Transforming growth factor (TGF)-β is a multifunctional cytokine expressed by almost every tissue and cell type. The signal transduction of TGF-β can stimulate diverse cellular responses and is particularly critical to embryonic development, wound healing, tissue homeostasis, and immune homeostasis in health. The dysfunction of TGF-β can play key roles in many diseases, and numerous targeted therapies have been developed to rectify its pathogenic activity. In the past decades, a large number of studies on TGF-β signaling have been carried out, covering a broad spectrum of topics in health, disease, and therapeutics. Thus, a comprehensive overview of TGF-β signaling is required for a general picture of the studies in this field. In this review, we retrace the research history of TGF-β and introduce the molecular mechanisms regarding its biosynthesis, activation, and signal transduction. We also provide deep insights into the functions of TGF-β signaling in physiological conditions as well as in pathological processes. TGF-β-targeting therapies which have brought fresh hope to the treatment of relevant diseases are highlighted. Through the summary of previous knowledge and recent updates, this review aims to provide a systematic understanding of TGF-β signaling and to attract more attention and interest to this research area.

Immune regulatory cytokines in seminal plasma of healthy men: A scoping review and analysis of variance

OBJECTIVE

Seminal plasma cytokines are associated with fertility and reproductive health, but progressing their clinical utility is hampered by absence of reference data on concentration ranges of relevant cytokines in healthy men. We employed a systematic approach to assemble current evidence on the concentrations of immune regulatory cytokines present in seminal plasma (SP) of normozoospermic and/or fertile men and evaluated the impact of different platform methodologies for cytokine quantification.

EVIDENCE REVIEW

A systematic literature search was performed utilising PubMed, Web of Science and Scopus. Databases were searched from inception until 30th June 2022 inclusive, using combinations of keywords pertaining to seminal fluid and cytokines, and was restricted to human participants. Original data with values reported as concentration of specific cytokines in SP of men clearly defined as fertile or normozoospermic were extracted from studies written in English.

RESULTS

A total of 3769 publications were initially identified, of which 118 fulfilled the eligibility criteria for inclusion. A total of 51 individual cytokines are detectable in SP of healthy men. The number of studies reporting on each cytokine range from 1 to >20. The reported concentrations for many cytokines linked with fertility status, including IL6, CXCL8/IL8, and TNFA, are highly variable between published studies. This is associated with the different immunoassay methodologies utilised and may be exacerbated by a lack of validation of assays to ensure suitability for SP assessment. Due to the large variation between studies, accurate reference ranges for healthy men cannot be determined from the published data.

CONCLUSIONS

The concentrations of cytokines and chemokines detected in SP is inconsistent and highly variable between studies and cohorts, limiting current capacity to define reference ranges for cytokine concentrations in fertile men. The lack of standardisation in methods used to process and store SP, and variation in platforms used to evaluate cytokine abundance, are factors contributing to the observed heterogeneity. To progress the clinical utility of SP cytokine analysis will require standardisation and validation of methodologies so that reference ranges for healthy fertile men can be defined.

The radiobiology of TGFβ

Ionizing radiation is a pillar of cancer therapy that is deployed in more than half of all malignancies. The therapeutic effect of radiation is attributed to induction of DNA damage that kills cancers cells, but radiation also affects signaling that alters the composition of the tumor microenvironment by activating transforming growth factor β (TGFβ). TGFβ is a ubiquitously expressed cytokine that acts as biological lynchpin to orchestrate phenotypes, the stroma, and immunity in normal tissue; these activities are subverted in cancer to promote malignancy, a permissive tumor microenvironment and immune evasion. The radiobiology of TGFβ unites targets at the forefront of oncology-the DNA damage response and immunotherapy. The cancer cell intrinsic and extrinsic network of TGFβ responses in the irradiated tumor form a barrier to both genotoxic treatments and immunotherapy response. Here, we focus on the mechanisms by which radiation induces TGFβ activation, how TGFβ regulates DNA repair, and the dynamic regulation of the tumor immune microenvironment that together oppose effective cancer therapy. Strategies to inhibit TGFβ exploit fundamental radiobiology that may be the missing link to deploying TGFβ inhibitors for optimal patient benefit from cancer treatment.

Progress of Statin Therapy in the Treatment of Idiopathic Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a type of interstitial lung disease (ILD) characterized by the proliferation of fibroblasts and aberrant accumulation of extracellular matrix. These changes are accompanied by structural destruction of the lung tissue and the progressive decline of pulmonary function. In the past few decades, researchers have investigated the pathogenesis of IPF and sought a therapeutic approach for its treatment. Some studies have shown that the occurrence of IPF is related to pulmonary inflammatory injury; however, its specific etiology and pathogenesis remain unknown, and no effective treatment, with the exception of lung transplantation, has been identified yet. Several basic science and clinical studies in recent years have shown that statins, the traditional lipid-lowering drugs, exert significant antifibrotic effects, which can delay the progression of IPF and impairment of pulmonary function. This article is aimed at summarizing the current understanding of the pathogenesis of IPF, the progress of research on the use of statins in IPF models and clinical trials, and its main molecular targets.

Into the Tissues: Extracellular Matrix and Its Artificial Substitutes: Cell Signalling Mechanisms

The existence of orderly structures, such as tissues and organs is made possible by cell adhesion, i.e., the process by which cells attach to neighbouring cells and a supporting substance in the form of the extracellular matrix. The extracellular matrix is a three-dimensional structure composed of collagens, elastin, and various proteoglycans and glycoproteins. It is a storehouse for multiple signalling factors. Cells are informed of their correct connection to the matrix via receptors. Tissue disruption often prevents the natural reconstitution of the matrix. The use of appropriate implants is then required. This review is a compilation of crucial information on the structural and functional features of the extracellular matrix and the complex mechanisms of cell-cell connectivity. The possibilities of regenerating damaged tissues using an artificial matrix substitute are described, detailing the host response to the implant. An important issue is the surface properties of such an implant and the possibilities of their modification.

TGF-β Signaling: A Therapeutic Target to Reinstate Regenerative Plasticity in Vascular Dementia?

Vascular dementia (VaD) is the second leading form of memory loss after Alzheimer's disease (AD). Currently, there is no cure available. The etiology, pathophysiology and clinical manifestations of VaD are extremely heterogeneous, but the impaired cerebral blood flow (CBF) represents a common denominator of VaD. The latter might be the result of atherosclerosis, amyloid angiopathy, microbleeding and micro-strokes, together causing blood-brain barrier (BBB) dysfunction and vessel leakage, collectively originating from the consequence of hypertension, one of the main risk factors for VaD. At the histopathological level, VaD displays abnormal vascular remodeling, endothelial cell death, string vessel formation, pericyte responses, fibrosis, astrogliosis, sclerosis, microglia activation, neuroinflammation, demyelination, white matter lesions, deprivation of synapses and neuronal loss. The transforming growth factor (TGF) β has been identified as one of the key molecular factors involved in the aforementioned various pathological aspects. Thus, targeting TGF-β signaling in the brain might be a promising therapeutic strategy to mitigate vascular pathology and improve cognitive functions in patients with VaD. This review revisits the recent understanding of the role of TGF-β in VaD and associated pathological hallmarks. It further explores the potential to modulate certain aspects of VaD pathology by targeting TGF-β signaling.

The Dynamic Roles of TGF-β Signalling in EBV-Associated Cancers

The transforming growth factor-β (TGF-β) signalling pathway plays a critical role in carcinogenesis. It has a biphasic action by initially suppressing tumorigenesis but promoting tumour progression in the later stages of disease. Consequently, the functional outcome of TGF-β signalling is strongly context-dependent and is influenced by various factors including cell, tissue and cancer type. Disruption of this pathway can be caused by various means, including genetic and environmental factors. A number of human viruses have been shown to modulate TGF-β signalling during tumorigenesis. In this review, we describe how this pathway is perturbed in Epstein-Barr virus (EBV)-associated cancers and how EBV interferes with TGF-β signal transduction. The role of TGF-β in regulating the EBV life cycle in tumour cells is also discussed.

Regulation of the Bioavailability of TGF-β and TGF-β-Related Proteins

The bioavailability of members of the transforming growth factor β (TGF-β) family is controlled by a number of mechanisms. Bona fide TGF-β is sequestered into the matrix in a latent state and must be activated before it can bind to its receptors. Here, we review the molecules and mechanisms that regulate the bioavailability of TGF-β and compare these mechanisms with those used to regulate other TGF-β family members. We also assess the physiological significance of various latent TGF-β activators, as well as other extracellular modulators of TGF-β family signaling, by examining the available in vivo data from knockout mouse models and other biological systems.

Transforming Growth Factor-Beta and Oxidative Stress Interplay: Implications in Tumorigenesis and Cancer Progression

Transforming growth factor-beta (TGF-β) and oxidative stress/Reactive Oxygen Species (ROS) both have pivotal roles in health and disease. In this review we are analyzing the interplay between TGF-β and ROS in tumorigenesis and cancer progression. They have contradictory roles in cancer progression since both can have antitumor effects, through the induction of cell death, senescence and cell cycle arrest, and protumor effects by contributing to cancer cell spreading, proliferation, survival, and metastasis. TGF-β can control ROS production directly or by downregulating antioxidative systems. Meanwhile, ROS can influence TGF-β signaling and increase its expression as well as its activation from the latent complex. This way, both are building a strong interplay which can be taken as an advantage by cancer cells in order to increment their malignancy. In addition, both TGF-β and ROS are able to induce cell senescence, which in one way protects damaged cells from neoplastic transformation but also may collaborate in cancer progression. The mutual collaboration of TGF-β and ROS in tumorigenesis is highly complex, and, due to their differential roles in tumor progression, careful consideration should be taken when thinking of combinatorial targeting in cancer therapies.

Development of a novel multiplexed assay for quantification of transforming growth factor-β (TGF-β)

Changes in activity or levels of transforming growth factor-β (TGF-β) are associated with a variety of diseases; however, measurement of TGF-β in biological fluids is highly variable. TGF-β is biologically inert when associated with its latency-associated peptide (LAP). Most available immunoassays require exogenous activation by acid/heat to release TGF-β from the latent complex. We developed a novel electrochemiluminescence-based multiplexed assay on the MesoScale Discovery® platform that eliminates artificial activation, simultaneously measures both active TGF-β1 and LAP1 and includes an internal control for platelet-derived TGF-β contamination in blood specimens. We optimized this assay to evaluate plasma levels as a function of activation type and clinical specimen preparation. We determined that breast cancer patients' plasma have higher levels of circulating latent TGF-β (LTGF-β) as measured by LAP1 than healthy volunteers (p < 0.0001). This assay provides a robust tool for correlative studies of LTGF-β levels with disease, treatment outcomes and toxicity with a broad clinical applicability.

Transforming growth factor-Beta and urokinase-type plasminogen activator: dangerous partners in tumorigenesis-implications in skin cancer

Transforming growth factor-beta (TGF-β) is a pleiotropic factor, with several different roles in health and disease. TGF-β has been postulated as a dual factor in tumor progression, since it represses epithelial tumor development in early stages, whereas it stimulates tumor progression in advanced stages. During tumorigenesis, cancer cells acquire the capacity to migrate and invade surrounding tissues and to metastasize different organs. The urokinase-type plasminogen activator (uPA) system, comprising uPA, the uPA cell surface receptor, and plasminogen-plasmin, is involved in the proteolytic degradation of the extracellular matrix and regulates key cellular events by activating intracellular signal pathways, which together allow cancer cells to survive, thus, enhancing cell malignance during tumor progression. Due to their importance, uPA and its receptor are tightly transcriptionally regulated in normal development, but are deregulated in cancer, when their activity and expression are related to further development of cancer. TGF-β regulates uPA expression in cancer cells, while uPA, by plasminogen activation, may activate the secreted latent TGF-β, thus, producing a pernicious cycle which contributes to the enhancement of tumor progression. Here we review the specific roles and the interplay between TGF-β and uPA system in cancer cells and their implication in skin cancer.

Unchaining the beast; insights from structural and evolutionary studies on TGFβ secretion, sequestration, and activation

TGFβ is secreted in a latent state and must be "activated" by molecules that facilitate its release from a latent complex and allow binding to high affinity cell surface receptors. Numerous molecules have been implicated as potential mediators of this activation process, but only a limited number of these activators have been demonstrated to play a role in TGFβ mobilisation in vivo. Here we review the process of TGFβ secretion and activation using evolutionary data, sequence conservation and structural information to examine the molecular mechanisms by which TGFβ is secreted, sequestered and released. This allows the separation of more ancient TGFβ activators from those factors that emerged more recently, and helps to define a potential hierarchy of activation mechanisms.

Fibrogenic reactions in lung disease

Fibrogenic lung reactions occur as a common phenotype shared among disorders of heterogeneous etiologies. Even with a common etiology, the extent and pattern of fibrosis vary greatly among individuals, even within families, suggesting complex gene-environment interactions. The search for mechanisms shared among all fibrotic lung diseases would represent a major advance in the identification of therapeutic targets that could have a broad impact on lung health. Although it is difficult to grasp all of the complexities of the varied cell types and cytokine networks involved in lung fibrogenic responses, and to predict the biologic responses to the overexpression or deficiency of individual cytokines, a large body of evidence converges on a single common theme: the central importance of the transforming growth factor beta (TGF-beta) pathway. Therapies that act upstream or downstream of TGF-beta activation have the therapeutic potential to treat all fibrogenic responses in the lung.

Integrin-mediated transforming growth factor-beta activation, a potential therapeutic target in fibrogenic disorders

A subset of integrins function as cell surface receptors for the profibrotic cytokine transforming growth factor-beta (TGF-beta). TGF-beta is expressed in an inactive or latent form, and activation of TGF-beta is a major mechanism that regulates TGF-beta function. Indeed, important TGF-beta activation mechanisms involve several of the TGF-beta binding integrins. Knockout mice suggest essential roles for integrin-mediated TGF-beta activation in vessel and craniofacial morphogenesis during development and in immune homeostasis and the fibrotic wound healing response in the adult. Amplification of integrin-mediated TGF-beta activation in fibrotic disorders and data from preclinical models suggest that integrins may therefore represent novel targets for antifibrotic therapies.

Tissue level, activation and cellular localisation of TGF-beta1 and association with survival in gastric cancer patients

Transforming growth factor-β1 (TGF-β1), a tumour suppressing as well as tumour-promoting cytokine, is stored as an extracellular matrix-bound latent complex. We examined TGF-β1 activation and localisation of TGF-β1 activity in gastric cancer. Gastric tumours showed increased stromal and epithelial total TGF-β1 staining by immunohistochemistry. Active TGF-β1 was present in malignant epithelial cells, but most strongly in smooth muscle actin expressing fibroblasts. Normal gastric mucosa from the same patient showed some staining for total, and little for active TGF-β1. Active TGF-β1 levels were determined by ELISA on tissue homogenates, confirming a strong increase in active TGF-β1 in tumours compared to corresponding normal mucosa. Moreover, high tumour TGF-β1 activity levels were significantly associated with clinical parameters, including worse survival of the patients. Total and active TGF-β1 levels were not correlated, suggesting a specific activation process. Of the different proteases tested, active TGF-β1 levels were only correlated with urokinase activity levels. The correlation with urokinase activity suggests a role for plasmin in TGF-β1 activation in the tumour microenvironment, resulting in transformation of resident fibroblasts to tumour promoting myofibroblasts. In conclusion we have shown localisation and clinical relevance of TGF-β1 activity levels in gastric cancer.

The bioactivity of transforming growth factor-beta1 can be regulated via binding to dermal collagens in mink lung epithelial cells

BACKGROUND

The bioactivity of transforming growth factor-beta1 (TGF-beta1) is known to be regulated by some components of the extracellular matrix (ECM), but the possibility that it might be regulated by collagen, the richest ECM component, has never been previously reported.

OBJECTIVE

This study was designed to investigate the possible role that different types of collagens might play on the bioactivity of TGF-beta1.

METHODS

The interaction of 125I-TGF-beta1 and various types of collagen was examined by a solid-phase assay and by a co-precipitation assay. The bioactivity of TGF-beta1 was assessed by a proliferation assay in which mink lung epithelial cells were examined in the presence and absence of collagens.

RESULTS

Activated native dimeric TGF-beta1 bound to type I collagen in a dose-dependent manner, while monomeric TGF-beta1 bound poorly to the collagen. Type III collagen, and type I gelatin, a heat-denatured type I collagen, also showed a similar interaction with TGF-beta1, however, type IV collagen showed a weak interaction. In the presence of types I and III collagens, the inhibitory effect of TGF-beta1 on the proliferation of mink lung epithelial cells was sustained, thus suggesting that the bioactivity of TGF-beta1 had been enhanced. Type I gelatin also enhanced the inhibition of cell growth, but its effect was weak in comparison with that of type I collagen. The amount of TGF-beta1 which remained intact in the conditioned medium after incubation with MLEC in the presence of types I and III collagens was more than that incubated without collagen.

CONCLUSIONS

Our results suggest that types I and III collagens, the two most abundant components of the interstitial collagens, can potentially bind to activated TGF-beta1 and regulate the bioactivity of this growth factor, thereby possibly maintaining the biologically available TGF-beta1 level.

In vitro bioassay for transforming growth factor-beta using XTT method

Research in the cytokine field has grown exponentially in recent years, and the validity of such studies relies heavily on the appropriate measurement of levels of cytokines in various biological samples. Transforming growth factor (TGF)-beta, a hormonally active polypeptide found in normal and transformed tissue, is a potent regulator of cell growth and differentiation. The most widely used bioassay for TGF-beta is the inhibition of the proliferation of mink lung epithelial cells. Though detection of [3H]thymidine incorporation is more sensitive than the MTT assay, it presents some disadvantages due to the safety and disposal problems associated with radioisotopes. In this study, we attempted to ascertain the experimental conditions which could be used for measuring the in vitro biological activity of TGF-beta in a safer and more sensitive way compared with the currently available methods. We compared the commonly used method, the MTT assay, to the XTT assay using different parameters including cell number, incubation time and the wave length used for detecting the product. We examined the anti-proliferative activities of TGF-beta in three different cell lines: Mv-1-Lu mink lung epithelial cells, MCF10A human breast epithelial cells and H-ras-transformed MCF10A cells. Herein, we present an experimental protocol which provides the most sensitive method of quantifying the biological activity of TGF-beta, with a detection limit of as low as 10 pg/ml: Mv-1-Lu or H-ras MCF10 A cells (1 x 10(5)/well) were incubated with TGF-beta at 37 degrees C in a humidified CO2 incubator for 24 hr followed by XTT treatment and determination of absorbance at 450 or 490 nm. Our results may contribute to the establishment of an in vitro bioassay system, which could be used for the satisfactory quantitation of TGF-beta.

The tumor microenvironment: a potential arbitrator of the tumor suppressive and promoting actions of TGFbeta

Transforming growth factor beta (TGFbeta) members are secreted in biologically inactive complexes that must be activated in order to enable binding to their cell surface receptors. Interestingly, many of the proteins that can activate TGFbeta have been implicated in either suppressing or promoting tumorigenesis. Included among these are matrix proteins (thrombospondin-1), receptors (integrins alphanubeta6 and alphanubeta8) and proteases (matrix metalloproteases and plasmin). These proteins cannot only activate TGFbeta, but can also modulate cell responsiveness to TGFbeta. In this section, we review data highlighting the complexity and bidirectionality of TGFbeta matrix interactions within the tumor microenvironment, and propose that these dynamic interactions are a critical spatial and temporal determinant of the effects of TGFbeta on tumorigenesis.

Cell proliferating effect of latent transforming growth factor-beta1 is cell membrane dependent

The mechanism of in vivo activation of transforming growth factor-beta1 (TGF-beta1), which is critical to its role in many physiological and pathological conditions, is not fully understood. To explore the mechanism by which dermal fibroblasts respond to latent TGF-beta1 directly, the efficacy of either latent TGF-beta1 (LTGF-beta1) alone or LTGF-beta1 plus cell membranes isolated from fibroblasts, mink lung, and one skin-related (Sk23) and two skin-unrelated (U251 and D54MG) transformed cell lines was examined using the mink lung epithelial cell (Mv1Lu) inhibition assay. As a source of LTGF-beta1, PA317 cells were transfected with previously constructed pLin-TGF-beta1 or pLin vectors with no TGF-beta1 insert. LTGF-beta1 expressing PA317 cells were then enriched by growth in the presence of 0.5 mg G-418 for 6-10 days. Eight out of 53 colonies of cells expressing high levels of LTGF-beta1 were selected and their conditioned media were removed after 3 days and used to evaluate the latency and bioactivity of TGF-beta1 using ELISA and Mv1Lu growth inhibition assay, respectively. The level of TGF-beta1 was 19-fold greater (21.4 +/- 0.4 vs. 1.1 +/- 0.2 ng/ml) in conditioned medium derived from pLin-TGF-beta1 transfected cells than that of control. These conditioned media were then used for the subsequent cell proliferating experiments. The results showed that latent TGF-beta1, which proved to be inactive in an Mv1Lu inhibition assay, significantly stimulates fibroblast cell proliferation compared to that of control in a dose-dependent fashion. In another set of experiments, cells were treated with either active (acidified/neutralized) or latent TGF-beta1 and the results showed a significant increase in cell proliferation in response to low concentrations of active TGF-beta1. However, high concentrations of active TGF-beta1 markedly suppressed fibroblast proliferation. These dual effects were in contrast to a steady increase in fibroblast proliferation found in response to latent TGF-beta1. To explore why LTGF-beta1 has a differential proliferating effect on epithelial and fibroblast cell proliferation, cell membranes from these cells were isolated and incubated with PA317-conditioned medium containing LTGF-beta1 and then added to mink lung cells. Only isolated fibroblast cell membranes incubated with LTGF-beta1 inhibited Mv1Lu cells. To examine whether the LTGF-beta1 cell proliferating activity is unique to dermal fibroblasts or is a general phenomenon, in similar experimental conditions cell membranes from several cell lines, U251, D54MG, and SK23, were isolated, incubated with LTGF-beta1, and then added to an Mv1Lu inhibition assay. The proliferation of Mv1Lu epithelial cells was significantly (1547 +/- 269 vs. 3568 +/- 23) inhibited with SK23, but not U251 cell membranes plus LTGF-beta1 relative to that of control. The inhibitory effect of SK23 plus LTGF-beta1 was cell membrane dose-dependent. In conclusion, the result of this study shows that LTGF-beta1 may directly modulate cell proliferation of those cells that possess a cell membrane associated LTGF-beta1 activation mechanism.

Latent-TGF-beta: an overview

The latency associated with the transforming growth factor-betas (TGF-betas) was discovered in 1984. Since the two publications on this subject in that year, there has been on average over sixty reports in which latency was the dominant theme for each of the past 10 years, proof enough of the interest in this field of growth factor research. As the mature 25 kD forms of the TGF-betas are required for them to exert their many, diverse biological effects, it was inevitable that an explanation of the structure and of the activation of the latent complexes be sought. This overview provides a description of these essential points. Now that it has been clearly shown that dysregulation of particular components of the TGF-beta signalling pathway is implicated in many human diseases, the activation of the latent TGF-beta complexes has taken on added importance. Technical improvements enable the distinction of active and latent TGF-beta proteins in vivo and have started to reveal anomalies in the control of activation in relation to various pathological situations.

Cell density-dependent proliferative effects of transforming growth factor (TGF)-beta 1, beta 2, and beta 3 in human chondrosarcoma cells HCS-2/8 are associated with changes in the expression of TGF-beta receptor type I

In this study, the growth properties of the human chondrosarcoma cell line HCS-2/8, its response to transforming growth factor (TGF)-beta isoforms 1, 2, and 3, and its expression of TGF-beta receptors I and II were examined. We demonstrated that these tumor cells are not contact-inhibited and that they can proliferate in the absence of additional serum growth factors. In sparse cultures, all TGF-beta forms inhibited the growth of HCS-2/8 cells, whereas they induced a 2-fold increase of DNA synthesis in serum-fed confluent cultures. In serum-free confluent conditions only TGF-beta 1 stimulated the proliferation rate, whereas TGF-beta 2 was without effect and TGF-beta 3 was rather inhibitory. This bimodal effect of TGF-beta forms was associated with a greater level of TGF-beta receptor 1 mRNA in confluent HCS-2/8 than in sparse cultures, suggesting that the growth response to TGF-beta forms is dependent on the receptor profile expressed.

Transforming growth factor-beta signal transduction in epithelial cells

Transforming growth factor (TGF)-beta is a natural and potent growth inhibitor of a variety of cell types, including epithelial, endothelial, and hematopoietic cells. The ability of TGF-beta to potently inhibit the growth of many solid tumors of epithelial origin, including breast and colon carcinomas, is of particular interest. However, many solid tumor cells become refractory to the growth inhibitory effects of TGF-beta due to defects in TGF-beta signaling pathways. In addition, TGF-beta may stimulate the invasiveness of tumor cells via the paracrine effects of TGF-beta. Accordingly, in order to develop more effective anticancer therapeutics, it is necessary to determine the TGF-beta signal transduction pathways underlying the growth inhibitory effects and other cellular effects of TGF-beta in normal epithelial cells. Thus far, two primary signaling cascades downstream of the TGF-beta receptors have been elucidated, the Sma and mothers against decapentaplegic homologues and the Ras/mitogen-activated protein kinase pathways. The major objective of this review is to summarize TGF-beta signaling in epithelial cells, focusing on recent advances involving the Sma and mothers against decapentaplegic homologues and Ras/mitogen-activated protein kinase pathways. This review is particularly timely in that it provides a comprehensive summary of both signal transduction mechanisms and the cell cycle effects of TGF-beta.

Latency, activation, and binding proteins of TGF-beta

The TGF-beta superfamily of growth factors consists of an increasing number of different polypeptide modulators of cell growth, differentiation, and morphogenesis. Three mammalian isoforms have been molecularly cloned. Numerous ways to regulate the expression of the TGF-beta genes have been identified. TGF-betas are, for example, subject to regulation by retinoids, steroid hormones, and vitamin D. A characteristic feature in the biology of TGF-betas is that they are usually secreted from cells in latent forms. The large latent complex consists of the small latent complex (TGF-beta and its propeptide) and a high molecular weight protease resistant binding protein, latent TGF-beta binding protein (LTBP). LTBPs are required for the proper folding and secretion of TGF-beta. TGF-beta is not just secreted from cultured cells but is deposited via LTBPs to the pericellular space, namely to the extracellular matrix. Release of these complexes and activation by proteases is under tight regulation and provides a means to rapidly increase local concentrations of TGF-beta. Biological events, where enhanced or focal proteolysis and activation of latent TGF-beta takes place, include cell invasion, tissue remodeling, and wound healing.

Transforming growth factor-beta1 in the cerebrospinal fluid of patients with subarachnoid hemorrhage: titers derived from exogenous and endogenous sources

Transforming growth factor-beta1 (TGF-beta1) is a fibrogenic cytokine that is involved in postinjury repair and is implicated in the etiology of postsubarachnoid hemorrhage (SAH) chronic communicating hydrocephalus. TGF-beta1 was measured by enzyme-linked immunosorbant assay (ELISA) in sequential samples of cerebrospinal fluid (CSF) in 11 patients with hydrocephalus after SAH; levels were seen to be biphasically elevated and sources were investigated. TGF-beta1 levels were compared with albumin levels that estimated CSF blood content. Control samples from nonhemorrhagic hydrocephalics were tested similarly. Mean total TGF-beta1 levels were elevated to 4400+/-3435 (+/-SD) pg/mL greater than control levels of 97+/-42 at 1 to 2 days posthemorrhage. Thereafter, levels fell to 714+/-401 by 5 to 6 days posthemorrhage, then rose to a second peak of 1667+/-774 at 9 to 10 days posthemorrhage, remaining significantly increased until 19 days posthemorrhage (P = 0.007). The first peak probably derived from extravasated platelets and correlated with increased albumin levels in the CSF. The second TGF-beta1 peak rose greater than CSF albumin levels that had stabilized at this time, and thus was attributed to a tissue-specific response rather than a re-bleed. TGF-beta1 was detected in the choroid secretory epithelium from controls, but levels were greater in SAH patients at 10 to 12 days posthemorrhage. The authors conclude that the elevated levels of TGF-beta1 in CSF after SAH are derived initially from blood and later from endogenous sources such as the choroid plexus.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.

Cellular response to latent TGF-beta1 is facilitated by insulin-like growth factor-II/mannose-6-phosphate receptors on MS-9 cells

This study was conducted to explore the mechanism of activation of TGF-beta1 which is critical to its role in many physiological and pathological conditions. We have previously demonstrated that latent TGF-beta1 modulates ECM through interaction with IGF-II/M6P receptors on dermal fibroblasts. In this report, we provide evidence that large (270 kDa) but not small (46 kDa) M6P receptors facilitate the cellular response to LTGF-beta1 released from genetically modified cells. As a source of LTGF-beta1, PA317 cells were transfected with either pLin-TGF-beta1 vector or pLin vector with no TGF-beta1 insert using calcium phosphate precipitation. Conditioned medium from transfected cells was removed after 3 days and used to evaluate the latency and bioactivity of TGF-beta1 using ELISA and mink lung epithelial cell growth inhibition assay, respectively. The level of TGF-beta1 was 20-fold greater (2142 +/- 369 vs 102 +/- 23 pg/ml) in conditioned medium derived from pLin-TGF-beta1-transfected cells than in that of controls. Various volumes of this conditioned medium were then used to treat MS-9, SR-2, and MS cells bearing the large, small, and no IGF-II/M6P receptors, respectively, for 24 h. [(3)H]Thymidine incorporation, used as an index for cell proliferation, showed a markedly lower level of proliferation in MS-9 cells in response to a given concentration of LTGF-beta1 than was seen in SR-2 and MS cells. Interestingly, under similar experimental conditions, either addition of M6P at 1 mM concentration or anti-TGF-beta1 antibody abrogated the MS-9 cell proliferative response to LTGF-beta1. In contrast, the inhibitory response of these three cell strains to heat-activated conditioned medium was the same. As another measure of LTGF-beta1-induced cellular response, the expression of mRNA for pro alpha1(I) of type I collagen was also evaluated. A marked increase in expression of this transcript in MS-9 cells in response to LTGF-beta1 was observed. To further examine the possible correlation between the large IGF-II/M6P receptors and cellular responses to LTGF-beta1, expression of IGF-II/M6P receptors at the protein and mRNA levels were evaluated by ligand binding and RT-PCR, respectively. Using (125)I-IGF-II as a ligand, the number of specific IGF-II/M6P receptors was found to be threefold greater on MS-9 than on SR-2 and MS cells. This finding was consistent with the level of IGF-II/M6P receptor mRNA detected by RT-PCR in MS-9 cells. In conclusion, the result of this study shows a direct link between large but not small IGF-II/M6P receptors on MS-9 cells and their response to LTGF-beta1.

Insulin-like growth factor-II/mannose 6 phosphate receptors facilitate the matrix effects of latent transforming growth factor-beta1 released from genetically modified keratinocytes in a fibroblast/keratinocyte co-culture system

This study was conducted to explore the mechanism of activation of transforming growth factor-beta1 (TGF-beta1) which is critical to its role in many physiological and pathological conditions. To date, almost all reports concerning TGF-beta1 activation delineated that release of mature TGF-beta1 from latency associated protein (LAP) is required for its activation. We report that latent TGF-beta1 (LTGF-beta1) released from TGF-beta1 genetically modified keratinocytes grown in the top chamber of a co-culture system functions as a fibrogenic factor through interaction with insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptors of human dermal fibroblasts grown in the lower chamber of this system. Following successful transduction, the pLin-LTGF-beta1 vector was amplified in PA31 7 packaging cells which possess viral structural proteins for vector in the presence of neomycin. Conditioned medium derived from packaging cells containing competent viral particles was then used to transduce either keratinocytes or fibroblasts grown in the upper chamber of a co-culture system, in which a 0.4 microm porous membrane separates the two chambers. In this way, LTGF-beta1 produced by transduced cells in the upper chamber is released and diffuses into the lower chambers where dermal fibroblasts are grown. Conditioned medium from the lower chamber was removed 3 days later and used to evaluate the latency and bioactivity of TGF-beta1 using enzyme-linked immunosorbent assay (ELISA) and mink lung (Mv1 Lu) epithelial growth inhibition assay. Cells were also harvested and used for RNA extraction. The results of these experiments showed that 1) the TGF-beta1-LAP complex, which was latent in traditionally used mink lung growth inhibition assay, directly modulated the expression of collagenase, type I, and type III collagen mRNA by dermal fibroblasts; 2) this stimulation was inhibited by M6P in a dose-dependent manner; 3) the TGF-beta1-LAP inhibits Mv1Lu epithelial cells only when this complex was incubated with cell membranes isolated from dermal fibroblasts; and 4) LTGF-beta1 activation seems to occur through a conformational alteration rather than by release of the mature TGF-beta1 from LAP in our co-cultured system. This conformational alteration seems to occur through the interaction of the TGF-beta1-LAP complex with the IGF-II/M6P receptors. Thus, the quantity of IGF-II/M6P receptors is important in cellular response to LTGF-beta1 in any physiological and pathological conditions.

Latent transforming growth factor-beta binding proteins (LTBPs)--structural extracellular matrix proteins for targeting TGF-beta action

Growth factors of the transforming growth factor-beta family are potent regulators of the extracellular matrix formation, in addition to their immunomodulatory and regulatory roles for cell growth. TGF-beta s are secreted from cells as latent complexes containing TGF-beta and its propeptide, LAP (latency-associated peptide). In most cells LAP is covalently linked to an additional protein, latent TGF-beta binding protein (LTBP), forming the large latent complex. LTBPs are required for efficient secretion and correct folding of TGF-beta s. The secreted large latent complexes associate covalently with the extracellular matrix via the N-termini of the LTBPs. LTBPs belong to the fibrillin-LTBP family of extracellular matrix proteins, which have a typical repeated domain structure consisting mostly of epidermal growth factor (EGF)-like repeats and characteristic eight cysteine (8-Cys) repeats. Currently four different LTBPs and two fibrillins have been identified. LTBPs contain multiple proteinase sensitive sites, providing means to solubilize the large latent complex from the extracellular matrix structures. LTBPs are now known to exist both as soluble molecules and in association with the extracellular matrix. An important consequence of this is LTBP-mediated deposition and targeting of latent, activatable TGF-beta into extracellular matrices and connective tissues. LTBPs have a dual function, they are required both for the secretion of the small latent TGF-beta complex as well as directing bound latent TGF-beta to extracellular matrix microfibrils. However, it is not known at present whether LTBPs are capable of forming microfibrils independently, or whether they are a part of the fibrillin-containing fibrils. Most LTBPs possess RGD-sequences, which may have a role in their interactions with the cell surface. At least LTBP-1 is chemotactic to smooth muscle cells, and is involved in vascular remodelling. Analyses of the expressed LTBPs have revealed considerable variations throughout the molecules, generated both by alternative splicing and utilization of multiple promoter regions. The significance of this structural diversity is mostly unclear at present.

TGF-β1 in Aplysia: Role in Long-Term Changes in the Excitability of Sensory Neurons and Distribution of TβR-II-Like Immunoreactivity

Exogenous recombinant human transforming growth factor β-1 (TGF-β1) induced long-term facilitation ofAplysia sensory-motor synapses. In addition, 5-HT-induced facilitation was blocked by application of a soluble fragment of the extracellular portion of the TGF-β1 type II receptor (TβR-II), which presumably acted by scavenging an endogenous TGF-β1-like molecule. Because TβR-II is essential for transmembrane signaling by TGF-β, we sought to determine whether Aplysia tissues contained TβR-II and specifically, whether neurons expressed the receptor. Western blot analysis of Aplysia tissue extracts demonstrated the presence of a TβR-II-immunoreactive protein in several tissue types. The expression and distribution of TβR-II-immunoreactive proteins in the central nervous system was examined by immunohistochemistry to elucidate sites that may be responsive to TGF-β1 and thus may play a role in synaptic plasticity. Sensory neurons in the ventral–caudal cluster of the pleural ganglion were immunoreactive for TβR-II, as well as many neurons in the pedal, abdominal, buccal, and cerebral ganglia. Sensory neurons cultured in isolation and cocultured sensory and motor neurons were also immunoreactive. TGF-β1 affected the biophysical properties of cultured sensory neurons, inducing an increase of excitability that persisted for at least 48 hr. Furthermore, exposure to TGF-β1 resulted in a reduction in the firing threshold of sensory neurons. These results provide further support for the hypothesis that TGF-β1 plays a role in long-term synaptic plasticity in Aplysia.

Transforming growth factor-beta1 and fibrosis in congenital muscular dystrophies

We evaluated transforming growth factor-beta1 (TGF-beta1) expression in the muscle of four laminin alpha2-negative, four laminin alpha2-positive and seven partial laminin alpha2-deficient congenital muscular dystrophy (CMD) patients, and compared it to Duchenne muscular dystrophy (DMD) patients and controls. TGF-beta1 mRNA levels in skeletal muscle from laminin alpha2-negative and laminin alpha2-positive CMD patients were significantly greater than in controls (P < 0.05 and P < 0.005, respectively), while in partial laminin alpha2-deficient muscular dystrophy patients the amount was not significantly higher than in controls (P > 0.1). The TGF-beta1 values were lower than those found in DMD, although the extent of fibrosis was greater in CMD than in DMD and controls. Our findings suggest that TGF-beta1 is involved in CMD muscle fibrosis, but differently from what we observed in DMD muscles as it seems not to be the major player in connective tissue proliferation.

Extracellular matrix-associated transforming growth factor-beta: role in cancer cell growth and invasion

Growth factors of the transforming growth factor-beta (TGF-beta) family inhibit the proliferation of epithelial, endothelial, and hematopoietic cells, and stimulate the synthesis of extracellular matrix components. TGF-beta s are secreted from cells in high-molecular-mass protein complexes that are composed of three proteins, the mature TGF-beta-dimer, the TGF-beta propeptide dimer, or latency-associated protein (LAP), and the latent TGF-beta binding protein (LTBP). Mature TGF-beta is cleaved from its propeptide during secretion, but the proteins remain associated by noncovalent interactions. LTBP is required for efficient secretion and processing of latent TGF-beta and it binds to LAP via disulfide bond(s). LTBP is a component of extracellular matrix microfibrils, and it targets the latent TGF-beta complex to the extracellular matrix. TGF-beta signaling is initiated by proteolytic cleavage of LTBP that results in the release of the latent TGF-beta complex from the extracellular matrix. TGF-beta is activated by dissociation of LAP from the mature TGF-beta. Subsequent signaling involves binding of active TGF-beta to its type II cell surface receptors, which phosphorylate and activate type I TGF-beta receptors. Type I receptors, in turn, phosphorylate cytoplasmic transcriptional activator proteins Smad2 and Smad3, inducing their translocation to the nucleus. Recent evidence suggests that acquisition of resistance to TGF-beta growth inhibition plays a major role in the progression of epithelial and hematopoietic cell malignancies. The role of secretion of TGF-beta in tumorigenesis is more complex. The secretion of TGF-beta s by tumor cells may contribute to autocrine growth inhibition, but on the other hand, it may also promote invasion, metastasis, angiogenesis, and even immunosuppression. Tumor cells may also fail to deposit LTBP:TGF-beta complexes to the extracellular matrix. The elucidation of the mechanisms of the release of TGF-beta from the matrix and its subsequent activation aids the understanding of the pathophysiologic roles of TGF-beta in malignant growth, and allows the development of therapeutic agents that regulate the activity of TGF-beta.

Effects of hypertonic stress on transforming growth factor-beta activity in normal rat kidney cells

Hypertonicity is known to modulate the expression of some genes and the action of several cytokines. We evaluated whether hypertonicity would increase the expression and/or activity of transforming growth factor-beta (TGF-beta) in normal rat kidney (NRK) cells. The bioassay for TGF-beta showed that mature TGF-beta activity was significantly increased when the cells were cultured in a hypertonic medium (500 mOsm/kg). Comparing to the isotonic medium, hypertonicity accelerated the increase in TGF-beta activity during the initial 24 hours after changing the medium. The activity was increased as the medium osmolality increased from 300 to 500 mOsm/kg. Raffinose was found to be the most effective in increasing TGF-beta activity. NaCl, glucose, and mannitol also increased TGF-beta activity. In contrast, total TGF-beta (mature and latent) activity and TGF-beta mRNA abundance did not change significantly, suggesting that hypertonicity activated TGF-beta without affecting the synthesis of TGF-beta. To determine whether collagen synthesis was increased by hypertonicity, we examined [3H] proline incorporation into NRK cells cultured in hypertonic medium. Proline incorporation increased in an osmolality-dependent manner. Raffinose was also the most effective solute at increasing the proline incorporation. Furthermore, anti-TGF-beta antibody prevented the increase in proline incorporation induced by hypertonicity. These results suggest that hypertonicity promotes the processing of latent TGF-beta to the biologically active form, resulting in the stimulation of collagen synthesis in NRK cells.

Regulation and interactions of transforming growth factor-beta with cardiovascular cells: implications for development and disease

1. Transforming growth factors-beta (TGF-beta) are multifunctional proteins that regulate cell growth, differentiation, migration and extracellular matrix production and have an important role in embryonic development and tissue remodelling. 2. The diverse biological actions of TGF-beta are elicited following their interaction with type I and type II TGF-beta receptors, both of which are transmembrane serine/threonine kinases, suggesting an important role for protein phosphorylation in the mechanism of action of these cytokines on the growth of cells and their extracellular environment. 3. Alterations in TGF-beta gene expression and action in various cell types associated with the cardiovascular system may contribute to the pathophysiology of a number of diseases, such as hypertension, atherosclerosis and restenosis, as well as the development of cardiac abnormalities.

Expression of transforming growth factor-beta 1 in dystrophic patient muscles correlates with fibrosis. Pathogenetic role of a fibrogenic cytokine

Duchenne muscular dystrophy is a fatal disorder characterized by progressive muscular weakness, wasting, and severe muscle contractures in later disease stages. Muscle biopsy reveals conspicuous myofiber degeneration and fibrosis substituting muscle tissue. We quantitatively determined mRNA of the potent fibrogenic cytokine transforming growth factor-beta 1 by quantitative PCR in 15 Duchenne muscular dystrophy, 13 Becker muscular dystrophy, 11 spinal muscular atrophy patients, and 16 controls. Higher transforming growth factor-beta 1 expression was greater in Duchenne muscular dystrophy patients than controls (P = 0.012) and Becker patients (P = 0.03). Fibrosis was significantly more prominent in Duchenne muscular dystrophy than Becker muscular dystrophy, spinal muscular atrophy, and controls. The proportion of connective tissue in muscle biopsies increased progressively with age in Duchenne muscular dystrophy patients, while transforming growth factor-beta 1 levels peaked at 2 and 6 yr of age. Transforming growth factor-beta 1 protein was also detected by immunocytochemistry and immunoblotting. Our findings suggest that transforming growth factor-beta 1 stimulates fibrosis in Duchenne muscular dystrophy. Expression of transforming growth factor-beta 1 in the early stages of Duchenne muscular dystrophy may be critical in initiating muscle fibrosis and antifibrosis treatment could slow progression of the disease, increasing the utility of gene therapy.

Human mast cell chymase and leukocyte elastase release latent transforming growth factor-beta 1 from the extracellular matrix of cultured human epithelial and endothelial cells

Monolayer cultures of human epithelial and endothelial cells were used to study the association of latent transforming growth factor-beta 1 (TGF-beta 1) to extracellular matrices and its release and activation during matrix degradation. Human umbilical vein endothelial cells and embryonic lung fibroblasts produced relatively high levels of TGF-beta 1, its propeptide (beta 1-latency-associated protein), and latent TGF-beta-binding protein and incorporated latent TGF-beta 1 into their matrices as shown by immunoblotting. Amnion epithelial cells produced lower levels of these proteins. Confluent cultures of epithelial cells were exposed to matrix-degrading proteases and glycosidases. Mast cell chymase, leukocyte elastase, and plasmin efficiently released matrix-bound latent TGF-beta 1 complexes, while chondroitinase ABC and heparitinases were ineffective. The ability of the proteases to activate recombinant latent TGF-beta 1 was tested using growth inhibition assays and a novel sodium deoxycholate-polyacrylamide gel electrophoresis followed by immunoblotting. Sodium deoxycholate solubilized M(r) 25,000 TGF-beta 1 but did not dissociate high M(r) latent TGF-beta 1 complexes, allowing separation of these forms by polyacrylamide gel electrophoresis. Mast cell chymase and leukocyte elastase did not activate latent TGF-beta 1, suggesting that its release from matrix and activation are controlled by different mechanisms. The release of TGF-beta from the matrix by leukocyte and mast cell enzymes may contribute to the accumulation of connective tissue in inflammation.

Thrombospondin causes activation of latent transforming growth factor-beta secreted by endothelial cells by a novel mechanism

Thrombospondin (TSP) forms specific complexes with transforming growth factor-beta (TGF-beta) in the alpha granule releasate of platelets and these TSP-TGF-beta complexes inhibit the growth of bovine aortic endothelial cells (BAE). In these studies, we report that TSP stripped of associated TGF-beta (sTSP) retained growth inhibitory activity which was partially reversed by a neutralizing antibody specific for TGF-beta. Since BAE cells secrete latent TGF-beta, we determined whether sTSP activates the latent TGF-beta secreted by BAE cells. Cells were cultured with or without sTSP and then the conditioned medium was tested for the ability to support TGF-beta-dependent normal rat kidney (NRK) colony formation in soft agar. Medium conditioned with sTSP showed a dose- and time-dependent ability to stimulate BAE-secreted TGF-beta activity, reaching maximal activation by 1-2 h with 0.4 micrograms/ml (0.9 nM) sTSP. The sTSP-mediated stimulation of TGF-beta activity is not dependent on serum factors and is not a general property of extracellular matrix molecules. The sTSP-mediated stimulation of TGF-beta activity was blocked by a mAb specific for sTSP and by neutralizing antibodies to TGF-beta. Activation of BAE cell secreted latent TGF-beta by sTSP can occur in the absence of cells and apparently does not require interactions with cell surface molecules, since in conditioned medium removed from cells and then incubated with sTSP, activation occurs with kinetics and at levels similar to what is seen when sTSP is incubated in the presence of cells. Serine proteases such as plasmin are not involved in sTSP-mediated activation of TGF-beta. Factors that regulate the conversion of latent to active TGF-beta are keys to controlling TGF-beta activity. These data suggest that TSP is a potent physiologic regulator of TGF-beta activation.

Production of multiple growth factors by a human non-small cell lung carcinoma cell line

The 1PT cell line, derived from an undifferentiated bronchial carcinoma, produced, in conditioned medium, immunoreactive basic fibroblast growth factor (bFGF), insulin-like growth factors I and II (IGF-I and IGF-II), epidermal growth factor (EGF), transforming growth factor alpha (TGF alpha), and transforming growth factor beta-2 (TGF beta 2) in its latent form, but not platelet-derived growth factor (PDGF), tumour necrosis factor alpha (TNF alpha), or transforming growth factor beta-1 (TGF beta 1). Comparative studies of growth stimulation of human umbilical vein (HUV) endothelial cells indicated that the growth factors detected in 1PT-conditioned medium do not solely account for its proliferative effects on these cells. These results support previous characterization studies [1,2] that suggest the production of a potentially novel tumour-derived endothelial cell growth factor by the 1PT cell line.

Production of autostimulatory growth factors by the human carcinoma line, RPMI 2650

The human carcinoma line RPMI 2650 produces autocrine factors; they are detected by the ability of RPMI 2650 conditioned medium (CM) to stimulate growth in soft agar of RPMI 2650 cells plated at low density. The autocrine activity in crude CM can be fractionated by ultrafiltration into a lower molecular weight (MW) fraction (R1-30), which concentrates molecules in the 1000-30,000 Da range; and a higher MW fraction (R30) with molecules greater than 30,000 Da in a more concentrated form. R1-30 is labile to acid, base, and heat treatment, whereas R30 is stable to (and sometimes activated by) these treatments. Boiling of R30, however, renders it labile to acid, base, and trypsin treatments. CM can be separated into a weakly heparin-binding fraction (with stability properties similar, but not identical, to R1-30), and a non-heparin binding fraction (with stability properties similar to R30). RPMI 2650 cells secrete transforming growth factor (TGF)alpha- and TGF beta-like molecules, but the R1-30 fraction can be distinguished from these TGFs, and from most other known growth factors, by its unusual combination of acid lability and weak affinity for heparin. Since the R30/non-heparin binding fraction is rendered labile by boiling or acid treatment, it may represent a bound or conformationally stable form of a growth factor.

Identification of multiple active growth factors in basement membrane Matrigel suggests caution in interpretation of cellular activity related to extracellular matrix components

We have recently demonstrated the formation of interconnecting canalicular cell processes in bone cells upon contact with basement membrane components. Here we have determined whether growth factors in the reconstituted basement membrane (Matrigel) were active in influencing the cellular network formation. Various growth factors including transforming growth factor beta (TGF-beta), epidermal growth factor (EGF), insulin-like growth factor 1, bovine fibroblast growth factor (bFGF), and platelet-derived growth factor (PDGF) were identified in Matrigel. Exogenous TGF-beta blocked the cellular network formation. Conversely, addition of TGF-beta 1 neutralizing antibodies to Matrigel stimulated the cellular network formation. bFGF, EGF, and PDGF all promoted cellular migration and organization on Matrigel. Addition of bFGF to MC3T3-E1 cells grown on Matrigel overcame the inhibitory effect of TGF-beta. Some TGF-beta remained bound to type IV collagen purified from the Engelbreth-Holm-Swarm tumor matrix. These data demonstrate that reconstituted basement membrane contains growth factors which influence cellular behavior, suggesting caution in the interpretation of experiments on cellular activity related to Matrigel, collagen type IV, and possibly other extracellular matrix components.

1,25-Dihydroxy-vitamin-D3 enhances antiproliferative effect and transcription of TGF-beta1 on human keratinocytes in culture

Both TGF-beta and 1,25-dihydroxy-vitamin-D3 (1,25(OH)2D3) have been reported to decrease the proliferation of normal human keratinocytes. The effect and expression of TGF-beta in keratinocytes treated with 1,25(OH)2D3 was investigated. Human keratinocytes were grown in the presence of various concentrations of TGF-beta and/or 1,25(OH)2D3 prior to enumeration. TGF-beta, alone, has a half maximal dose of inhibition (ED50) of approximately 750 pg/ml after seven days in culture in Keratinocyte Growth Medium (KGM; Clonetics) supplemented with 1.5 mM calcium. When 1,25(OH)2D3 (10(-7)M) was also added to cultures with various concentrations of TGF-beta, the ED50 shifted an average of 2-fold less. The presence of TGF-beta (10 pg/ml) augmented the potency of 1,25(OH)2D3 by at least 10-fold. In keratinocyte cultures, the antiproliferative effect of the two compounds together is synergistic. In keratinocytes grown for 1 week in the presence of 1,25(OH)2D3 at 10(-6)M, the TGF-beta 1 message increased approximately 5-fold. An increase is detected within 2 hours of exposure to 1,25(OH)2D3. There was only a 50% increase in the levels of TGF-beta 2 and no detection of TGF-beta 3. When keratinocyte cultures were treated with 1,25(OH)2D3 and neutralizing antibodies to TGF-beta, the induced-antiproliferative activity was blocked by more than 50%. The keratinocytes produced more active than latent TGF-beta after growth with high doses of 1,25(OH)2D3.

Ultrastructure of alpha 2-macroglobulins

New results concerning the ultrastructure of human alpha 2-macroglobulin (alpha 2M) molecules are presented in connection and comparison with the historical, the current and our own most recent, even unpublished results on the structure and function of alpha 2M and related proteins. The electron microscopic approach uses classical negative staining, combined with the new imaging mode "Electron Energy Loss Spectroscopy", which provides unusual contrast, resolution and readability of the electron micrographs. Immuno- and cryoelectron microscopy, as well as image processing has provided new data necessary to the building of tentative 3D models of the molecule. A model for the native tetrameric alpha 2M is described for the first time, and tries to explain and gather the various observations, sometimes contradictory, taken from different laboratories. A revised version for a model of the methylamine- and proteinase-transformed forms of alpha 2M is also shown. The probable positions of the bait regions and the thiol esters are given on both models. We confirm that alpha 2M is a twin trap capable of inactivating one or two proteinases by partial immobilization. Preliminary results on the production of crystals of alpha 2M-chymotrypsin complexes are also presented. A critical analysis of our models is presented in comparison with others. The technical limitations reached with some techniques and some possible extensions of future research in the field are also presented.

Activation of human platelet-derived latent transforming growth factor-beta 1 by human glioblastoma cells. Comparison with proteolytic and glycosidic enzymes

Transforming growth factor-beta (TGF-beta), a regulator of cell growth and differentiation, is secreted by most cultured cells in latent form (L-TGF-beta). Activation of L-TGF-beta can be achieved by various physico-chemical treatments, including acidification, alkalinization, heating and chaotropic agents. Proposed physiological activators include proteinases and glycosidases, which, however, only lead to limited activation (15-20% of the total TGF-beta activity after acidic activation). In the present study L-TGF-beta 1 partially purified from human platelets was not activated by treatment with neuraminidase or the proteinases plasmin, endoproteinase Arg-C, elastase and chymotrypsin. The mechanism of activation of L-TGF-beta was further assessed by using the human glioblastoma cell line 308, which releases biologically active TGF-beta 2. Factor(s) secreted by 308 glioblastoma cells were found to be able to activate partially purified L-TGF-beta 1 from human platelets. Our finding may prove to constitute a physiologically relevant mechanism for the activation of latent forms of TGF-beta in vivo.