Transforming growth factor beta isoform 2-specific high affinity binding to native alpha 2-macroglobulin. Chimeras identify a sequence that determines affinity for native but not activated alpha 2-macroglobulin

IL-1β and TGFβ2 synergistically induce endothelial to mesenchymal transition in an NFκB-dependent manner

Endothelial to mesenchymal transition (EndMT) contributes to fibrotic diseases. The main inducer of EndMT is TGFβ signaling. TGFβ2 is the dominant isoform in the physiological embryonic EndMT, but its role in the pathological EndMT in the context of inflammatory co-stimulation is not known. The aim of this study was to investigate TGFβ2-induced EndMT in the context of inflammatory IL-1β signaling. Co-stimulation with IL-1β and TGFβ2, but not TGFβ1, caused synergistic induction of EndMT. Also, TGFβ2 was the only TGFβ isoform that was progressively upregulated during EndMT. External IL-1β stimulation was dispensable once EndMT was induced. The inflammatory transcription factor NFκB was upregulated in an additive manner by IL-1β and TGFβ2 co-stimulation. Co-stimulation also led to the nuclear translocation of NFκB which was sustained over long-term treatment. Activation of NFκB was indispensable for the co-induction of EndMT. Our data suggest that the microenvironment at the verge between inflammation (IL-1β) and tissue remodeling (TGFβ2) can strongly promote the process of EndMT. Therefore our findings provide new insights into the mechanisms of pathological EndMT.

A meta-analysis of microarray-based gene expression studies of olfactory bulb-derived olfactory ensheathing cells

Genome wide transcriptional profiling and large scale proteomics have emerged as two powerful methods to dissect the molecular properties of specific neural tissues or cell types on a global scale. Several genome-wide transcriptional profiling and proteomics studies have been published on cultured olfactory ensheathing cells (OEC). In this article we present a meta-analysis of all five published and publicly available micro-array gene expression datasets of cultured early-passage-OB-OEC with other cell types (Schwann cells, late-passage-OB-OEC, mucosa-OEC, an OEC cell line, and acutely dissected OEC). The aim of this meta-analysis is to identify genes and molecular pathways that are found in multiple instead of one isolated study. 454 Genes were detected in at least three out of five microarray datasets. In this "Top-list", genes involved in the biological processes "growth of neurites", "blood vessel development", "migration of cells" and "immune response" were strongly overrepresented. By applying network analysis tools, molecular networks were constructed and Hub-genes were identified that may function as key genes in the above mentioned interrelated processes. We also identified 7 genes (ENTPD2, MATN2, CTSC, PTHLH, GLRX1, COL27A1 and ID2) with uniformly higher or lower expression in early-passage-OB-OEC in all five microarray comparisons. These genes have diverse but intriguing roles in neuroprotection, neurite extension and/or tissue repair. Our meta-analysis provides novel insights into the molecular basis of OB-OEC-mediated neural repair and can serve as a repository for investigators interested in the molecular biology of OEC. This article is part of a Special Issue entitled: Understanding olfactory ensheathing glia and their prospect for nervous system repair.

Extracellular angiogenic growth factor interactions: an angiogenesis interactome survey

Angiogenesis plays a key role in various physiological and pathological processes, including inflammation and tumor growth. Numerous angiogenic growth factors (AGFs) have been identified. Usually, the angiogenic process is assumed to represent the outcome of a straightforward interaction of AGFs with specific signalling receptors of the endothelial cell (EC) surface. Actually, the mechanisms by which AGFs induce neovascularization are much more complex. Indeed, angiogenesis is the result of the simultaneous actions of various AGFs and angiogenesis modulators; multiple EC surface receptors with different structure and biological properties are engaged by AGFs to exert a full angiogenic response; AGFs bind a variety of free and immobilized proteins, polysaccharides, and complex lipids of the extracellular milieu that affect AGF integrity, stability, and bioavailability; some of the AGF-binding molecules interact also with AGF receptors. In this review the authors summarize literature data and discuss the current knowledge about the extracellular molecules able to interact with AGFs, thus representing possible key regulators of the angiogenesis process and targets/templates for the development of novel antiangiogenic drugs. This work represents an attempt to highlight common theme in the AGF interactome that occurs at the extracellular level during neovascularization.

Functional role of TGF beta in Alzheimer's disease microvascular injury: lessons from transgenic mice

Recent studies have implicated pro- and anti-inflammatory cytokines as integral to Alzheimer's disease (AD) pathogenesis. Among them, transforming growth factor-beta (TGF-beta) is emerging as an important factor in regulating inflammatory responses. This multifunctional cytokine might be centrally involved in several aspects of AD pathogenesis by regulating beta-amyloid precursor protein synthesis and processing, plaque formation, astroglial and microglial response and neuronal cell death. Among all of these potential roles, studies in transgenic and infusion animal models have shown that TGF-beta may primarily contribute to AD pathogenesis by influencing A beta production and deposition, which in turn might result in damage to the brain microvasculature. The lessons learned from these models are of great interest not only for understanding of the role of TGF-beta in AD, but also for future treatments where testing of anti-inflammatory agents such as ibuprofen and an amyloid vaccine hold great promise. In this regard, further elucidation of the signal pathways by which TGF-beta exerts its effect in AD might lead to specific targets for further therapeutic intervention.

Interaction of lecithin:cholesterol acyltransferase (LCAT).alpha 2-macroglobulin complex with low density lipoprotein receptor-related protein (LRP). Evidence for an alpha 2-macroglobulin/LRP receptor-mediated system participating in LCAT clearance

The reaction of lecithin:cholesterol acyltransferase (LCAT) with high density lipoproteins (HDL) is of critical importance in reverse cholesterol transport, but the structural and functional pathways involved in the regulation of LCAT have not been established. We present evidence for the direct binding of LCAT to alpha(2)-macroglobulin (alpha(2)M) in human plasma to form a complex 18.5 nm in diameter. Forty percent of plasma LCAT-HDL was associated with alpha(2)M; moreover, most of the LCAT in cerebrospinal fluid and in the medium of cultured human hepatoma cell line was associated with alpha(2)M. Purified recombinant human LCAT (rLCAT) labeled with (125)I bound to native and methylamine-activated alpha(2)M (alpha(2)M-MA) in vitro in a time- and concentration-dependent manner, and this binding did not depend on the presence of lipid. rLCAT bound to alpha(2)M-MA with greater affinity than to alpha(2)M. Furthermore, rLCAT did not activate alpha(2)M as phosphatidylcholine-specific phospholipase C does. Reconstituted HDL particles (LpA-I) inhibited the binding of rLCAT to alpha(2)M more efficiently than native HDL(3) did. LCAT associated with alpha(2)M was enzymatically inactive under both endogenous and exogenous assay conditions. Purified rLCAT alone did not bind to low density lipoprotein receptor-related protein (LRP) as lipoprotein lipase (LPL) does; however, when rLCAT was combined with alpha(2)M-MA to form a complex, binding, internalization, and degradation of rLCAT took place in LRP-expressing cells (LRP (+/+)) but not in cells deficient in LRP (LRP (-/-)). It is concluded that the binding of LCAT to alpha(2)M inhibits its enzymatic activity. Furthermore, the finding supports the possibility that the LRP receptor can act in vivo to mediate clearance of the LCAT-alpha(2)M complex and may significantly influence the bioavailability of LCAT.

NMR solution structure of the receptor binding domain of human alpha(2)-macroglobulin

Human alpha(2)-macroglobulin-proteinase complexes bind to their receptor, the low density lipoprotein receptor-related protein (LRP), through a discrete 138-residue C-terminal receptor binding domain (RBD), which also binds to the beta-amyloid peptide. We have used NMR spectroscopy on recombinantly expressed uniformly (13)C/(15)N-labeled human RBD to determine its three-dimensional structure in solution. Human RBD is a sandwich of two antiparallel beta-sheets, one four-strand and one five-strand, and also contains one alpha-helix of 2.5 turns and an additional 1-turn helical region. The principal alpha-helix contains two lysine residues on the outer face that are known to be essential for receptor binding. A calcium binding site (K(d) approximately 11 mM) is present in the loop region at one end of the beta-sandwich. Calcium binding principally affects this loop region and does not significantly perturb the stable core structure of the domain. The structure and NMR assignments will enable us to examine in solution specific binding of RBD to domains of the receptor and to beta-amyloid peptide.

The trypsin-inhibitory efficiency of human alpha 2-macroglobulin in the presence of alpha 1-proteinase inhibitor: evidence for the formation of an alpha 2-macroglobulin--alpha 1-proteinase inhibitor complex

The inhibition of bovine pancreatic trypsin was studied at pH 7, 25 degrees C, using mixtures of purified human alpha 2-macroglobulin (alpha 2M) and alpha 1-proteinase inhibitor (alpha 1 PI). The partitioning of the enzyme between the two inhibitors was determined by comparing control esterase activity, assayed with N-benzoyl-L-arginine ethyl ester as substrate, with that remaining after incubation with inhibitory mixtures. (At [I]0 > [E]0, remaining esteratic activity reflects the concentration of alpha 2M-associated enzyme (alpha 2M-E*) and the concentration of alpha 1PI-associated, inactive enzyme (alpha 1PI-E*) is given by the difference, [E]0-[alpha 2M-E*].) The pattern of product distribution was found to be incompatible with an inhibitory model involving parallel, second-order reactions of E with alpha 2M and alpha 1PI. The data pointed to complex formation between the two inhibitors, limiting the level of alpha 2M readily available for reaction with E. Analysis based on the binding equilibrium, alpha 2M (dimeric unit) + alpha 1PI reversible alpha 2M-alpha 1PI, yielded Kd = 2.1 +/- 0.3 microM. Complex formation between alpha 2M and alpha 1PI was verified by gel permeation experiments. alpha 2M was found to restrict the volume of distribution of alpha 1PI in Sephadex G200 beds. Kd, deduced from gel permeation behaviour, was 0.8 +/- 0.32 microM. Preliminary kinetic experiments with dialyzed plasma suggested that the alpha 2M-alpha 1PI interaction is effective also in vivo. Given Kd and the mean plasma levels of the two inhibitors ([alpha 2M] = 2 microM; [alpha 1PI] = 36 microM), it was estimated that > 90% of alpha 2M in human circulation must be complexed to alpha 1PI and lack immediate antiproteinase activity.

Distinct functional domains of TGF-beta bind receptors on endothelial cells

Transforming growth factor-beta (TGF-beta) is a multi-functional regulator of cell growth and differentiation. Three distinct isoforms of TGF-beta exist having similar, but not identical actions. TGF-beta 1, but not TGF-beta 2, binds to T beta RII and also to endoglin, a cell surface protein abundant on endothelial cells. In contrast, the affinity constant of TGF-beta 2 for alpha 2-macroglobulin is 10-fold greater than that of TGF-beta 1. TGF-beta 2 also binds better than TGF-beta 1 to a glycosyl phosphatidylinositol (GPI)-linked binding protein expressed on vascular endothelial cells. Using chimeric TGF-beta molecules, in which selected regions of TGF-beta 1 have been exchanged for the corresponding region of TGF-beta 2, we demonstrate here that amino acids 92-95 or 94-98 of TGF-beta determine isoform specific binding to endoglin. In contrast, exchange of only amino acids 95 and 98 did not alter TGF-beta specificity. Isoform specific binding to a GPI-linked protein on EJG endothelial cells was modulated by a region containing amino acids 40-68, although exchange of only amino acids 40-47 did not confer isoform specific binding. Significantly, the 92-98 region also modulates binding of TGF-beta to the type II receptor whereas isoform specific binding to alpha 2-macroglobulin requires concerted exchange of amino acids 45 and 47. Taken together, these results show that at least three different functional domains are important modulators of TGF-beta interaction with binding proteins and receptors.

Localization of the binding site for transforming growth factor-beta in human alpha2-macroglobulin to a 20-kDa peptide that also contains the bait region

alpha2-Macroglobulin (alpha2M) functions as a major carrier of transforming growth factor-beta (TGF-beta) in vivo. The goal of this investigation was to characterize the TGF-beta-binding site in alpha2M. Human alpha2M, which was reduced and denatured to generate 180-kDa subunits, bound TGF-beta1, TGF-beta2, and NGF-beta in ligand blotting experiments. Cytokine binding was not detected with bovine serum albumin that had been reduced and alkylated, and only minimal binding was detected with purified murinoglobulin. To localize the TGF-beta-binding site in alpha2M, five cDNA fragments, collectively encoding amino acids 122-1302, were expressed as glutathione S-transferase (GST) fusion proteins. In ligand blotting experiments, TGF-beta2 bound only to the fusion protein (FP3) that includes amino acids 614-797. FP3 bound 125I-TGF-beta1 and 125I-TGF-beta2 in solution, preventing the binding of these growth factors to immobilized alpha2M-methylamine (alpha2M-MA). The IC50 values were 33 +/- 5 and 26 +/- 6 nM for TGF-beta1 and TGF-beta2, respectively; these values were comparable with or lower than those determined with native alpha2M or alpha2M-MA. A GST fusion protein that includes amino acids 798-1082 of alpha2M (FP4) and purified GST did not inhibit the binding of TGF-beta to immobilized alpha2M-MA. FP3 (0.2 microM) neutralized the activity of TGF-beta1 and TGF-beta2 in fetal bovine heart endothelial (FBHE) cell proliferation assays; FP4 was inactive in this assay. FP3 also increased NO synthesis by RAW 264.7 cells, mimicking an alpha2M activity that has been attributed to the neutralization of endogenously synthesized TGF-beta. Thus, we have isolated a peptide corresponding to 13% of the alpha2M sequence that binds TGF-beta and neutralizes the activity of TGF-beta in two separate biological assays.

Mutational analysis of a transforming growth factor-beta receptor binding site

Transforming growth factor-beta s (TGF-beta 1, -beta 2, -beta 3) are important regulators of cell growth and differentiation which share approximately 70% identical amino acids. Using LS513 colorectal cells, which are growth inhibited by TGF-beta 1 (ED50 of 100 pM), but are refractory to TGF-beta 2 (ED50 of 50,000 to 100,000 pM), we have determined that amino acids 92-98 of TGF-beta specify growth inhibition. The chimeric protein TGF-beta 1/beta 2(92-98), in which amino acids 92-98 of TGF-beta 1 were exchanged for the corresponding amino acids of TGF-beta 2, was indistinguishable from TGF-beta 2 at inhibiting growth of LS513 cells. In contrast, both TGF-beta 1/beta 2(92-95) and TGF-beta 1/beta 2(94-98) inhibited the growth of LS513 cells with an ED50 of approximately 1000 pM. TGF-beta 1/beta 2(95-98), in which amino acids 95-98 of TGF-beta 1 have been replaced with the corresponding amino acids of TGF-beta 2, had full activity and was indistinguishable from TGF-beta 1. Receptor cross-linking experiments demonstrated that binding of the chimeras to the type I and type II receptors of LS513 cells was consistent with their biological activity. TGF-beta 1/beta 2(95-98), TGF-beta 1/beta 2(92-95) and TGF-beta 1/beta 2(94-98) were each similar to TGF-beta 2 in that they failed to bind to the soluble Type II receptor in a solid-phase assay. These results demonstrate that amino acids 92-98 are involved in the interaction between TGF-beta and its signaling receptors and they show that modest changes within this region can substantially alter biological response.

Chemical modification of alpha2-macroglobulin to generate derivatives that bind transforming growth factor-beta with increased affinity

alpha2-Macroglobulin (alpha2M) binds a number of cytokines, including transforming growth factor-beta1 (TGF-beta1) and TGF-beta2. The affinity of these interactions depends on the alpha2M conformation. In this investigation, we treated human alpha2M with cis-dichlorodiammineplatinum (II) (cis-Pt), a crosslinking reagent that partially 'locks' the alpha2M conformation, and then with methylamine to generate a preparation (alpha2M-P/M) consisting of stable alpha2M conformational intermediates. alpha2M-P/M bound TGF-beta1 and TGF-beta2 with higher affinity than any other form of alpha2M studied to date. The equilibrium dissociation constants were 14 and 2 nM for TGF-beta1 and TGF-beta2, respectively. alpha2M-P/M, at 100 nM, neutralized the activity of TGF-beta1 by about 75% in an endothelial cell proliferation assay. The equivalent concentration of native alpha2M or methylamine-modified alpha2M had no effect. These studies demonstrate that the potential of alpha2M as a cytokine carrier and neutralizer may not be fully realized in either the native or completely activated conformations.

Purification and properties of placental prolactin-related protein-I

We used sucrose density gradient centrifugation, size exclusion chromatography, and high-pressure reversed-phase chromatography in the purification of bovine prolactin-related protein-I (bPRP-I) to homogeneity from a secretory granule-enriched fraction of fetal cotyledon. Amino terminal sequence was unambiguous, consistent with the nucleic acid sequence of the cDNA 50 codons distal to the initial AUG in the open reading frame, and began with the residues: RKSFTDRFMNAASLSHDFY. This is distinct from the signal peptide cleavage site predicted by the algorithm of von Heijne (1986) as well as that expected by comparison with other members of the growth hormone/prolactin family of hormones. The level of bPRP-I in uterine fluid was sufficient to detect by Western blot of unfractionated material and estimated as at least 0.65 microM. In contrast, bPRP-I was undetectable in the serum by this method. Interaction of [125I]-bPRP-I with high molecular weight serum components interfered with its measurement by radio-immunoassay, and could be replicated with purified alpha 2-macroglobulin with an apparent KD of about 0.41 microM. Thus, the bPRP-I gene product is processed secreted and distributed in a manner consistent with a paracrine action at the materno-fetal interface.

Transforming growth factor beta s and wound healing

The Transforming Growth Factor beta superfamily (TGF beta) is one of the most complex groups of cytokines with widespread effects on many aspects of growth and development. The TGF beta isoforms and other family members, e.g. Activins and BMPs, have diverse effects in similar physiological situations. TGF beta is involved in the wound healing process. The three mammalian isoforms (TGF beta 1, 2 and 3) and recently other family members, e.g. Activin, have been localised in healing wounds. Manipulation of the ratios of TGF beta superfamily members, particularly the ratio of TGF beta 1 relative to TGF beta 3, reduces scarring and fibrosis. Such manipulations include reducing the levels of TGF beta 1/TGF beta 2 using neutralising antibodies or preventing the activation of TGF beta s. In chronic or impaired wounds the exogenous addition of TGF beta superfamily members accelerates aspects of the healing process. This review summarises evidence for the role of TGF beta superfamily members in wound healing and how modulation of TGF beta levels can prevent scarring and fibrosis.

Differential regulation of TGF-beta 2 by hormones in rat uterus and mammary gland

Previous work from this laboratory has shown that transforming growth factor beta 2 (TGF-beta 2) mRNA is abundant in the pregnant uterus. In the present study, we examined the synthesis and secretion of TGF-beta 1,2 and 3 in the rat uterus and mammary gland and show differential secretion and expression of TGF-beta 2 in a tissue specific manner. Elevated levels of TGF-beta 2 were detected in late pregnant maternal plasmas (> 100 pM), and in the milk (> 500 pM) during early lactation. High concentrations of TGF-beta 2 (> 200 pM) were also detected in uterine fluids collected from ovariectomized adult rats after high dose estrogen treatment. TGF-beta 2 mRNA levels were elevated in lobuloalveolar epithelial cells isolated from pregnant mammary gland. Three major transcripts of 3.5, 4.0, and 4.7 kb were seen, of which the 4.7 kb, dominates. Mammary glands of estrogen treated ovariectomized rats showed a similar pattern of TGF-beta 2 transcripts. In contrast, four major TGF-beta 2 mRNA transcripts of 5.7, 4.7, 4.0, and 3.5 kb, with the dominant species of 4.0 and 5.7 kb, were observed in uteri from the estrogen treated animals up to 48 h after the last estrogen injection. This suggests that TGF-beta 2 is regulated in a tissue specific manner. We conclude that the secretion of TGF-beta 2 is tightly regulated by hormones and that estrogen and prolactin are critical factors in the tissue-specific regulation of the local production of TGF-beta 2 in the mammary gland and female reproductive tract.

Binding affinity of transforming growth factor-beta for its type II receptor is determined by the C-terminal region of the molecule

Transforming growth factor-beta (TGF-beta) isoforms have differential binding affinities for the TGF-beta type II receptor (TbetaRII). In most cells, TGF-beta1 and TGF-beta3 bind to TbetaRII with much higher affinity than TGF-beta2. Here, we report an analysis of the effect of TGF-beta structure on its binding to TbetaRII by using TGF-beta mutants with domain deletions, amino acid replacements, and isoform chimeras. Examination of the binding of TGF-beta mutants to the recombinant extracellular domain of TbetaRII by a solid-phase TGF-beta/TbetaRII assay demonstrated that only those TGF-beta mutants containing the C terminus of TGF-beta1 (TGF-beta1-(Delta69-73), TGF-beta1-(Trp71), and TGF-beta2/beta1-(83-112)) bind with high affinity to TbetaRII, similar to native TGF-beta1. Moreover, replacement of only 6 amino acids in the C terminus of TGF-beta1 with the corresponding sequence of TGF-beta2 (TGF-beta1/beta2-(91-96)) completely eliminated the high affinity binding of TGF-beta1. Proliferation of fetal bovine heart endothelial (FBHE) cells was inhibited to a similar degree by all of the TGF-beta mutants. However, recombinant soluble TbetaRII blocked the inhibition of FBHE cell proliferation induced by TGF-beta mutants retaining the C terminus of TGF-beta1, consistent with the high binding affinity between these TGF-beta molecules and TbetaRII. It was further confirmed that the TGF-beta2 mutant with its C terminus replaced by that of TGF-beta1 (TGF-beta2/beta1-(83-112)) competed as effectively as TGF-beta1 with 125I-TGF-beta1 for binding to membrane TbetaRI and TbetaRII on FBHE cells. These observations clearly indicate that the domain in TGF-beta1 responsible for its high affinity binding to TbetaRII, both the soluble and membrane-bound forms, is located at C terminus of the molecule.

Murine alpha-macroglobulins demonstrate divergent activities as neutralizers of transforming growth factor-beta and as inducers of nitric oxide synthesis. A possible mechanism for the endotoxin insensitivity of the alpha2-macroglobulin gene knock-out mouse

alpha2-Macroglobulin null mice demonstrate increased resistance to endotoxin challenge (Umans, L., Serneels, L., Overbergh, L., Van Leuven, F., and Van den Berghe, H. (1995) J. Biol. Chem. 270, 19778-19785). We hypothesized that this phenotype might reflect the function of murine alpha2M (malpha2M) as a neutralizer of transforming growth factor-beta (TGF-beta) and inducer of nitric oxide synthesis in vivo. When incubated with wild-type mouse plasma, TGF-beta1 and TGF-beta2 bound only to malpha2M. Alternative TGF-beta-binding proteins were not detected in plasma from alpha2M(-/-) mice. Wild-type mouse plasma, but not plasma from alpha2M(-/-) mice, inhibited TGF-beta1 binding to TGF-beta receptors on fibroblasts. Purified malpha2M bound TGF-beta1 and TGF-beta2 with similar affinity; the KD values were 28 +/- 4 and 33 +/- 4 nM, respectively. Murinoglobulin, the second murine alpha-macroglobulin, bound both TGF-beta isoforms with 30-fold lower affinity. Malpha2M counteracted the activities of TGF-beta1 and TGF-beta2 in an endothelial cell growth assay. Malpha2M also induced NO synthesis when incubated with RAW 264.7 cells, an activity which probably results from the neutralization of autocrine TGF-beta activity. Human alpha2M induced NO synthesis comparably to malpha2M; however, MUG had no effect. These studies demonstrate that the ability to neutralize TGF-beta is a property of malpha2M, which is not redundant in the murine alpha-macroglobulin family or in murine plasma. Malpha2M is the only murine alpha-macroglobulin that promotes NO synthesis. The absence of malpha2M, in alpha2M(-/-) mice, may allow TGF-beta to more efficiently suppress excessive iNOS expression following endotoxin challenge.

Alpha 2-macroglobulin functions as a cytokine carrier to induce nitric oxide synthesis and cause nitric oxide-dependent cytotoxicity in the RAW 264.7 macrophage cell line

Nitric oxide (NO) is an important mediator of macrophage activities. We studied the regulation of macrophage NO synthesis by alpha 2-macroglobulin (alpha 2M), a proteinase inhibitor and carrier of certain growth factors, including transforming growth factor-beta (TGF-beta). Native alpha 2M and the alpha 2M receptor-recognized derivative, alpha 2M-methylamine (alpha 2M-MA), increased nitrite generation by the RAW 264.7 murine macrophage cell line. The level of nitrite accumulation, which is an index of NO synthesis, was comparable to that observed with interferon-gamma. Native alpha 2M and alpha 2M-MA also increased inducible nitric oxide synthase (iNOS) mRNA levels and substantially reduced the number of viable cells, as determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium/succiny l dehydrogenase assay or trypan blue exclusion. At slightly higher alpha 2M concentrations, [3H]thymidine incorporation was inhibited. All of these activities were counteracted nearly completely when the iNOS competitive inhibitor NG-monomethyl-L-arginine was included. By in situ nick translation, native alpha 2M and alpha 2M-MA increased the percentage of cells with detectable single strand chromatin nicks from 4 to 12 and 17%, respectively. This change suggested apoptosis; however, electron microscopy studies demonstrated variability in the morphology of injured cells. To determine the mechanism by which alpha 2M increases macrophage NO synthesis, we studied proteolytic alpha 2M derivatives that retain partial activity. A 600-kDa derivative that retains growth factor binding activity increased RAW 264.7 cell NO synthesis and iNOS mRNA levels comparable to native alpha 2M and alpha 2M-MA. The purified 18-kDa alpha 2M receptor-binding fragment had no effect on NO synthesis or iNOS expression. Thus, the growth factor-carrier activity of alpha 2M and not its receptor-binding activity is essential for NO synthesis regulation. A TGF-beta-neutralizing antibody mimicked the activity of alpha 2M, increasing RAW 264.7 cell NO synthesis and decreasing cellular viability. These studies demonstrate that alpha 2M can regulate macrophage NO synthesis and profoundly affect cellular function without gaining entry into the cell and without binding specific plasma membrane receptors.