Masayuki Funaba

Activin A Functions as a Th2 Cytokine in the Promotion of the Alternative Activation of Macrophages

Activin A, a member of the TGF-β superfamily, is a pluripotent growth and differentiation factor. In this study, we report that murine Th cells produce activin A upon activation. Activin activity in the cultured CD4+ T cells was induced by anti-CD3 cross-linking. Activin βA mRNA level was increased in response to activation, indicating that activin production in CD4+ T cells is regulated at the mRNA level. Activin production was detected exclusively in CD4+CD25− T cells, but not in CD4+CD25+ regulatory T cells. When CD4+ T cells were differentiated into Th cell subsets, higher activin secretion was detected when cultured under Th2-skewing conditions. The mRNA level of activin βA was abundant in Th2, but not in Th1 cells. Furthermore, secretion of activin was significantly higher in activated Th2 clones than in Th1 clones. The activin βA-proximal promoter contains a binding site for c-Maf, a Th2-specific transcriptional factor, at close proximity with an NF-AT binding site. c-Maf was able to synergize with NF-AT to transactivate activin βA gene, and both factors are implicated in activin βA transcription in Th2 cells. Activin A induced macrophages to express arginase-1 (M-2 phenotype), whereas it inhibited inducible NO synthase expression (M-1 phenotype) induced by IFN-γ. Taken together, these observations suggest that activin A is a novel Th2 cytokine that promotes differentiation of macrophages toward the M-2 phenotype.

Altered function of murine mast cells in response to lipopolysaccharide and peptidoglycan

Toll-like receptors (TLRs) recognize and signal the presence of bacterial components such as lipopolysaccharide (LPS) and peptidoglycan (PG) as a part of innate immunity. Our previous studies revealed that mast cells function as effector cells in the protection of mice against lethal enterobacterial infections. In this study, we examined both the gene expression of molecules involved in TLR signaling and the effects of LPS and PG in bone marrow-derived cultured mast cells (BMCMCs). The mRNA expression of TLR2, TLR4 and TLR6 was detected in BMCMCs. CD14, MD-2 and MyD88, which are also involved in TLR pathway, were also expressed. Neither LPS nor PG affected degranulation in BMCMCs, but release of tumor necrosis factor increased slightly in response to LPS and PG. Both LPS and PG enhanced expression of pro-matrix metalloproteinase 9 (pro-MMP-9) in a dose-dependent manner, and DNA fragmentation was induced by LPS, but not by PG. These results suggest that mast cells are the targets of LPS and PG, and that the functions of these molecules produced exclusively by bacteria partly overlap, but are distinct.

Role of activin A in murine mast cells: modulation of cell growth, differentiation, and migration

Activins, members of the transforming growth factor‐β (TGF‐β) superfamily, are potent growth and differentiation factors. Our previous studies revealed that activin A, a homodimer of inhibin/activin βA, was induced in mast cells and peritoneal macrophages in response to their activation. In the present study, we examined the roles of activin A in murine bone marrow‐derived, cultured mast cell progenitors (BMCMCs), which expressed gene transcripts for molecules involved in activin signaling, suggesting that BMCMCs could be target cells of activin A. Treatment of activin A inhibited 3‐[4,5‐dimethylthiazol‐2‐yl]‐2,5‐diphenyltetrazolium bromide uptake into BMCMCs in a dose‐dependent manner. The IC50 concentration was 2.1 nM, which was less potent than 185 pM TGF‐β1. Activin A treatment caused morphological changes toward the differentiated cells at 2 nM and up‐regulated mRNA of mouse mast cell protease‐1 (mMCP‐1), a marker enzyme of mature mucosal mast cells, at 1 nM. Activin A also showed activity in inducing migration of BMCMCs; the optimal concentration for maximal migration was 10 pM, which was much lower than the concentrations to inhibit cell growth and to activate the mMCP‐1 gene. Taking the present results together with our previous results, it is suggested that activin A secreted from activated immune cells recruits mast cell progenitors to sites of inflammation and that with increasing activin A concentration, the progenitors differentiate into mature mast cells. Thus, activin A may positively regulate the functions of mast cells as effector cells of the immune system.

Transcriptional Activation of Mouse Mast Cell Protease-7 by Activin and Transforming Growth Factor-β Is Inhibited by Microphthalmia-associated Transcription Factor

Previous studies have revealed that activin A and transforming growth factor-β1 (TGF-β1) induced migration and morphological changes toward differentiation in bone marrow-derived cultured mast cell progenitors (BMCMCs). Here we show up-regulation of mouse mast cell protease-7 (mMCP-7), which is expressed in differentiated mast cells, by activin A and TGF-β1 in BMCMCs, and the molecular mechanism of the gene induction of mmcp-7. Smad3, a signal mediator of the activin/TGF-β pathway, transcriptionally activated mmcp-7. Microphthalmia-associated transcription factor (MITF), a tissue-specific transcription factor predominantly expressed in mast cells, melanocytes, and heart and skeletal muscle, inhibited Smad3-mediated mmcp-7 transcription. MITF associated with Smad3, and the C terminus of MITF and the MH1 and linker region of Smad3 were required for this association. Complex formation between Smad3 and MITF was neither necessary nor sufficient for the inhibition of Smad3 signaling by MITF. MITF inhibited the transcriptional activation induced by the MH2 domain of Smad3. In addition, MITF-truncated N-terminal amino acids could associate with Smad3 but did not inhibit Smad3-mediated transcription. The level of Smad3 was decreased by co-expression of MITF but not of dominant-negative MITF, which resulted from proteasomal protein degradation. The changes in the level of Smad3 protein were paralleled by those in Smad3-mediated signaling activity. These findings suggest that MITF negatively regulates Smad-dependent activin/TGF-β signaling in a tissue-specific manner.

Immunolocalization of Type I or Type II Activin Receptors in the Rat Brain

We have studied immunolocalization of activin receptors in the central nervous system using polyclonal antibodies (IgG) to type I (50–55 kDa, ActRI), type II (70–75 kDa, ActRII) or a subtype of type II known as type IIB (ActRIIB) receptors of activin. A total of 7 antisera to rat activin receptors was generated, i.e. 3 kinds of antisera to the extracellular domain (ActRI(81–89), ActRII(91–100), or ActRIIB(90–99)) and 4 antisera to the kinase domain (ActRI(323–333), ActRII(307–319), ActRII(407–420) or ActRIIB(306–319)). The region of aa 407–420 of ActRII is identical with that of ActRIIB. At first, we characterized these antibodies by Western blot analysis using ovarian proteins fractionated by preparative SDS‐PAGE. All antibodies to ActRII and ActRIIB specifically reacted with 75 kDa‐proteins which could also bind to activin‐A. Anti‐ActRII(91–100) antibody also reacted with 62 kDa‐proteins which were capable of binding with activin‐A. Although no positive reactions to anti‐ActRI(81–89) antibody were seen in ovarian proteins, a positive reaction was detected at 52 kDa only when the proteins were deglycosylated. By use of these antibodies, immunolocalization of activin receptors was examined in the rat brain. The patterns of expression of activin type I and type II receptors were different. Positive reactions to anti‐ActRII(91–100) antibody were detected in neurons of the cerebral cortex, hippocampus, medial amygdala and thalamus. In the hypothalamus, some neurons of the supraoptic nucleus were weakly stained, and widely scattered neurons of the lateral hypothalamic area were moderately stained. On the contrary, the most intense reactions to anti‐ActRI(81–89) antibody were detected in neurons of the lateral hypothalamic area. In addition, many neurons of the cerebral cortex were also stained, but neurons of the hippocampus and the amygdala were not stained. These results suggest that activin may have physiological roles not only for hypothalamic neuroendocrinological and feeding‐related systems as suggested previously but may also have functions in cortical and limbic pathways as a neuromodulator or for maintenance of neurons.

Degranulation in RBL-2H3 cells: regulation by calmodulin pathway

Involvement of the calmodulin pathway in Ca2+‐induced degranulation was evaluated in RBL‐2H3 mast cells. Pretreatment of RBL‐2H3 cells with a calmodulin antagonist, W‐13, blocked ionomycin‐dependent release of β‐hexosaminidase into the supernatant, although W‐13 treatment alone slightly but significantly increased the release. Ca2+/calmodulin activates various protein kinases and phosphatases including myosin‐light chain kinase (MLCK), calmodulin‐dependent protein kinases (CaMKs), and calcineurin. When RBL‐2H3 cells were pretreated with a MLCK inhibitor, ML‐7, or a CaMKs inhibitor, KN‐93, the ionomycin‐dependent release of β‐hexosaminidase into the supernatant was inhibited. In addition, pretreatment with calcineurin inhibitors, cyclosporin A and FR901725, resulted in blockage of the ionomycin‐dependent release of β‐hexosaminidase into the supernatant. Our results indicate that Ca2+/calmodulin, activated calmodulin, is indispensable for Ca2+‐induced degranulation, and that within the calmodulin pathways, at least MLCK, CaMKs and calcineurin positively regulate the release of granules initiated by increasing cytosolic Ca2+concentrations in RBL‐2H3 cells.

A dual role of activin A in regulating immunoglobulin production of B cells

Here, we report that activin A has a dual role in regulating Ig production of murine B cells. Activated B cells secrete activin activity by increasing activin A and decreasing follistatin expression. B cells also express type I and type II activin receptors, suggesting that they are targets of activin. Pretreatment of naïve B cells with activin A and subsequent activation by LPS resulted in increased cell growth and IgG production. In contrast, no significant effect was observed when activin A was added to naïve B cells simultaneously with LPS, indicating that activin A acts on resting but not activated B cells. In addition, activin A did not induce B cells to produce IgE, even when added prior to activation; however, in vivo antigen‐specific IgE production was reduced significantly by neutralization of circulating activin A. These findings indicate that activin A plays an important role in Th2‐mediated immune responses by enhancing antibody production through two distinct modes: acts directly on resting B cells to elicit full functions of activated B cells and acts indirectly on activated B cells through modulation of other immune cells.

Calcium-regulated expression of activin A in RBL-2H3 mast cells.

The present study examined the regulatory expression of activin A, a potent growth and differentiation factor, in rat basophilic leukemia (RBL-2H3) mast cells. Treatment of RBL-2H3 cells sensitized with anti-dinitrophenyl IgE with multivalent dinitrophenyl led to a clear increase in RT-PCR products of inhibin/activin beta(A). The steady-state mRNA of inhibin/activin beta(A) was also induced by increasing cytosolic Ca(2+) concentration with ionomycin, which required de novo protein synthesis, and was regulated at the transcriptional level. Pretreatment of RBL-2H3 cells with antagonists or inhibitors for the calmodulin pathway blocked ionomycin-dependent inhibin/activin beta(A) transcription and mRNA induction, suggesting the involvement of calmodulin-dependent kinase (CaMK) and calcineurin. The ionomycin-dependent inhibin/activin beta(A) induction was also partially blocked by preincubation with c-Jun NH(2)-terminal kinase (JNK) and p38 kinase inhibitors, but not with MEK1 inhibitor. These results suggest that inhibin/activin beta(A) gene activation is achieved by the JNK and p38 kinase activation through the calmodulin pathway in mast cells.

Expression and transcriptional activity of alternative splice variants of Mitf exon 6

Microphthalmia-associated transcription factor (Mitf) is a tissue-specific transcription factor. At least nine distinct mouse isoform mRNAs are encoded by alternative splicing of the first exon of Mitf (Mitf-A, -B, -C, -D, -E, -H, -J, -M, and -mc), while exons 2–9 of all Mitf isoforms examined to date are identical. In addition, alternative splice variants of exon 6a encoding 6 amino acid proximal to the basic region of the protein are known in Mitf-A, -H, and -M. In this study, we identified alternative splice variants of exon 6a in other Mitf isoforms (Mitf-E, -J, and -mc) in melanocytes, mast cells, macrophages, and heart. We also compared the transcriptional activity of Mitf variants containing exon 6a to that of Mitf variants that did not contain exon 6a. PCR-RFLP analysis revealed that expression of Mitf with exon 6a was comparable with that of Mitf without exon 6a, irrespective of the specificity of the first exon, or cell type, although Mitf isoforms with different first exons were expressed in a cell type-dependent manner. Luciferase-based reporter assays revealed that transcription of Tyrosinase, which is known Mitf-regulated gene, was elicited more efficiently by expression of Mitf isoforms containing exon 6a, compared to isoforms that did not contain exon 6a. However, when transcription of Tyrp-1, Mmcp-6, and PAI-1 was examined, no significant differences were detected between Mitf isoforms with exon 6a and those without exon 6a, except for Tyrp-1 transcription by Mitf-D/E isoform. These results reveal a diverse pattern of gene expression and different transcriptional activities of Mitf isoforms, suggesting discrete regulation of gene transcription in specific tissues by Mitf.

Activin A: serum levels and immunohistochemical brain localization in rats given diets deficient in L-lysine or protein.

When a L-lysine (Lys)-deficient diet is given to rats, Lys in plasma and brain declines and rats will then select a Lys solution from among other L-amino acids (AAs). The recording of single-unit activity in the lateral hypothalamic area of these rats suggested that neural plasticity occurred, specifically responding to the deficient nutrient, Lys, centrally and during ingestion of AA. Possible neurotrophic factors in serum from rats with or without deficiency of either protein or Lys was assayed by Hydra japonica. An increase in serum inhibin and activin A was observed in rats fed a Lys-sufficient and nonprotein diet, respectively. However, serum activin A-like activity was severely suppressed under Lys deficiency. Additionally, the immunohistochemical distribution of activin A in the brain was found in the nucleus tractus solitarius, the area postrema, and the arcuate nucleus. These facts indicate that ingestion of Lys-deficient or nonprotein diet caused a change in serum levels of activin A as a possible neurotrophic factor. This release may elicit plasticity in the sensitivity of neurons to deficient AA in the nuclei that could selectively drive ingestive behavior for its particular AA (e.g., Lys) to maintain AA homeostasis.