Inhibition of the BET family of epigenetic reader proteins: A novel principle for modulating gene expression in IgE-activated mast cells.

INTRODUCTION: The BET family of bromodomain-containing proteins constitute epigenetic readers that bind to acetylated lysine residues of core histones, thereby translating epigenetic histone marks to effects on gene expression. BET inhibitors are currently emerging as promising therapeutic agents for treatment of various pathological conditions. Here, we explored the potential of using BET inhibition to modulate IgE-mediated responses in mast cells. METHODS: We assessed the effects of BET inhibitors PFI-1, I-BET151, and I-BET762 on responses downstream of mast cell activation through IgE receptor cross-linking. RESULTS: BET inhibitors were neither toxic for mast cells (at doses up to 20 µM), nor did they prevent IgE-mediated mast cell degranulation. However, we found that BET inhibition, in particular by I-BET151, suppressed IL-6 gene expression and IL-6 protein release in response to IgE-mediated mast cell activation. This was observed in both bone marrow-derived mast cells (BMMCs) and in mature peritoneal-cell derived mast cells. Further analysis showed that BET inhibition also suppressed the expression of a number of additional genes of those that were upregulated by IgE receptor cross-linking, including IL-3, IL-7R, CCR1, and ADAMTS9. However, BET inhibition was selective, i.e., several genes that were upregulated by IgE receptor cross-linking were not affected by BET inhibitors. CONCLUSIONS: These findings suggest that BET inhibition can interfere with the upregulated expression of selected genes in mast cells activated by IgE receptor cross-linking. Further, our findings introduce the concept of utilizing epigenetic mechanisms for modulating mast cell function in the context of IgE-driven disease.

Activated mast cells promote differentiation of B cells into effector cells.

Based on the known accumulation of mast cells (MCs) in B cell-dependent inflammatory diseases, including rheumatoid arthritis, we hypothesized that MCs directly modulate B cells. We show here that degranulated, and to a lesser extent naïve or IgE-sensitized, MCs activate both naïve and B cell receptor-activated B cells. This was shown by increased proliferation, blast formation, and expression of CD19, MHC class II and CD86 in the B cells. Further, MCs stimulated the secretion of IgM and IgG in IgM(+) B cells, indicating that MCs can induce class-switch recombination in B cells. We also show that coculture of MCs with B cells promotes surface expression of L-selectin, a homing receptor, on the B cells. The effects of MCs on B cells were partly dependent on cell-cell contact and both follicular and marginal zone B cells could be activated by MCs. Our findings suggest that degranulated MCs support optimal activation of B cells, a finding that is in line with in vivo studies showing that MCs frequently degranulate in the context of B-cell driven pathologies such as arthritis. Together, our findings show that MCs have the capacity to differentiate B cells to effector cells.

Damnacanthal inhibits IgE receptor-mediated activation of mast cells.

Damnacanthal, an anthraquinone obtained from the noni fruit (Morinda citrifolia L.), has been described to possess anti-cancer and anti-inflammatory properties. Since mast cells are key players in various inflammatory conditions as well as in cancer, we considered the possibility that the biological actions of damnacanthal, at least partly, could be due to effects on mast cells. Many of the biological activities of mast cells are mediated by IgE receptor cross-linking, which results in degranulation with release of preformed granule mediators, as well as de novo synthesis and release of additional compounds. Here we show that damnacanthal has profound inhibitory activity on mast cell activation through this pathway. The release of the granule compounds beta-hexosaminidase and tryptase release was completely abrogated by damnacanthal at doses that were non-toxic to mast cells. In addition, damnacanthal inhibited activation-dependent pro-inflammatory gene induction, as well as cytokine/chemokine release in response to mast cell stimulation. The mechanism underlying damnacanthal inhibition was linked to impaired phosphorylation of Syk and Akt. Furthermore, damnacanthal inhibited mast cell activation in response to calcium ionophore A23187. Altogether, the data presented here demonstrate that damnacanthal inhibits mast cell activation induced by different stimuli and open a new window for the use of this compound as a mast cell stabilizer.

Mast cells are activated by Staphylococcus aureus in vitro but do not influence the outcome of intraperitoneal S. aureus infection in vivo.

Staphylococcus aureus is a major pathogen that can cause a broad spectrum of serious infections including skin infections, pneumonia and sepsis. Peritoneal mast cells have been implicated in the host response towards various bacterial insults and to provide mechanistic insight into the role of mast cells in intraperitoneal bacterial infection we here studied the global effects of S. aureus on mast cell gene expression. After co-culture of peritoneal mast cells with live S. aureus we found by gene array analysis that they up-regulate a number of genes. Many of these corresponded to pro-inflammatory cytokines, including interleukin-3, interleukin-13 and tumour necrosis factor-α. The cytokine induction in response to S. aureus was confirmed by ELISA. To study the role of peritoneal mast cells during in vivo infection with S. aureus we used newly developed Mcpt5-Cre(+) × R-DTA mice in which mast cell deficiency is independent of c-Kit. This is in contrast to previous studies in which an impact of mast cells on bacterial infection has been proposed based on the use of mice whose mast cell deficiency is a consequence of defective c-Kit signalling. Staphylococcus aureus was injected intraperitoneally into mast-cell-deficient Mcpt5-Cre(+) × R-DTA mice using littermate mast-cell-sufficient mice as controls. We did not observe any difference between mast-cell-deficient and control mice with regard to weight loss, bacterial clearance, inflammation or cytokine production. We conclude that, despite peritoneal mast cells being activated by S. aureus in vitro, they do not influence the in vivo manifestations of intraperitoneal S. aureus infection.

Nuclear receptor 4a3 (nr4a3) regulates murine mast cell responses and granule content.

Nuclear receptor 4a3 (Nr4a3) is a transcription factor implicated in various settings such as vascular biology and inflammation. We have recently shown that mast cells dramatically upregulate Nuclear receptor 4a3 upon activation, and here we investigated the functional impact of Nuclear receptor 4a3 on mast cell responses. We show that Nuclear receptor 4a3 is involved in the regulation of cytokine/chemokine secretion in mast cells following activation via the high affinity IgE receptor. Moreover, Nuclear receptor 4a3 negatively affects the transcript and protein levels of mast cell tryptase as well as the mast cell's responsiveness to allergen. Together, these findings identify Nuclear receptor 4a3 as a novel regulator of mast cell function.

Active caspase-3 is stored within secretory compartments of viable mast cells.

Caspase-3 is a main executioner of apoptotic cell death. The general notion is that, in viable cells, caspase-3 is found as a cytosolic inactive proenzyme and that caspase-3 activation is largely confined to processes associated with cell death. In this study, we challenge this notion by showing that enzymatically active caspase-3 is stored in viable mast cells. The enzymatically active caspase-3 was undetectable in the cytosol of viable cells, but was recovered in subcellular fractions containing secretory granule-localized proteases. Moreover, active caspase-3 was rapidly released into the cytosolic compartment after permeabilization of the secretory granules. Using a cell-permeable substrate for caspase-3, the presence of active caspase-3-like activity in granule-like compartments close to the plasma membrane was demonstrated. Moreover, it was shown that mast cell activation caused release of the caspase-3 to the cell exterior. During the course of mast cell differentiation from bone marrow cells, procaspase-3 was present in cells of all stages of maturation. In contrast, active caspase-3 was undetectable in bone marrow precursor cells, but increased progressively during the process of mast cell maturation, its accumulation coinciding with that of a mast cell-specific secretory granule marker, mouse mast cell protease 6. Together, the current study suggests that active caspase-3 can be stored within secretory compartments of viable mast cells.

ADAMTS: novel proteases expressed by activated mast cells.

Here we show that mast cells (MCs) express the metalloproteases of the A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) family, and that ADAMTS expression is influenced by MC activation. Co-culture of MCs with live Gram-positive bacteria caused a profound induction of ADAMTS-9 and -6, as well as down-regulated expression of ADAMTS-5. Similar patterns were also seen after MC activation with calcium ionophore and by immunoglobulin E receptor crosslinking. Moreover, ADAMTS-5, -6 and -9 were all induced by activation of terminally differentiated murine peritoneal MCs and in a human MC line. ADAMTS-9 up-regulation in response to immunoglobulin E receptor crosslinking was strongly dependent on Gö6976-sensitive protein kinase C and partly dependent on nuclear factor of activated T cells and nuclear factor kappa-light-chain-enhancer of activated B cells, respectively. The expression of ADAMTS-5, -6 and -9 was closely linked to MC maturation, as shown by their strong induction during the differentiation of bone marrow precursor cells into mature MCs. ADAMTS family members have been shown to possess aggrecanase activity. Accordingly, MCs were shown to express aggrecanase activity. Finally, ADAMTS-5 protein was detected in MCs by immunocytochemistry. Taken together, the present study reveals ADAMTS expression by MCs and that MC activation regulates the expression of these proteases, thus implicating the ADAMTS family of proteases in MC function.

Polyamines are present in mast cell secretory granules and are important for granule homeostasis.

BACKGROUND: Mast cell secretory granules accommodate a large number of components, many of which interact with highly sulfated serglycin proteoglycan (PG) present within the granules. Polyamines (putrescine, spermidine and spermine) are absolutely required for the survival of the vast majority of living cells. Given the reported ability of polyamines to interact with PGs, we investigated the possibility that polyamines may be components of mast cell secretory granules. METHODOLOGY/PRINCIPAL FINDINGS: Spermidine was released by mouse bone marrow derived mast cells (BMMCs) after degranulation induced by IgE/anti-IgE or calcium ionophore A23187. Additionally, both spermidine and spermine were detected in isolated mouse mast cell granules. Further, depletion of polyamines by culturing BMMCs with α-difluoromethylornithine (DFMO) caused aberrant secretory granule ultrastructure, impaired histamine storage, reduced serotonin levels and increased ß-hexosaminidase content. A proteomic approach revealed that DFMO-induced polyamine depletion caused an alteration in the levels of a number of proteins, many of which are connected either with the regulated exocytosis or with the endocytic system. CONCLUSIONS/SIGNIFICANCE: Taken together, our results show evidence that polyamines are present in mast cell secretory granules and, furthermore, indicate an essential role of these polycations during the biogenesis and homeostasis of these organelles.

Polyamines affect histamine synthesis during early stages of IL-3-induced bone marrow cell differentiation.

Mast cells synthesize and store histamine, a key immunomodulatory mediator. Polyamines are essential for every living cell. Previously, we detected an antagonistic relationship between the metabolisms of these amines in established mast cell and basophilic cell lines. Here, we used the IL-3-driven mouse bone marrow-derived mast cell (BMMC) culture system to further investigate this antagonism in a mast cell model of deeper physiological significance. Polyamines and histamine levels followed opposite profiles along the bone marrow cell cultures leading to BMMCs. alpha-Difluoromethylornithine (DFMO)-induced polyamine depletion resulted in an upregulation of histidine decarboxylase (HDC, the histamine-synthesizing enzyme) expression and activity, accompanied by increased histamine levels, specifically during early stages of these cell cultures, where an active histamine synthesis process occurs. In contrast, DFMO did not induce any effect in either HDC activity or histamine levels of differentiated BMMCs or C57.1 mast cells, that exhibit a nearly inactive histamine synthesis rate. Sequence-specific DNA methylation analysis revealed that the DFMO-induced HDC mRNA upregulation observed in early bone marrow cell cultures is not attributable to a demethylation of the gene promoter caused by the pharmacological polyamine depletion. Taken together, the results support an inverse relationship between histamine and polyamine metabolisms during the bone marrow cell cultures leading to BMMCs and, moreover, suggest that the regulation of the histamine synthesis occurring during the early stages of these cultures depends on the concentrations of polyamines.

The polyamine and histamine metabolic interplay in cancer and chronic inflammation.

PURPOSE OF REVIEW: To provide an update on the major research contributing to deciphering the metabolic interplay of polyamines/histamine and its impact in cancer and chronic inflammation. RECENT FINDINGS: The most recent and relevant findings that might reflect a link between the polyamines/histamine metabolic interplay and the development of cancer and chronic inflammation-related diseases include: the observation that histamine catabolism is downregulated in the colonic mucosa of patients with colonic adenoma; the finding that some polyamine and histamine-related metabolites are different between a breast cancer cell line and a reference mammary epithelial cell line; and the demonstration of the critical role that mast cells (a cell type in which the polyamine/histamine metabolic interplay has been confirmed) play in the development of pancreatic tumors. There is still, however, a lack of specific studies elucidating the exact contribution of the polyamine/histamine metabolic interplay in these clinical settings. SUMMARY: In mammalian cells, a polyamine/histamine metabolic interplay has been extensively proven; however, its ultimate effect on human health largely depends on the cell type and environment. Information on this topic is currently fragmented in the literature. In order to develop efficient intervention strategies, it will be necessary to establish an integrated and holistic view of the role of the polyamine/histamine metabolic interplay in each pathological state.

Serotonin and histamine storage in mast cell secretory granules is dependent on serglycin proteoglycan.

BACKGROUND: Serotonin and histamine are components of human and rodent mast cell secretory granules. OBJECTIVE: Serotonin and histamine are stored in the same compartment as serglycin proteoglycan. Here we addressed the possibility that serglycin may be involved in their storage and/or release. METHODS: The storage and release of histamine and serotonin was studied in bone marrow-derived mast cells (BMMCs) and in peritoneal mast cells from wild-type or serglycin-/- mice. RESULTS: Both serotonin and histamine storage in BMMCs was positively correlated with the degree of mast cell differentiation, and the amount of stored amine was reduced in serglycin-/- BMMCs compared with wild-type controls. The amounts of histamine/serotonin stored were reflected by the expression levels of histidine decarboxylase and tryptophan hydroxylase 1, respectively. Calcium ionophore activation resulted in serotonin/histamine release both from wild-type and serglycin-/- BMMCs. Interestingly, serotonin release was induced in cells lacking intracellular stores of serotonin, suggesting de novo synthesis. The knockout of serglycin affected the levels of stored and released mast cell serotonin and histamine to an even larger extent in in vivo-derived mast cells than in BMMCs. CONCLUSION: These results establish a previously assumed, but not proven, role of serglycin in storage of histamine and, further, establish for the first time that serotonin storage in mast cells is dependent on serglycin proteoglycan.