Enhanced transglutaminase 2 expression in response to stress-related catecholamines in macrophages

Differential Expression of Tissue Transglutaminase Splice Variants in Peripheral Blood Mononuclear Cells of Primary Progressive Multiple Sclerosis Patients

Multiple Sclerosis (MS) is an inflammatory and neurodegenerative disorder of the central nervous system (CNS) characterized by inflammation and immune cell infiltration in the brain parenchyma. Tissue transglutaminase (TG2), a calcium-dependent cross-linking enzyme, has been shown to be present in infiltrating MHC-II positive cells in lesions of patients suffering from MS. Moreover, TG2 mRNA levels in peripheral blood mononuclear cells (PBMC)-derived from primary progressive (PP)-MS patients correlated with clinical parameters, thus highlighting the importance of TG2 in MS pathology. In the present study, we further characterized TG2 expression by measuring the mRNA levels of full-length TG2 and four TG2 alternative splice variants in PBMCs derived from PP-MS patients and healthy control (HC) subjects. In PP-MS-derived PBMCs, TG2 variant V4b was significantly higher expressed, and both V4a and V4b variants were relatively more expressed in relation to full-length TG2. These observations open new avenues to unravel the importance of TG2 alternative splicing in the pathophysiology of PP-MS.

Monocyte-derived tissue transglutaminase in multiple sclerosis patients: reflecting an anti-inflammatory status and function of the cells?

BACKGROUND

Leukocyte infiltration into the central nervous system is an important feature of multiple sclerosis (MS) pathology. Among the infiltrating cells, monocytes comprise the largest population and are considered to play a dual role in the course of the disease. The enzyme tissue transglutaminase (TG2), produced by monocytes, plays a central role in monocyte adhesion/migration in animal models of MS. In the present study, we questioned whether TG2 expression is altered in monocytes from MS patients compared to healthy control (HC) subjects. Moreover, we determined the inflammatory status of these TG2-expressing monocytes, what inflammatory factor regulates TG2 expression, and whether TG2 can functionally contribute to their adhesion/migration processes.

METHODS

Primary human monocytes from MS patients and HC subjects were collected, RNA isolated and subjected to qPCR analysis. Human THP-1 monocytes were lentivirally transduced with TG2 siRNA or control and treated with various cytokines. Subsequently, mRNA levels of inflammatory factors, adhesion properties, and activity of RhoA were analyzed in interleukin (IL)-4-treated monocytes.

RESULTS

TG2 mRNA levels are significantly increased in monocytes derived from MS patients compared to HC subjects. In addition, correlation analyses indicated that TG2-expressing cells display a more anti-inflammatory, migratory profile in MS patients. Using THP-1 monocytes, we observed that IL-4 is a major trigger of TG2 expression in these cells. Furthermore, knockdown of TG2 expression leads to a pro-inflammatory profile and reduced adhesion/migration properties of IL-4-treated monocytes.

CONCLUSIONS

TG2-expressing monocytes in MS patients have a more anti-inflammatory profile. Furthermore, TG2 mediates IL-4-induced anti-inflammatory status in THP-1 monocytes, adhesion, and cytoskeletal rearrangement in vitro. We thus propose that IL-4 upregulates TG2 expression in monocytes of MS patients, driving them into an anti-inflammatory status.

Tranilast inhibits interleukin-33 production by macrophages

Tranilast is an anti-allergy medication that inhibits the release of chemical mediators such as histamine. However, the mechanisms underlying its anti-allergy effects are not fully understood. Interleukin (IL)-33, a novel member of the IL-1 cytokine family, promotes T helper type 2 immune responses and plays a pathogenic role in allergic disorders. In the present study, we examined the effects of tranilast on IL-33 production by RAW264.7 macrophages. Lipopolysaccharide (LPS) increased both IL-33 mRNA expression and IL-33 protein synthesis. Tranilast significantly inhibited LPS-induced IL-33 protein production by RAW264.7 macrophages in a dose-dependent manner; these same effects were observed on IL-33 mRNA levels in RAW264.7 macrophages and a primary culture of macrophages. LPS markedly activated Akt in RAW264.7 macrophages, whereas tranilast suppressed LPS-induced Akt activation. The effects of tranilast on Akt activation appeared to be responsible for the decrease in IL-33 production. Our present findings suggest that the inhibition of IL-33 production by tranilast might contribute to the anti-allergy effects of this medication.

Cyclic AMP signaling enhances lipopolysaccharide sensitivity and interleukin-33 production in RAW264.7 macrophages

While it has been suggested that IL-33 plays pathogenic roles in various disorders, the factors that stimulate IL-33 production are poorly characterized. In the present study, the effect of cyclic adenosine monophosphate (cAMP) signaling on IL-33 production in RAW264.7 macrophages in response to various doses of LPS was examined. High-dose LPS treatment induced IL-33 and TNF protein production in RAW264.7 macrophages. In contrast, low-dose LPS failed to induce IL-33 production while significantly inducing TNF production. In the presence of the membrane-permeable cAMP analog 8-Br-cAMP, low-dose LPS induced vigorous IL-33 production. This phenomenon was consistent with amounts of mRNA. Similarly, the cAMP-increasing agent adrenaline also enhanced the sensitivity of RAW264.7 macrophages to LPS as demonstrated by IL-33 production. The protein kinase A (PKA) inhibitor H89 blocked the effects of 8-Br-cAMP and adrenaline on IL-33 production, suggesting that PKA is involved in IL-33 induction. Taken together, cAMP-mediated signaling pathway appears to enhance the sensitivity of RAW264.7 macrophages to LPS with respect to IL-33 production. Our findings suggest that stress events and the subsequent secretion of adrenaline enhance macrophage production via IL-33; this process may be associated with the pathogenesis of various disorders involving IL-33.