DPP-4 Inhibition Leads to Decreased Pancreatic Inflammatory Profile and Increased Frequency of Regulatory T Cells in Experimental Type 1 Diabetes

Emerging Role of Dipeptidyl Peptidase-4 in Autoimmune Disease

Dipeptidyl-peptidase IV (DPP4), originally identified as an aminopeptidase in 1960s, is an ubiquitously expressed protease presented as either a membrane-bound or soluble form. DPP4 cleaves dipeptide off from the N-terminal of its substrates, altering the bioactivity of its substrates. Subsequent studies reveal that DPP4 is also involved in various cellular processes by directly binding to a number of ligands, including adenosine deaminase, CD45, fibronectin, plasminogen, and caveolin-1. In recent years, many novel functions of DPP4, such as promoting fibrosis and mediating virus entry, have been discovered. Due to its implication in fibrotic response and immunoregulation, increasing studies are focusing on the potential role of DPP4 in inflammatory disorders. As a moonlighting protein, DPP4 possesses multiple functions in different types of cells, including both enzymatic and non-enzymatic functions. However, most of the review articles on the role of DPP4 in autoimmune disease were focused on the association between DPP4 enzymatic inhibitors and the risk of autoimmune disease. An updated comprehensive summary of DPP4’s immunoregulatory actions including both enzymatic dependent and independent functions is needed. In this article, we will review the recent advances of DPP4 in immune regulation and autoimmune rheumatic disease.

Combination of vitamin D and dipeptidyl peptidase-4 inhibitors (VIDPP-4i) as an immunomodulation therapy for autoimmune diabetes

Type 1 diabetes (T1D) and latent autoimmune diabetes in adults (LADA) represent the most common types of autoimmune diabetes and are characterized by different age of onset, degrees of immune-mediated destruction of pancreatic beta cells and rates of disease progression towards insulin dependence. Several immunotherapies aimed to counteract autoimmune responses against beta cells and preserve beta-cell function are currently being investigated, particularly in T1D. Preliminary findings suggest a potential role of combination therapy with vitamin D and dipeptidyl peptidase-4 (DPP-4) inhibitors (VIDPP-4i) in preserving beta-cell function in autoimmune diabetes. This manuscript aims to provide a comprehensive overview of the immunomodulatory properties of vitamin D and DPP-4 inhibitors, as well as the rationale for investigation of their combined use as an immunomodulation therapy for autoimmune diabetes.

Characterization of DPP-IV Inhibitory Peptides Using an In Vitro Cell Culture Model of the Intestine

Here, we characterize the activities of two depeptidyl peptidase-IV (DPP-IV) inhibitory peptides, VLATSGPG and LDKVFER, using the Caco-2 monolayer model for the intestine. VLATSGPG and LDKVFR inhibited the DPP-IV in the cells via a mixed-type inhibition mode, with in situ IC₅₀ values of 207.3 and 148.5 μM, respectively. Furthermore, VLATSGPG and LDKVFR were transported intact across the cells, with P_app values of 2.41 ± 0.16 and 4.23 ± 0.29 × 10^-7 cm/s, respectively. Fragmented peptides were identified in the basolateral side of the membrane. Two of these, GPG and VLA, exhibited high inhibitory activities of 83.6 ± 3.3 and 58.5 ± 2.5%, respectively, at 100 μM concentration. Although 3 mM VLATSGPG and LDKVFR were transported across the epithelium in a concentration-dependent manner, their transport did not damage the tight junction proteins, ZO-1 and occludin. This study demonstrates that the two peptides potentially regulate DPP-IV activity in the intestine.

Macrophage inflammatory state in Type 1 diabetes: triggered by NLRP3/iNOS pathway and attenuated by docosahexaenoic acid

Type 1 diabetes mellitus (T1D) is a chronic autoimmune disease characterized by insulin-producing pancreatic β-cell destruction and hyperglycemia. While monocytes and NOD-like receptor family-pyrin domain containing 3 (NLRP3) are associated with T1D onset and development, the specific receptors and factors involved in NLRP3 inflammasome activation remain unknown. Herein, we evaluated the inflammatory state of resident peritoneal macrophages (PMs) from genetically modified non-obese diabetic (NOD), NLRP3-KO, wild-type (WT) mice and in peripheral blood mononuclear cells (PBMCs) from human T1D patients. We also assessed the effect of docosahexaenoic acid (DHA) on the inflammatory status. Macrophages from STZ-induced T1D mice exhibited increased inflammatory cytokine/chemokine levels, nitric oxide (NO) secretion, NLRP3 and iNOS protein levels, and augmented glycolytic activity compared to control animals. In PMs from NOD and STZ-induced T1D mice, DHA reduced NO production and attenuated the inflammatory state. Furthermore, iNOS and IL-1β protein expression levels and NO production were lower in the PMs from diabetic NLRP3-KO mice than from WT mice. We also observed increased IL-1β secretion in PBMCs from T1D patients and immortalized murine macrophages treated with advanced glycation end products and palmitic acid. The present study demonstrated that the resident PMs are in a proinflammatory state characterized by increased NLRP3/iNOS pathway-mediated NO production, up-regulated proinflammatory cytokine/chemokine receptor expression and altered glycolytic activity. Notably, ex vivo treatment with DHA reverted the diabetes-induced changes and attenuated the macrophage inflammatory state. It is plausible that DHA supplementation could be employed as adjuvant therapy for treating individuals with T1D.

Emerging Roles of Dipeptidyl Peptidase-4 Inhibitors in Delaying the Progression of Type 1 Diabetes Mellitus

Type 1 diabetes mellitus (T1DM) results from the immune cell-mediated destruction of functional pancreatic β-cells. In the presymptomatic period, T1DM is characterized by the presence of two or more autoantibodies against the islet cells in patients without glycemic decompensation. Therapeutic strategies that can modify the autoimmune process could slow the progression of T1DM. Dipeptidyl peptidase-4 (DPP-4) or CD26, a multifunctional serine protease with a dual function (regulatory protease and binding protein), can modulate inflammation and immune cell-mediated β-cell destruction. CD26 is involved in T-cell co-stimulation, migration, memory development, thymic maturation, and emigration patterns. DPP-4 degrades the peptide hormones GLP-1 and GIP. In addition to regulating glucose metabolism, DPP-4 exerts anti-apoptotic, regenerative, and proliferative effects to promote β-cell mass expansion. GLP-1 receptor signaling may regulate murine lymphocyte proliferation and maintenance of peripheral regulatory T-cells. In patients with T1DM, the serum DPP-4 activity is upregulated. Several studies have suggested that the upregulated DPP-4 activity is correlated with T1DM pathophysiology. DPP-4, which is preferentially expressed on the Th1 surface, can promote the polarization of Th1 immunity, a prerequisite for T1DM development. CD26 inhibition can suppress T-cell proliferation and Th1 cytokine production and stimulate tumor growth factor beta-1 (TGF-β1) secretion, which plays an important role in the regulation of autoimmunity in T1DM. Studies on humans or animal models of T1DM have suggested that DPP-4 inhibitors can improve β-cell function and attenuate autoimmunity in addition to decreasing insulin dependence. This review summarizes the emerging roles of DPP-4 inhibitors in potentially delaying the progression of T1DM.

Long-term inhibition of dipeptidyl-peptidase 4 reduces islet infiltration and downregulates IL-1β and IL-12 in NOD mice

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DPP-4 inhibition reduced CD4+T cells infiltration and ameliorated insulitis.

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DPP-4 inhibition downregulated serum IL-1β and IL-12.

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LPS increased CD11b+ cells to infiltrate into islets.

Dipeptidyl-peptidase 4 (DPP-4) inhibitor (sitagliptin) is a novel anti-hyperglycemia drug in the treatment of type 2 diabetes. However, its potential in type 1 diabetes is still unclear. Recent studies show that increased infection, especially respiratory tract infection, is significantly associated with DPP-4 inhibitors. In this study, we aimed to explore the effects of long-term inhibition of DPP- 4 on innate immunity in type 1 diabetes. Forty mice were randomly divided into 4 groups (n = 10 in each group): control group, lipopolysaccharide (LPS) group, sitagliptin group and sitagliptin + LPS group. The concentrations of IL-1β, IL-2, IL-4, IL-5, IL-6, IL-10, IL-12, TNF-α and IFN-γ were measured with Mesco Scale Discovery multiplexed-assay kit. Immunohistochemistry staining of pancreases was performed and insulitis scores for each islet were determined. The results showed that DPP-4 inhibition has no effect on incident rate of diabetes and metabolic parameters in NOD mice. Long-term inhibition of DPP-4 reduced CD4+T cells to infiltrate into islets and ameliorated insulitis in NOD mice. DPP-4 inhibition downregulated serum interleukin IL-1β and IL-12 in NOD mice. However, it had no significant effect on LPS-induced IL-1β, IL-6, IL-10, IL-12, tumor necrosis factor (TNF)-α and interferon (IFN)-γ in NOD mice. In conclusion, Long-term inhibition of DPP-4 exists anti-inflammatory effect in type 1 diabetes probably by reducing CD4+T cells to infiltrate into islets and downregulating L-1β and IL-12 in serum.

Klotho Ameliorates Cellular Inflammation via Suppression of Cytokine Release and Upregulation of miR-29a in the PBMCs of Diagnosed Alzheimer's Disease Patients

Alzheimer's disease (AD) is the most common neurodegenerative disorder characterized by neural inflammation and oxidative stress. In the current study, the protective effects of klotho and linagliptin treatment on human peripheral blood mononuclear cells (PBMCs) of AD patients and healthy controls (HCs) are assessed through measurement of inflammatory cytokines, signaling proteins, and miRNA expression. Sixteen diagnosed AD patients and sixteen HCs were enrolled in the study. Blood samples were obtained and PBMCs were isolated. PBMCs were treated with klotho at different concentrations (0.5, 1, and 2 nM) and linagliptin (50 μM). The concentration of interleukin-1β (IL-1β), interleukin 6 (IL-6), tumor necrosis factor-alpha (TNF-α), epsilon isoform of protein kinase C (PKCε), phosphorylated cyclic AMP response element binding (pCREB), and Wnt1 were measured by ELISA. The expression of miR-29a and miR-195 was detected by real-time PCR. The results showed that klotho significantly reduced IL-1β, IL-6, and TNF-α levels in both groups of the experiment. Linagliptin also remarkably reduced TNF-α levels in the AD group. Moreover, klotho caused the downregulation of Wnt1 in the PBMCs of both groups and the upregulation of the pCREB in HCs. Meanwhile, klotho induced miR-29a expression in the PBMCs of HCs, while miR-29a expression was induced in the AD group by klotho and linagliptin. The current findings revealed that klotho alleviates inflammation in human PBMCs, probably through the suppression of inflammatory cytokines and the upregulation of miR-29a, and part of its beneficial effect is mediated through appropriate modulation of the Wnt1/pCREB signaling cascade. In addition, linagliptin exerts protective effects by reducing TNF-α and inducing miR-29a expression in PBMCs.