A Randomized Controlled Clinical Trial in Healthy Older Adults to Determine Efficacy of Glycine and N-Acetylcysteine Supplementation on Glutathione Redox Status and Oxidative Damage.

Glycine and cysteine are non-essential amino acids that are required to generate glutathione, an intracellular tripeptide that neutralizes reactive oxygen species and prevents tissue damage. During aging glutathione demand is thought to increase, but whether additional dietary intake of glycine and cysteine contributes towards the generation of glutathione in healthy older adults is not well understood. We investigated supplementation with glycine and n-acetylcysteine (GlyNAC) at three different daily doses for 2 weeks (low dose: 2.4 g, medium dose: 4.8 g, or high dose: 7.2 g/day, 1:1 ratio) in a randomized, controlled clinical trial in 114 healthy volunteers. Despite representing a cohort of healthy older adults (age mean = 65 years), we found significantly higher baseline levels of markers of oxidative stress, including that of malondialdehyde (MDA, 0.158 vs. 0.136 µmol/L, p < 0.0001), total cysteine (Cysteine-T, 314.8 vs. 276 µM, p < 0.0001), oxidized glutathione (GSSG, 174.5 vs. 132.3 µmol/L, p < 0.0001), and a lower ratio of reduced to oxidized glutathione (GSH-F:GSSG) (11.78 vs. 15.26, p = 0.0018) compared to a young reference group (age mean = 31.7 years, n = 20). GlyNAC supplementation was safe and well tolerated by the subjects, but did not increase levels of GSH-F:GSSG (end of study, placebo = 12.49 vs. 7.2 g = 12.65, p-value = 0.739) or that of total glutathione (GSH-T) (end of study, placebo = 903.5 vs. 7.2 g = 959.6 mg/L, p-value = 0.278), the primary endpoint of the study. Post-hoc analyses revealed that a subset of subjects characterized by high oxidative stress (above the median for MDA) and low baseline GSH-T status (below the median), who received the medium and high doses of GlyNAC, presented increased glutathione generation (end of study, placebo = 819.7 vs. 4.8g/7.2 g = 905.4 mg/L, p-value = 0.016). In summary GlyNAC supplementation is safe, well tolerated, and may increase glutathione levels in older adults with high glutathione demand. Clinical Trial Registration: https://clinicaltrials.gov/ct2/show/NCT05041179, NCT05041179.

Adipokines enhance oleic acid-induced proliferation of vascular smooth muscle cells by inducing CD36 expression.

Adipose tissue is not only releasing lipids but also various adipokines that are both dysregulated in the obese state and may contribute to obesity-associated vascular dysfunction and cardiovascular risk. We have previously shown that the combination of adipocyte-conditioned medium (CM) and oleic acid (OA) increases proliferation of human vascular smooth muscle cells (VSMC) in a synergistic way. We identified vascular endothelial growth factor (VEGF) as a component within CM that is responsible for most of the observed effects. In this study, we investigate novel mechanisms that underlie the combined effects of adipokine and oleic acid-induced proliferation of VSMC. Oleic acid leads to significant lipid accumulation in VSMC that is further enhanced by the combined treatment with CM. Accordingly CM stimulates CD36 expression in VSMC while OA is not affecting CD36. Silencing of CD36 was established and prevents lipid accumulation in all tested conditions. CD36 silencing also abrogates CM- and OA-induced proliferation and considerably reduces proliferation induced by the combination of CM and OA. At the same time, VEGF secretion and VEGF-receptor 1 (VEGF-R1) by VSMC was not affected by CD36 silencing. However, VEGF was not able to induce any proliferation in VSMC after CD36 silencing that also blunted VEGF-induced extracellular signal-regulated kinase (ERK) activation. Finally, combined silencing of CD36 together with a blocking antibody against VEGF prevented most of CMOA-induced proliferation. In conclusion, our results demonstrate that CD36 is mediating CM-induced proliferation of VSMC. Induction of CD36 by adipokines enhances the response of VSMC towards VEGF and OA.

Differential impact of oleate, palmitate, and adipokines on expression of NF-κB target genes in human vascular smooth muscle cells.

It is widely accepted that obesity is a major risk factor for the development of atherosclerosis. In this context, adipose tissue produces a variety of adipokines and releases free fatty acids, contributing to a chronic-low grade inflammation state implicated in vascular complications. In this study, we investigated the role of adipokines, oleic acid (OA), palmitic acid (PA), and the combinations on activation of NF-κB target genes in human vascular smooth muscle cells (SMC) to assess the hypothesis of synergistic interactions between these molecules. Adipocyte-conditioned medium (CM), generated from human adipocytes, in combination with low concentrations of OA, but not PA, induces SMC proliferation and activation of the transcription factor NF-κB in a synergistic way. Combined treatment of CM and OA further regulates a set of downstream NF-κB target genes including angiopoietin-1, activin A, and MMP-1, all critically involved in SMC dysfunction. This suggests that the lipotoxic potential of fatty acids is substantially enhanced by the presence of adipocyte-derived factors.

Identification and validation of novel adipokines released from primary human adipocytes.

Adipose tissue is a major endocrine organ, releasing signaling and mediator proteins, termed adipokines, via which adipose tissue communicates with other organs. Expansion of adipose tissue in obesity alters adipokine secretion, which may contribute to the development of metabolic diseases. Although recent profiling studies have identified numerous adipokines, the amount of overlap from these studies indicates that the adipokinome is still incompletely characterized. Therefore, we conducted a complementary protein profiling on concentrated conditioned medium derived from primary human adipocytes. SDS-PAGE/liquid chromatography-electrospray ionization tandem MS and two-dimensional SDS-PAGE/matrix-assisted laser desorption ionization/time of flight MS identified 347 proteins, 263 of which were predicted to be secreted. Fourty-four proteins were identified as novel adipokines. Furthermore, we validated the regulation and release of selected adipokines in primary human adipocytes and in serum and adipose tissue biopsies from morbidly obese patients and normal-weight controls. Validation experiments conducted for complement factor H, αB-crystallin, cartilage intermediate-layer protein, and heme oxygenase-1 show that the release and expression of these factors in adipocytes is regulated by differentiation and stimuli, which affect insulin sensitivity, as well as by obesity. Heme oxygenase-1 especially reveals to be a novel adipokine of interest. In vivo, circulating levels and adipose tissue expression of heme oxygenase-1 are significantly increased in obese subjects compared with lean controls. Collectively, our profiling study of the human adipokinome expands the list of adipokines and further highlights the pivotal role of adipokines in the regulation of multiple biological processes within adipose tissue and their potential dysregulation in obesity.

Dipeptidyl peptidase 4 is a novel adipokine potentially linking obesity to the metabolic syndrome.

OBJECTIVE: Comprehensive proteomic profiling of the human adipocyte secretome identified dipeptidyl peptidase 4 (DPP4) as a novel adipokine. This study assessed the functional implications of the adipokine DPP4 and its association to the metabolic syndrome. RESEARCH DESIGN AND METHODS: Human adipocytes and skeletal and smooth muscle cells were used to monitor DPP4 release and assess the effects of soluble DPP4 on insulin signaling. In lean and obese subjects, depot-specific expression of DPP4 and its release from adipose tissue explants were determined and correlated to parameters of the metabolic syndrome. RESULTS: Fully differentiated adipocytes exhibit a substantially higher release of DPP4 compared with preadipocytes or macrophages. Direct addition of DPP4 to fat and skeletal and smooth muscle cells impairs insulin signaling. A fivefold higher level of DPP4 protein expression was seen in visceral compared with subcutaneous fat of obese patients, with no regional difference in lean subjects. DPP4 serum concentrations significantly correlated with adipocyte size. By using adipose tissue explants from lean and obese subjects, we observed a twofold increase in DPP4 release that strongly correlated with adipocyte volume and parameters of the metabolic syndrome and was decreased to the lean level after weight reduction. DPP4 released from adipose tissue correlated positively with an increasing risk score for the metabolic syndrome. CONCLUSIONS: DPP4 is a novel adipokine that may impair insulin sensitivity in an autocrine and paracrine fashion. Furthermore, DPP4 release strongly correlates with adipocyte size, potentially representing an important source of DPP4 in obesity. Therefore, we suggest that DPP4 may be involved in linking adipose tissue and the metabolic syndrome.

Oleic acid and adipokines synergize in inducing proliferation and inflammatory signalling in human vascular smooth muscle cells.

In the context of obesity, perivascular fat produces various adipokines and releases free fatty acids, which may induce inflammation and proliferation in the vascular wall. In this study we investigated how adipokines, oleic acid (OA) and the combined treatment regulate human vascular smooth muscle cell (hVSMC) proliferation and migration and the underlying signalling pathways. Adipocyte-conditioned media (CM) generated from human adipocytes induces a prominent proliferation and migration of hVSMC. Autocrine action of adiponectin totally abolishes CM-induced proliferation. Furthermore, OA but not palmitic acid induces proliferation of hVSMC. CM itself does not contain fatty acids, but CM in combination with OA markedly enhances proliferation of hVSMC in a synergistic way. Both the nuclear factor (NF)-κB and the mammalian target of rapamycin (mTOR) pathway were synergistically activated under these conditions and found to be essential for hVSMC proliferation. Expression of iNOS and production of nitric oxide was only enhanced by combined treatment inducing a marked release of VEGF. Combination of OA and VEGF induces an additive increase of hVSMC proliferation. We could show that the combination of CM and OA led to a synergistic proliferation of hVSMC. Expression of iNOS and production of nitric oxide were only enhanced under these conditions and were paralleled by a marked release of VEGF. These results suggest that the combined elevated release of fatty acids and adipokines by adipose tissue in obesity might be critically related to hVSMC dysfunction, vascular inflammation and the development of atherosclerosis.