The TWEAK/Fn14/CD163 axis-implications for metabolic disease.

TWEAK (tumor necrosis factor-like weak inducer of apoptosis) is a member of the TNF superfamily that controls a multitude of cellular events including proliferation, migration, differentiation, apoptosis, angiogenesis, and inflammation. TWEAK control of these events is via an expanding list of intracellular signalling pathways which include NF-κB, ERK/MAPK, Notch, EGFR and AP-1. Two receptors have been identified for TWEAK - Fn14, which targets the membrane bound form of TWEAK, and CD163, which scavenges the soluble form of TWEAK. TWEAK appears to elicit specific events based on the receptor to which it binds, tissue type in which it is expressed, specific extrinsic conditions, and the presence of other cytokines. TWEAK signalling is protective in healthy tissues, but in chronic inflammatory states become detrimental to the tissue. Consistent data show a role for the TWEAK/FN14/CD163 axis in metabolic disease, chronic autoimmune diseases, and acute ischaemic stroke. Low circulating concentrations of soluble TWEAK are predictive of poor cardiovascular outcomes in those with and without diabetes. This review details the current understanding of the TWEAK/Fn14/CD163 axis as one of the chief regulators of immune signalling and its cell-specific role in metabolic disease development and progression.

A20 controls expression of beta-cell regulatory genes and transcription factors.

TNFAIP3 encodes a zinc finger protein called A20, which has potent anti-inflammatory and anti-apoptotic properties. A20 promotes beta-cell survival and protects against islet graft rejection in experimental models. The current study sought to investigate the mechanisms underlying the protective role of A20 in the pancreatic beta-cell. Two islet cell types were used for experiments: the insulin-secreting BRIN-BD11 cell line and human islet cells. A20 was silenced using siRNA against TNFAIP3, and knockdown was confirmed by qPCR and immunostaining of cells. Cell viability, cytotoxicity and apoptosis were assessed using the ApotoxGlo assay. Glucose-stimulated insulin secretion and production of inflammatory cytokines (TNFa, IL1b and IFNg) were measured by ELISA. Expression of beta-cell regulatory genes (Abcc8, Kcnj11, Kcnq1, Gck, Scl2a2) and transcription factors (Hnf1a, Pdx1, Nkx6.1, Ngn3) was determined by qPCR. A20 deficiency increased apoptosis, impaired glucose-induced insulin secretion, and reduced expression of beta-cell regulatory genes and transcription factors. Addition of recombinant A20 normalized gene expression profiles. TNFa, IL1b and IFNg were elevated in A20 deficient cells and found to independently elicit changes in gene expression. Analysis of PCR array data suggests that A20 action in the beta cell is largely, although not exclusively, driven by the P65 subunit of NF-kB. The current report demonstrates a role for A20 in controlling beta-cell integrity and survival, which likely results from the regulation of inflammatory signalling. Of particular note is the impact that A20 deficiency has on the expression of transcription factors regulating the maturation and normal function of beta cells.

A transfer of carbon atoms from fatty acids to sugars and amino acids in yellow lupine (Lupinus luteus L.) seedlings.

The metabolism of 14C-acetate was investigated during the in vitro germination of yellow lupine seeds. Carbon atoms (14C) from the C-2 position of acetate were incorporated mainly into amino acids: aspartate, glutamate, and glutamine and into sugars: glucose, sucrose, and fructose. In contrast to this, 14C from the C-1 position of acetate was released mainly as 14CO2. Incorporation of 1-14C and 2-14C from acetate into amino acids and sugars in seedling axes was more intense when sucrose was added to the medium. However, in cotyledons where lipids are converted to carbohydrates, this process was inhibited by exogenous sucrose. Since acetate is the product of fatty acid beta-oxidation, our results indicate that, at least in lupine, seed storage lipids can be converted not only to sucrose, but mainly to amino acids. Inhibitory effects of sucrose on the incorporation of 14C from acetate into amino acids and sugars in cotyledons of lupine seedlings may be explained as the effect of regulation of the glyoxylate cycle by sugars.

Metabolic and ultrastructural responses of lupine embryo axes to sugar starvation.

Embryo axes isolated from germinating lupine seeds were cultivated in vitro for 24-96 h over media containing either 60 mmol/L sucrose or no sucrose. Ultrastructural studies showed that large vacuoles were accumulating in a central region of primary parenchyma cells in sucrose starved lupine embryo axes, whereas cytoplasm along with organelles were forced to a periphery of the cells. We suggest that the autolysis of cytoplasmic proteins contributes to the accumulation of the vacuoles and this suggestion is consistent with the results of the characterisation of protein content. The level of cytosolic proteins was reduced by 50% and the activity of cytosolic marker enzyme, PEP carboxylase, was reduced by 46% in starved embryos as compared to control. The mitochondria from starved tissues were not degraded. The level of mitochondrial proteins was reduced by only 10% and the activity of mitochondrial NAD-isocitrate dehydrogenase decreased by 8% as a result of starvation. As demonstrated by the results of Percoll density gradient centrifugation, sucrose starvation caused an increase of 49% in many of the higher density mitochondria fractions, whereas many of the lower density mitochondria fractions were decreased by 33%. The samples of mitochondria from starved embryo axes were determined to have higher respiration activity in the presence of glutamate and malate as compared to control samples. EPR-based analyses of free radicals showed the presence of free radicals with a signal at g = 2.0060 in embryo axes. The level of the radical was two times higher in sucrose-starved embryo axes than in control (the level of this radical increased in senescing plant tissues as well). The results of EPR-based quantitation of Mn2+ ions revealed that the level was a few times higher in starved material than in control. Starved embryo axes, however, do possess a number of adaptive mechanisms protecting them from oxidative damage. Densitometric analyses of gels revealed an increase in the activity of SOD in sugar-starved embryos, whereas CAT and POX activities were lower in axes grown without sucrose as compared to control. Superoxide dismutase, catalase and peroxidase zymogram analyses showed that synthesis of new isoforms was not induced by sugar starvation. An accumulation of phytoferritin was found in plastids of sucrose starved embryos. These results are discussed in relation to the metabolic changes observed in senescing plant tissues.