MitoNEET prevents iron overload-induced insulin resistance in H9c2 cells through regulation of mitochondrial iron.

Iron overload (IO) induces insulin resistance in H9c2 cardiomyoblast cells. Here, we used H9c2 cells overexpressing MitoNEET to examine the potential for protection against iron accumulation in the mitochondria and subsequent insulin resistance. In control H9c2 cells, IO was observed to increase mitochondrial iron content, reactive oxygen species (ROS) production, mitochondrial fission, and reduced insulin-stimulated Akt and ERK1/2 phosphorylation. IO did not significantly affect mitophagy, or mitochondrial content, however, an increase in peroxisome-proliferator-activated receptor gamma coactivator 1 alpha (PGC1α) protein expression, a key regulator of mitochondrial biogenesis, was observed. MitoNEET overexpression was able to attenuate the effects of IO on mitochondrial iron content, reactive oxygen species, mitochondrial fission, and insulin signaling. MitoNEET overexpression also upregulated levels of PGC1α protein. The mitochondria-targeted antioxidant, Skq1, prevented IO-induced ROS production and insulin resistance in control cells, indicating mitochondrial ROS plays a causal role in the onset of insulin resistance. The selective mitochondrial fission inhibitor, Mdivi-1, prevented IO-induced mitochondrial fission, however, it did not alleviate IO-induced insulin resistance. Collectively, IO causes insulin resistance in H9c2 cardiomyoblasts and this can be averted by reduction of mitochondrial iron accumulation and ROS production by overexpression of the MitoNEET protein.

Iron overload reduces adiponectin receptor expression via a ROS/FOXO1-dependent mechanism leading to adiponectin resistance in skeletal muscle cells.

Iron overload (IO) is a common yet underappreciated finding in metabolic syndrome (MetS) patients. With the prevalence of MetS continuing to rise, it is imperative to further elucidate cellular mechanisms leading to metabolic dysfunction. Adiponectin has many beneficial effects and is a therapeutic target for the treatment of MetS and cardiovascular diseases. IO positively correlates with reduced circulating adiponectin levels yet the impact of IO on adiponectin action is unknown. Here, we established a model of IO in L6 skeletal muscle cells and found that IO-induced adiponectin resistance. This was shown via reduced p38 mitogen-activated protein kinase phosphorylation in response to the small molecule adiponectin receptor (AdipoR) agonist, AdipoRon, in presence of IO. This correlated with reduced messenger RNA and protein levels of AdipoR1 and its facilitative signaling binding partner, APPL1. IO caused phosphorylation, nuclear extrusion, and thus inhibition of FOXO1, a known transcription factor regulating AdipoR1 expression. The antioxidant N-acetyl cystine attenuated the production of reactive oxygen species (ROS) by IO, and blunted its effect on FOXO1 phosphorylation and removal from the nucleus, as well as subsequent adiponectin resistance. In conclusion, our study identifies a ROS/FOXO1/AdipoR1 axis as a cause of skeletal muscle adiponectin resistance in response to IO. This new knowledge provides insight into a cellular mechanism with potential relevance to disease pathophysiology in MetS patients with IO.

Lipocalin-2 induces NLRP3 inflammasome activation via HMGB1 induced TLR4 signaling in heart tissue of mice under pressure overload challenge.

Lipocalin-2 (also known as NGAL) levels are elevated in obesity and diabetes yet relatively little is known regarding effects on the heart. We induced pressure overload (PO) in mice and found that lipocalin-2 knockout (LKO) mice exhibited less PO-induced autophagy and NLRP3 inflammasome activation than Wt. PO-induced mitochondrial damage was reduced and autophagic flux greater in LKO mice, which correlated with less cardiac dysfunction. All of these observations were negated upon adenoviral-mediated restoration of normal lipocalin-2 levels in LKO. Studies in primary cardiac fibroblasts indicated that lipocalin-2 enhanced priming and activation of NLRP3-inflammasome, detected by increased IL-1ß, IL-18 and Caspase-1 activation. This was attenuated in cells isolated from NLRP3-deficient mice or upon pharmacological inhibition of NLRP3. Furthermore, lipocalin-2 induced release of HMGB1 from cells and NLRP3-inflammasome activation was attenuated by TLR4 inhibition. We also found evidence of increased inflammasome activation and reduced autophagy in cardiac biopsy samples from heart failure patients. Overall, this study provides new mechanistic insight on the detrimental role of lipocalin-2 in the development of cardiac dysfunction.