Adipsin inhibits Irak2 mitochondrial translocation and improves fatty acid β-oxidation to alleviate diabetic cardiomyopathy

BACKGROUND

Diabetic cardiomyopathy (DCM) causes the myocardium to rely on fatty acid β-oxidation for energy. The accumulation of intracellular lipids and fatty acids in the myocardium usually results in lipotoxicity, which impairs myocardial function. Adipsin may play an important protective role in the pathogenesis of DCM. The aim of this study is to investigate the regulatory effect of Adipsin on DCM lipotoxicity and its molecular mechanism.

METHODS

A high-fat diet (HFD)-induced type 2 diabetes mellitus model was constructed in mice with adipose tissue-specific overexpression of Adipsin (Adipsin-Tg). Liquid chromatography-tandem mass spectrometry (LC-MS/MS), glutathione-S-transferase (GST) pull-down technique, Co-immunoprecipitation (Co-IP) and immunofluorescence colocalization analyses were used to investigate the molecules which can directly interact with Adipsin. The immunocolloidal gold method was also used to detect the interaction between Adipsin and its downstream modulator.

RESULTS

The expression of Adipsin was significantly downregulated in the HFD-induced DCM model (P < 0.05). Adipose tissue-specific overexpression of Adipsin significantly improved cardiac function and alleviated cardiac remodeling in DCM (P < 0.05). Adipsin overexpression also alleviated mitochondrial oxidative phosphorylation function in diabetic stress (P < 0.05). LC-MS/MS analysis, GST pull-down technique and Co-IP studies revealed that interleukin-1 receptor-associated kinase-like 2 (Irak2) was a downstream regulator of Adipsin. Immunofluorescence analysis also revealed that Adipsin was co-localized with Irak2 in cardiomyocytes. Immunocolloidal gold electron microscopy and Western blotting analysis indicated that Adipsin inhibited the mitochondrial translocation of Irak2 in DCM, thus dampening the interaction between Irak2 and prohibitin (Phb)-optic atrophy protein 1 (Opa1) on mitochondria and improving the structural integrity and function of mitochondria (P < 0.05). Interestingly, in the presence of Irak2 knockdown, Adipsin overexpression did not further alleviate myocardial mitochondrial destruction and cardiac dysfunction, suggesting a downstream role of Irak2 in Adipsin-induced responses (P < 0.05). Consistent with these findings, overexpression of Adipsin after Irak2 knockdown did not further reduce the accumulation of lipids and their metabolites in the cardiac myocardium, nor did it enhance the oxidation capacity of cardiomyocytes expose to palmitate (PA) (P < 0.05). These results indicated that Irak2 may be a downstream regulator of Adipsin.

CONCLUSIONS

Adipsin improves fatty acid β-oxidation and alleviates mitochondrial injury in DCM. The mechanism is related to Irak2 interaction and inhibition of Irak2 mitochondrial translocation.

STAT3 inhibition induced temozolomide-resistant glioblastoma apoptosis via triggering mitochondrial STAT3 translocation and respiratory chain dysfunction

Recent evidence has demonstrated that the signal transducer and activator of transcription 3 (STAT3) gene are abnormally active in glioblastoma multiforme (GBM), and this change is crucial for the tumor survival and chemotherapy-resistant. Certain preclinical pharmacology studies have focused on STAT3 phosphorylation and homodimerization, and have developed a class of salicylic acid-based inhibitors, which blocks the nuclear translocation-dependent canonical STAT3 signaling. In the present study, we demonstrated that the salicylic acid-based compound SH-4-54 was quite toxic to temozolomide (TMZ)-resistant GBM cells and could trigger apoptosis in these cells via enhancing mitochondrial translocation-dependent non-canonical STAT3 pathway. We demonstrated that incubation of TMZ-resistant GBM cells with SH-4-54 led to mitochondrial STAT3 (mitoSTAT3) activation and respiratory dysfunction reflected by disrupted (or suppressed) activities of oxidative phosphorylation complexes and oxygen consumption rate. Mechanistically, we proved that SH-4-54 could increase mitoSTAT3 transmembrane import via GRIM-19 and reinforce the association between mitoSTAT3 and mitochondrial transcription factor A (TFAM), indicating that SH-4-54 could facilitate the binding of mitoSTAT3 to mitochondria DNA (mtDNA) and negatively regulate mitochondrial-encoded genes, thus leading to the abnormal oxidation respiratory. Lastly, using GRIM-19 knockout cell line and subcutaneous xenotransplanted tumor model, we elaborately showed the enrichment of SH-4-54 in mitochondria by LC-MS/MS analysis. In conclusion, our data demonstrate thatthe salicylic acid-based compound SH-4-54 is quite effective in killing TMZ-resistant GBM cells and this cytotoxicity is attributed to mitoSTAT3 activation.

Enhanced Phosphorylation of Bax and Its Translocation into Mitochondria in the Brains of Individuals Affiliated with Alzheimer's Disease

Background:

Despite increased neuronal death, senile plaques, and neurofibrillary tangles observed in patients suffering from Alzheimer’s disease (AD), the detailed mechanism of cell death in AD is still poorly understood.

Method:

We hypothesized that p38 kinase activates and then phosphorylates Bax, leading to its translocation to mitochondria in AD brains compared to controls. The aim of this study was to investigate the role of p38 kinase in phosphorylation and sub-cellular localization of pro-apoptotic Bax in the frontal cortex of the brains from AD and control subjects. Increased oxidative stress in AD individuals compared to control was evaluated by measuring the levels of carbonylated proteins and oxidized peroxiredoxin, an antioxidant enzyme. The relative amounts of p38 kinase and phospho-Bax in mitochondria in AD brains and controls were determined by immunoblot analysis using the respective antibody against each protein following immunoprecipitation.

Results:

Our results showed that the levels of oxidized peroxiredoxin-SO₃ and carbonylated proteins are significantly elevated in AD brains compared to controls, demonstrating the increased oxidative stress.

Conclusion:

The amount of phospho-p38 kinase is increased in AD brains and the activated p38 kinase appears to phosphorylate Thr residue(s) of Bax, which leads to its mitochondrial translocation, contributing to apoptosis and ultimately, neurodegeneration.

Differential protein acetylation assists import of excess SOD2 into mitochondria and mediates SOD2 aggregation associated with cardiac hypertrophy in the murine SOD2-tg heart

SOD2 is the primary antioxidant enzyme neutralizing ^•O₂^- in mitochondria. Cardiac-specific SOD2 overexpression (SOD2-tg) induces supernormal function and cardiac hypertrophy in the mouse heart. However, the reductive stress imposed by SOD2 overexpression results in protein aggregation of SOD2 pentamers and differential hyperacetylation of SOD2 in the mitochondria and cytosol. Here, we studied SOD2 acetylation in SOD2-tg and wild-type mouse hearts. LC-MS/MS analysis indicated the presence of four acetylated lysines in matrix SOD2 and nine acetylated lysines in cytosolic SOD2 from the SOD2-tg heart. However, only one specific acetylated lysine residue was detected in the mitochondria of the wild-type heart, which was consistent with Sirt3 downregulation in the SOD2-tg heart. LC-MS/MS further detected hyperacetylated SOD2 with a signaling peptide in the mitochondrial inner membrane and matrix of the SOD2-tg heart, indicating partial arrest of the SOD2 precursor in the membrane during translocation into the mitochondria. Upregulation of HSP 70 and cytosolic HSP 60 enabled the translocation of excess SOD2 into mitochondria. In vitro acetylation of matrix SOD2 with Ac₂O deaggregated pentameric SOD2, restored the profile of cytosolic SOD2 hyperacetylation, and decreased matrix SOD2 activity. As revealed by 3D structure, acetylation of K89, K134, and K154 of cytosolic SOD2 induces unfolding of the tertiary structure and breaking of the salt bridges that are important for the quaternary structure, suggesting that hyperacetylation and HSP 70 upregulation maintain the unfolded status of SOD2 in the cytosol and mediate the import of SOD2 across the membrane. Downregulation of Sirt3, HSP 60, and presequence protease in the mitochondria of the SOD2-tg heart promoted protein misfolding that led to pentameric aggregation.

HPOB, an HDAC6 inhibitor, attenuates corticosterone-induced injury in rat adrenal pheochromocytoma PC12 cells by inhibiting mitochondrial GR translocation and the intrinsic apoptosis pathway

High levels of glucocorticoids (GCs) have been reported to damage normal hippocampal neurons, and such damage has been positively correlated with major depression (MD) and chronic stress. Our previous study showed that HDAC6 might be a potential target to regulate GC-induced glucocorticoid receptor (GR) translocation to the mitochondria and subsequent apoptosis. In the present study, we investigated the effect of HPOB, a selective HDAC6 inhibitor, on corticosterone (Cort)-induced apoptosis and explored the possible mechanism of action of HPOB in rat adrenal pheochromocytoma (PC12) cells, which possesses typical neuron features and expresses high levels of glucocorticoid receptors. We demonstrated that pre-treatment with HPOB remarkably reduced Cort-induced cytotoxicity and confirmed the anti-apoptotic effect of HPOB via the caspase-3 activity assay and H33342/PI and TUNEL double staining. Mechanistically, we demonstrated that HPOB reversed the Cort-induced elevation of GR levels in the mitochondria and blocked concomitant mitochondrial dysfunction and the intrinsic apoptosis pathway. Furthermore, HPOB was shown to attenuate expression of the multi-chaperone machinery (Hsp90-Hop-Hsp70) and cooperate with mitochondrial translocase of the outer/inner membrane (TOM/TIM) complex recruitment by triggering hyperacetylation of Hsps through HDAC6 inhibition. Considering all of these findings, the neuroprotective effect of HPOB demonstrated the crucial role of HDAC6 inhibition in reducing Cort-induced apoptosis in PC12 cells. The data further suggested that the anti-apoptotic activity of HDAC6 inhibition against the mitochondria-mediated impairment pathway might be mechanistically linked to the hyperacetylation of Hsps and consequent suppression of GR translocation to the mitochondria.