Depletion of Sirt3 leads to the impairment of adipogenic differentiation and insulin resistance via interfering mitochondrial function of adipose-derived human mesenchymal stem cells.

Upregulation of mitochondrial function and oxidative metabolism is a hallmark in the differentiation of stem cells. However, the mechanism underlying the metabolic reprogramming and upregulation of mitochondrial function during the differentiation of human mesenchymal stem cells (hMSCs) is largely unclear. Sirt3 has emerged as a sensor in regulating mitochondrial function and antioxidant defence system in cellular response to energy demand or environmental stimuli, but its roles in stem cell differentiation have not been fully understood. In this study, we used adipose-derived hMSCs (ad-hMSCs) to investigate the role of Sirt3 in adipogenic differentiation and in the function of mature adipocytes. We showed that at the early stage of adipogenic differentiation, Sirt3 upregulation is essential for the activation of biogenesis and bioenergetic function of mitochondria. In addition, we found that induction of Forkhead Box O 3a (FoxO3a), an upstream factor that regulates MnSOD gene transcription, is involved in the upregulation of antioxidant enzymes at the early stage of adipogenic differentiation. Silencing of Sirt3 by shRNA decreased the protein level of FoxO3a and subsequently downregulated a number of FoxO3a-mediated antioxidant enzymes and increased oxidative stress in ad-hMSCs after adipogenic induction. Importantly, depletion of Sirt3 compromised the ability of ad-hMSCs to undergo adipogenic differentiation and led to adipocyte dysfunction and insulin resistance. These findings suggest that Sirt3-mediated protein deacetylation plays an important role in regulating oxidative metabolism and antioxidant defence in stem cell differentiation, and that Sirt3 deficiency may be related to insulin resistance.

A novel role of ER stress signal transducer ATF6 in regulating enterovirus A71 viral protein stability.

BACKGROUND: Due to limited coding capacity of viral genome, enterovirus A71 (EV-A71) co-opts host nuclear proteins for its replication. Upon ER stress, the ER-localized 90 kDa activating transcription factor 6 (p90ATF6) is proteolytically cleaved to produce the transcriptionally active amino-terminal 50 kDa (p50ATF6) product where it enters the nucleus to activate a subset of unfolded protein response and ER-associated degradation (also known as ERAD) genes. During EV-A71 infection, however, this p50ATF6 product was not detected in the nucleus, and its downstream target genes were not activated. METHODS: We examined the role of ATF6 during EV-A71 infection, including its cleavage process and its role in viral life cycle by silencing or overexpressing ATF6. RESULTS: We showed that a potential cleavage in the middle of p90ATF6 produced an amino-terminal ~ 45 kDa fragment in a viral protease-independent but EV-A71-dependent manner. The disappearance of ATF6 was not restricted to a specific strain of EV-A71 or cell type, and was not simply caused by picornavirus-mediated global translational shutoff. This cleavage of ATF6, which was most likely mediated by the host response, was nevertheless independent of both cellular caspases and XBP1-associated proteasomes. The silencing of ATF6 expression by small interfering RNA suppressed viral titers due to reduced viral protein stability. This effect was markedly restored by the ectopic expression of p90ATF6. CONCLUSION: Our findings indicate that ATF6 plays a distinct role in viral protein stability and that the host uses different cleavage strategies, rather than conventional cleavage by generating p50ATF6, to combat viral infection.

Prognostic implications of miR-146b expression and its functional role in papillary thyroid carcinoma.

CONTEXT: Recent studies suggest that miR-146b deregulation in papillary thyroid carcinoma (PTC) was associated with advanced tumor characteristics. However, the influence of miR-146b expression on the prognosis of PTC remains unknown. We sought to correlate tumor expression levels of miR-146b with the prognosis of a previously reported PTC cohort and reveal the underlying mechanisms via a PTC cell line model. METHODOLOGY: Expression levels of miR-146b were assessed via quantitative real-time PCR in 71 cases of PTC with distinct clinico-pathogenetic characteristics. All patients were classified into the disease-free or active disease group, based on their medical records at the end of the follow-up period. In vitro gain-of-function experiments were performed in a BCPAP human papillary thyroid cancer cell line model, which harbored the homozygous mutation of BRAF. BCPAP cells were transfected with a mimic-miR-146b and nonspecific microRNA (miRNA) control to determine whether miR-146b overexpression promotes cell migration and invasion. Proliferation assay, colony formation assay, and chemotherapy-induced apoptosis were also determined. RESULTS: Multivariate logistic regression analysis demonstrated advanced tumor stage, presence of cervical lymph node metastasis, and miR-146b expression were independent risk factors for poor prognosis in PTC. Patients with higher miR-146b expression levels had significantly poorer overall survival compared with those with lower miR-146b levels. The associated hazard ratio was 3.92 (95% confidence interval, 1.73-8.86, log-rank P < .05). Overexpression of miR-146b significantly increased cell migration and invasiveness. Furthermore, miR-146b also increased resistance to chemotherapy-induced apoptosis. CONCLUSIONS: Our results suggest that miR-146b is a novel prognostic factor of PTC. Furthermore, in vitro functional studies provided the mechanistic explanation for miR-146b in tumor aggressiveness. These results enhance understanding of the molecular mechanisms involved in tumor aggressiveness in PTC, provide new prognostic biomarkers, and ultimately offer new leads for developing therapies for PTC.