Autophagy at the crossroads of metabolism and cellular defense

The Role and Mechanism of SIRT1 in Resveratrol-regulated Osteoblast Autophagy in Osteoporosis Rats

Osteoporosis is widely regarded as one of the typical aging-related diseases due to the impairment of bone remodeling. The silent information regulator of transcription1 (SIRT1) is a vital regulator of cell survival and life-span. SIRT1 has been shown to be activated by resveratrol treatment, and also has been proved to prevent aging-related diseases such as osteoporosis. However, the role of SIRT1 about autophagy or mitophagy of osteoblasts in resveratrol-regulated osteoporotic rats remains unclear. This study seeks to investigate the role of SIRT1 about autophagy or mitophagy in osteoblasts through PI3K/Akt signaling pathway in resveratrol-regulated osteoporotic rats. The vivo experiment results have revealed that resveratrol treatment significantly improved bone quality and reduced the levels of serum alkaline phosphatase and osteocalcin in osteoporotic rats. Moreover, Western bolt analysis showed that expression of SIRT1, LC3, and Beclin-1 in osteoblasts increased, while p-AKT and p-mTOR were downregulated in osteoporosis rats with high dose resveratrol treatment. On the other hand, resveratrol treatment increased the SIRT1 activity, LC3 and Beclin-1 mRNA expression in the dexamethasone (DEX)-treated osteoblasts. More mitophagosomes were observed in the DEX-treated osteoblasts with resveratrol. Meanwhile, the TOM20, Hsp60, p-Akt and p-mTOR activities were decreased in the DEX-treated osteoblasts with resveratrol. Resveratrol treatment did not change the p-p38 and p-JNK activities in the osteoblasts. These results revealed that resveratrol treatment protected osteoblasts in osteoporosis rats by enhancing mitophagy by mediating SIRT1 and PI3K/AKT/mTOR signaling pathway.

Autophagy as a target for glucocorticoid-induced osteoporosis therapy

Autophagy takes part in regulating the eukaryotic cells function and the progression of numerous diseases, but its clinical utility has not been fully developed yet. Recently, mounting evidences highlight an important correlation between autophagy and bone homeostasis, mediated by osteoclasts, osteocytes, bone marrow mesenchymal stem cells, and osteoblasts, and autophagy plays a vital role in the pathogenesis of glucocorticoid-induced osteoporosis (GIOP). The combinations of autophagy activators/inhibitors with anti-GIOP first-line drugs or some new autophagy-based manipulators, such as regulation of B cell lymphoma 2 family proteins and caspase-dependent clearance of autophagy-related gene proteins, are likely to be the promising approaches for GIOP clinical treatments. In view of the important role of autophagy in the pathogenesis of GIOP, here we review the potential mechanisms about the impacts of autophagy in GIOP and its association with GIOP therapy.

Colonic thioguanine pro-drug: Investigation of microbiome and novel host metabolism

Thiopurines are analogues of endogenous purines. They are pro-drugs which require the purine salvage pathway to convert them to the active drug nucleotides (TGN). These drugs are used to maintain clinical remission in patients with inflammatory bowel diseases. In our recent Gut paper, we showed that thioguanine worked quickly to improve colitis in the absence in the host animal of the key guanine salvage enzyme, hypoxanthine-guanine-phosphoribosyltransferase (HPRT). Current evidence favours the proposition that active drug delivery to the host lacking HPRT requires translocation of TGN-loaded bacteria across the inflamed mucosal barrier, and most likely delivery by phagocytosis. Alternatively, the efficacy of thioguanine in treating colitis could be mediated by modulation of the community of the microbiota in the intestine, or there are novel host pathways for conversion of the thioguanine pro-drug to TGN.

Colonic microbiota can promote rapid local improvement of murine colitis by thioguanine independently of T lymphocytes and host metabolism

OBJECTIVE

Mercaptopurine (MP) and pro-drug azathioprine are 'first-line' oral therapies for maintaining remission in IBD. It is believed that their pharmacodynamic action is due to a slow cumulative decrease in activated lymphocytes homing to inflamed gut. We examined the role of host metabolism, lymphocytes and microbiome for the amelioration of colitis by the related thioguanine (TG).

DESIGN

C57Bl/6 mice with or without specific genes altered to elucidate mechanisms responsible for TG's actions were treated daily with oral or intrarectal TG, MP or water. Disease activity was scored daily. At sacrifice, colonic histology, cytokine message, caecal luminal and mucosal microbiomes were analysed.

RESULTS

Oral and intrarectal TG but not MP rapidly ameliorated spontaneous chronic colitis in Winnie mice (point mutation in Muc2 secretory mucin). TG ameliorated dextran sodium sulfate-induced chronic colitis in wild-type (WT) mice and in mice lacking T and B lymphocytes. Remarkably, colitis improved without immunosuppressive effects in the absence of host hypoxanthine (guanine) phosphoribosyltransferase (Hprt)-mediated conversion of TG to active drug, the thioguanine nucleotides (TGN). Colonic bacteria converted TG and less so MP to TGN, consistent with intestinal bacterial conversion of TG to so reduce inflammation in the mice lacking host Hprt. TG rapidly induced autophagic flux in epithelial, macrophage and WT but not Hprt^-/- fibroblast cell lines and augmented epithelial intracellular bacterial killing.

CONCLUSIONS

Treatment by TG is not necessarily dependent on the adaptive immune system. TG is a more efficacious treatment than MP in Winnie spontaneous colitis. Rapid local bacterial conversion of TG correlated with decreased intestinal inflammation and immune activation.

Macroautophagy is dispensable for growth of KRAS mutant tumors and chloroquine efficacy

Macroautophagy is a key stress-response pathway that can suppress or promote tumorigenesis depending on the cellular context. Notably, Kirsten rat sarcoma (KRAS)-driven tumors have been reported to rely on macroautophagy for growth and survival, suggesting a potential therapeutic approach of using autophagy inhibitors based on genetic stratification. In this study, we evaluated whether KRAS mutation status can predict the efficacy to macroautophagy inhibition. By profiling 47 cell lines with pharmacological and genetic loss-of-function tools, we were unable to confirm that KRAS-driven tumor lines require macroautophagy for growth. Deletion of autophagy-related 7 (ATG7) by genome editing completely blocked macroautophagy in several tumor lines with oncogenic mutations in KRAS but did not inhibit cell proliferation in vitro or tumorigenesis in vivo. Furthermore, ATG7 knockout did not sensitize cells to irradiation or to several anticancer agents tested. Interestingly, ATG7-deficient and -proficient cells were equally sensitive to the antiproliferative effect of chloroquine, a lysosomotropic agent often used as a pharmacological tool to evaluate the response to macroautophagy inhibition. Moreover, both cell types manifested synergistic growth inhibition when treated with chloroquine plus the tyrosine kinase inhibitors erlotinib or sunitinib, suggesting that the antiproliferative effects of chloroquine are independent of its suppressive actions on autophagy.

Recent insights into the role of autophagy in the pathogenesis of rheumatoid arthritis

Autophagy appears to play a dual role in eukaryotic cells. It manifests cytoprotective effects through the regulation of catabolic processes and the clearance of pathogens; however, a correlation between autophagy and the pathogenesis of autoimmune/autoinflammatory conditions has recently been described. Autophagy has emerged as a mediator in the pathogenesis of RA. Autophagy may regulate apoptosis resistance and hyperplasia in synovial fibroblasts, promote osteoclastogenesis and stimulate osteoclast-mediated bone resorption through the delivery of citrullinated peptides to MHC compartments, which results in the activation of the innate and adaptive immune response, thereby resulting in RA. Given the likely importance of autophagy in the pathogenesis of RA, here we reviewed the detailed mechanisms concerning the pathogenicity of autophagy and autophagy proteins in RA.

The Crohn's disease-associated polymorphism in ATG16L1 (rs2241880) reduces SHIP gene expression and activity in human subjects

Crohn's disease (CD) is a polygenic immune-mediated disease characterized by gastrointestinal inflammation. Mice deficient in the hematopoietic-restricted SH2 domain-containing inositolpolyphosphate 5'-phosphatase (SHIP) develop spontaneous CD-like ileal inflammation. Intriguingly, SHIP mRNA is not upregulated in biopsies from patients with ileal CD despite immune cell infiltration, but SHIP's role in human CD remains unknown. We analyzed SHIP mRNA expression and activity in biopsies and peripheral blood mononuclear cells (PBMCs) from control and treatment-naive subjects with ileal CD, and demonstrated that SHIP mRNA and activity were lower in hematopoietic cells in ileal biopsies and PBMCs from subjects with CD. In all tissues from our patient cohort and in PBMCs from a second healthy control cohort, subjects homozygous for the autophagy-related 16-like protein (ATG16L1) CD-associated gene variant (rs2241880), had low SHIP mRNA expression and activity. SHIP protein expression increased during autophagy and SHIP upregulation was dependent on ATG16L1 and/or autophagy, as well as the ATG16L1 CD-associated gene variant. Finally, homozygosity for the ATG16L1 risk variant and low SHIP mRNA expression is inversely related to increased (LPS+ATP)-induced IL-1β production by PBMCs in our cohorts and was regulated by increased transcription of ILIB. These data suggest a novel mechanism by which the ATG16L1 CD-associated gene variant may predispose people to develop intestinal inflammation.

Induction of autophagy counteracts the anticancer effect of cisplatin in human esophageal cancer cells with acquired drug resistance

Cisplatin-based chemotherapy frequently resulted in acquired resistance. The underpinning mechanism of such resistance remains obscure especially in relation to autophagic response. This study thus investigated the role of autophagy in the anticancer activity of cisplatin in human esophageal cancer cells with acquired cisplatin resistance. In response to cisplatin treatment, EC109 cells exhibited substantial apoptosis and senescence whereas cisplatin-resistant EC109/CDDP cells exhibited resistance. In this respect, cisplatin increased ERK phosphorylation whose inhibition by MEK inhibitor significantly attenuated the cytotoxic and cytostatic effect of cisplatin. Notably, cisplatin preferentially induces autophagy in EC109/CDDP cells but not in EC109 cells. Moreover, the induction of autophagy was accompanied by the suppression of mTORC1 activity. Abolition of autophagy by pharmacological inhibitors or knockdown of ATG5/7 re-sensitized EC109/CDDP cells. Co-administration of an autophagy inhibitor chloroquine and cisplatin significantly suppressed tumor growth whereas cisplatin monotherapy failed to elicit anticancer activity in nude mice xenografted with EC109/CDDP cells. To conclude, our data implicate autophagic response as a key mechanism of acquired resistance to cisplatin, suggesting that autophagy is a novel target to improve therapy efficiency of cisplatin toward human esophageal cancers with acquired resistance.

Selective VPS34 inhibitor blocks autophagy and uncovers a role for NCOA4 in ferritin degradation and iron homeostasis in vivo

Cells rely on autophagy to clear misfolded proteins and damaged organelles to maintain cellular homeostasis. In this study we use the new autophagy inhibitor PIK-III to screen for autophagy substrates. PIK-III is a selective inhibitor of VPS34 that binds a unique hydrophobic pocket not present in related kinases such as PI(3)Kα. PIK-III acutely inhibits autophagy and de novo lipidation of LC3, and leads to the stabilization of autophagy substrates. By performing ubiquitin-affinity proteomics on PIK-III-treated cells we identified substrates including NCOA4, which accumulates in ATG7-deficient cells and co-localizes with autolysosomes. NCOA4 directly binds ferritin heavy chain-1 (FTH1) to target the iron-binding ferritin complex with a relative molecular mass of 450,000 to autolysosomes following starvation or iron depletion. Interestingly, Ncoa4(-/-) mice exhibit a profound accumulation of iron in splenic macrophages, which are critical for the reutilization of iron from engulfed red blood cells. Taken together, the results of this study provide a new mechanism for selective autophagy of ferritin and reveal a previously unappreciated role for autophagy and NCOA4 in the control of iron homeostasis in vivo.