What can naked mole-rats teach us about ameliorating hypoxia-related human diseases?

Ameliorating the deleterious impact of systemic or tissue-level hypoxia or ischemia is key to preventing or treating many human diseases and pathologies. Usefully, environmental hypoxia is also a common challenge in many natural habitats; animals that are native to such hypoxic niches often exhibit strategies that enable them to thrive with limited O2 availability. Studying how such species have evolved to tolerate systemic hypoxia offers a promising avenue of discovery for novel strategies to mitigate the deleterious effects of hypoxia in human diseases and pathologies. Of particular interest are naked mole-rats, which are among the most hypoxia-tolerant mammals. Naked mole-rats that tolerate severe hypoxia in a laboratory setting are also protected against clinically relevant mimics of heart attack and stroke. The mechanisms that support this tolerance are currently being elucidated but results to date suggest that metabolic rate suppression, reprogramming of metabolic pathways, and mechanisms that defend against deleterious perturbations of cellular signaling pathways all provide layers of protection. Herein, we synthesize and discuss what is known regarding adaptations to hypoxia in the naked mole-rat cardiopulmonary system and brain, as these systems comprise both the primary means of delivering O2 to tissues and the most hypoxia-sensitive organs in mammals.

Hypoxia-induced downregulation of RNA m6A protein machinery in the naked mole-rat heart.

Naked mole-rats, Heterocephalus glaber, are champion hypoxia-tolerant rodents that live under low oxygen conditions in their subterranean burrows. Detrimental effects of low oxygen can be mitigated through metabolic rate depression (MRD), metabolic reorganization, and global downregulation of nonessential cellular processes. Recent research has progressively implicated epigenetic modifications - rapid, reversible changes to gene expression that do not alter the DNA sequence itself - as major players in implementing and maintaining MRD. N6-adenosine (m6A) methylation is the most prevalent mammalian RNA modification and is responsible for pre-mRNA processing and mRNA export from the nucleus. Hence, m6A -mediated conformational changes alter the cellular fate of transcripts. The present study investigated the role of m6A RNA methylation responses to 24 h of hypoxia exposure in H. glaber cardiac tissue. Total protein levels of m6A writers/readers/erasers, m6A demethylase activity, and total m6A quantification were measured under normoxic vs. hypoxic conditions in H. glaber heart. While there was no change in either demethylase activity or total m6A content, many proteins of the m6A pathway were downregulated during hypoxia. Overall, m6A may not be a signature hypoxia-responsive characteristic in H. glaber heart, but downregulation of the protein machinery involved in m6A cycling points to an alternate biological involvement. Further research will explore other forms of RNA modifications and other epigenetic mechanisms to determine the controls on hypoxia endurance in this subterranean mammal.

Hypoxia impairs blood glucose homeostasis in naked mole-rat adult subordinates but not queens.

Naked mole-rats (NMRs) are among the most hypoxia-tolerant mammals and metabolize only carbohydrates in hypoxia. Glucose is the primary building block of dietary carbohydrates, but how blood glucose is regulated during hypoxia has not been explored in NMRs. We hypothesized that NMRs mobilize glucose stores to support anaerobic energy metabolism in hypoxia. To test this, we treated newborn, juvenile and adult (subordinate and queen) NMRs in normoxia (21% O2) or hypoxia (7, 5 or 3% O2), while measuring metabolic rate, body temperature and blood [glucose]. We also challenged animals with glucose, insulin or insulin-like growth factor-1 (IGF-1) injections and measured the rate of glucose clearance in normoxia and hypoxia. We found that: (1) blood [glucose] increases in moderate hypoxia in queens and pups, but only in severe hypoxia in adult subordinates and juveniles; (2) glucose tolerance is similar between developmental stages in normoxia, but glucose clearance times are 2- to 3-fold longer in juveniles and subordinates than in queens or pups in hypoxia; and (3) reoxygenation accelerates glucose clearance in hypoxic subordinate adults. Mechanistically, (4) insulin and IGF-1 reduce blood [glucose] in subordinates in both normoxia but only IGF-1 impacts blood [glucose] in hypoxic queens. Our results indicate that insulin signaling is impaired by hypoxia in NMRs, but that queens utilize IGF-1 to overcome this limitation and effectively regulate blood glucose in hypoxia. This suggests that sexual maturation impacts blood glucose handling in hypoxic NMR queens, which may allow queens to spend longer periods of time in hypoxic nest chambers.

Electrospun poly-l-lactic acid/polyvinyl alcohol nanofibers improved insulin-producing cell differentiation potential of human adipose-derived mesenchymal stem cells.

Combination of adipose-derived mesenchymal stem cells (ADSCs) and synthetic materials in terms of pancreatic tissue engineering can be considered as a treatment of diabetes. This study aimed to evaluate the differentiation of human ADSCs to pancreatic cells on poly-l-lactic acid/polyvinyl alcohol (PLLA/PVA) nanofibers as a three-dimensional (3D) scaffold. Mesenchymal stem cells (MSCs) were characterized for mesenchymal surface markers by flow cytometry. Then ADSCs were seeded on 3D scaffolds and treated with pancreatic differentiation medium. Immunostaining assay showed that ADSCs were very efficiently differentiated into a relatively homogeneous population of insulin-producing cells. Moreover, real-time RT-PCR results revealed that pancreas-specific markers were highly expressed in 3D scaffolds compared with their expression in tissue culture plates and this difference in expression level was significant. In addition, insulin and C-peptide secreted in response to varying concentrations of glucose in the 3D scaffold group was significantly higher than that in 2D culture. The results of the present study confirmed that PLLA/PVA scaffold seeded with ADSCs could be a suitable option in pancreatic tissue engineering.

The anticancer effects of pharmacological inhibition of autophagy in acute erythroid leukemia cells.

Although recent studies have reported different aspects of autophagy, from pro-survival to pro-death roles of this process in malignant cells, the underlying mechanisms by which autophagy inhibitors contribute toward the induction of programmed cell death in cancerous cells are still unclear. In the present study, we have attempted to explore some of the molecular features of pharmacological inhibition of autophagy in TF-1 cells (an acute erythroid leukemia model). Our findings indicated that ara-C induces autophagy (with alteration of LC3B, p62, and Beclin-1) in the cells; however, targeting autophagy by 3-methyladenine and chloroquine significantly increased caspase-dependent apoptosis and the sub-G1 compartment in ara-C-treated cells. Moreover, cell cycle analysis showed that 3-MA, as an early-stage autophagy inhibitor, could elevate the cell population in the G0/G1 cell cycle phase, which was associated with upregulation of p21 and p27 expressions. Interestingly, autophagy inhibition was also accompanied by downregulation of c-Myc gene and protein expression levels and upregulated levels of Bax and Bak gene expressions. In addition, following inhibition of autophagy, the levels of tumor-suppressive miRNA (i.e. miR-204) increased, whereas the values of oncogenic miRNAs (including miR-21, miR-221, miR-30a, and miR-17) decreased. Overall, our experiments indicate that autophagy inhibitors (especially chloroquine) seem to be promising agents for combination therapy in acute erythroid leukemia.