Humanin: A Novel Central Regulator of Peripheral Insulin Action

Decline in insulin action is a metabolic feature of aging and is involved in the development of age-related diseases including Type 2 Diabetes Mellitus (T2DM) and Alzheimer's disease (AD). A novel mitochondria-associated peptide, Humanin (HN), has a neuroprotective role against AD-related neurotoxicity. Considering the association between insulin resistance and AD, we investigated if HN influences insulin sensitivity.Using state of the art clamp technology, we examined the role of central and peripheral HN on insulin action. Continuous infusion of HN intra-cerebro-ventricularly significantly improved overall insulin sensitivity. The central effects of HN on insulin action were associated with activation of hypothalamic STAT-3 signaling; effects that were negated by co-inhibition of hypothalamic STAT-3. Peripheral intravenous infusions of novel and potent HN derivatives reproduced the insulin-sensitizing effects of central HN. Inhibition of hypothalamic STAT-3 completely negated the effects of IV HN analog on liver, suggesting that the hepatic actions of HN are centrally mediated. This is consistent with the lack of a direct effect of HN on primary hepatocytes. Furthermore, single treatment with a highly-potent HN analog significantly lowered blood glucose in Zucker diabetic fatty rats. Based upon the link of HN with two age-related diseases, we examined if there were age associated changes in HN levels. Indeed, the amount of detectable HN in hypothalamus, skeletal muscle, and cortex was decreased with age in rodents, and circulating levels of HN were decreased with age in humans and mice.We conclude that the decline in HN with age could play a role in the pathogenesis of age-related diseases including AD and T2DM. HN represents a novel link between T2DM and neurodegeneration and along with its analogues offers a potential therapeutic tool to improve insulin action and treat T2DM

The meta-epigenomic structure of purified human stem cell populations is defined at cis-regulatory sequences

The mechanism and significance of epigenetic variability in the same cell type between healthy individuals are not clear. Here we purify human CD34+ haematopoietic stem and progenitor cells (HSPCs) from different individuals and find that there is increased variability of DNA methylation at loci with properties of promoters and enhancers. The variability is especially enriched at candidate enhancers near genes transitioning between silent and expressed states, and encoding proteins with leukocyte differentiation properties. Our findings of increased variability at loci with intermediate DNA methylation values, at candidate poised enhancers and at genes involved in HSPC lineage commitment suggest that CD34+ cell subtype heterogeneity between individuals is a major mechanism for the variability observed. Epigenomic studies performed on cell populations, even when purified, are testing collections of epigenomes, or meta-epigenomes. Our findings show that meta-epigenomic approaches to data analysis can provide insights into cell subpopulation structure

Enhanced activation of a “nutrient-sensing” pathway with age contributes to insulin resistance

Calorie restriction improves life span whereas nutrient excess leads to obesity and unfavorable metabolic consequences, supporting the role for a cellular “nutrient sensor” in aging. Hexosamine biosynthetic pathway (HBP) is a candidate nutrient-sensing pathway. We hypothesized that altered nutrient sensing (by HBP) with age may provide a link among aging, nutrient flux, and insulin resistance. Using a hyperinsulinemic clamp in young rats, we show that experimental activation of HBP, through the systemic infusion of glucosamine, induced severe insulin resistance (36% decline in peripheral insulin action; P<0.05), increased adipose tissue gene expression of fat-derived peptides (PAI-1 by 4-fold, angiotensinogen 3-fold, leptin 2-fold, resistin 4-fold, and adiponectin 4-fold; P<0.01 compared with young saline-infused), and enhanced glycosylation of transcription factors, thus mimicking a physiological and biological phenotype of aging. We further demonstrate a greater activation of nutrient-sensing HBP with age in both old ad libitum-fed and calorie-restricted rats. Interestingly, old calorie-restricted animals rapidly develop insulin resistance when exposed to glucosamine, despite their “young” phenotype. These results suggest that altered nutrient sensing by HBP with age may be the link among nutrients, insulin resistance, and age-related diabetes.—Einstein, F. H., Fishman, S., Bauman, J., Thompson, R. F., Huffman, D. M., Atzmon, G., Barzilai, N., Muzumdar, R. H. Enhanced activation of a “nutrient-sensing” pathway with age contributes to insulin resistance

Comparison between surrogate indexes of insulin sensitivity/resistance and hyperinsulinemic euglycemic clamp estimates in rats

Assessing insulin resistance in rodent models gives insight into mechanisms that cause type 2 diabetes and the metabolic syndrome. The hyperinsulinemic euglycemic glucose clamp, the reference standard for measuring insulin sensitivity in humans and animals, is labor intensive and technically demanding. A number of simple surrogate indexes of insulin sensitivity/resistance have been developed and validated primarily for use in large human studies. These same surrogates are also frequently used in rodent studies. However, in general, these indexes have not been rigorously evaluated in animals. In a recent validation study in mice, we demonstrated that surrogates have a weaker correlation with glucose clamp estimates of insulin sensitivity/resistance than in humans. This may be due to increased technical difficulties in mice and/or intrinsic differences between human and rodent physiology. To help distinguish among these possibilities, in the present study, using data from rats substantially larger than mice, we compared the clamp glucose infusion rate (GIR) with surrogate indexes, including QUICKI, HOMA, 1/HOMA, log (HOMA), and 1/fasting insulin. All surrogates were modestly correlated with GIR (r = 0.34–0.40). Calibration analyses of surrogates adjusted for body weight demonstrated similar predictive accuracy for GIR among all surrogates. We conclude that linear correlations of surrogate indexes with clamp estimates and predictive accuracy of surrogate indexes in rats are similar to those in mice (but not as substantial as in humans). This additional rat study (taken with the previous mouse study) suggests that application of surrogate insulin sensitivity indexes developed for humans may not be appropriate for determining primary outcomes in rodent studies due to intrinsic differences in metabolic physiology. However, use of surrogates may be appropriate in rodents, where feasibility of clamps is an obstacle and measurement of insulin sensitivity is a secondary outcome

IGFBP-3 modulates the effects of HN on insulin sensitivity.

<p>A) IGFBP-3 binding of HN and HN analogues measured by densitometry of dot blots probed with radiolabeled IGFBP-3 (Ins = Insulin). B) Mouse embryonic fibroblasts generated from wild-type or IGFBP-3 knockout mice were incubated in serum-free media for 24 h followed by incubation with 100 nM HNG or HNGF6A for 24 h. Apoptosis was assessed by ELISA for fragmentation of histone-associated DNA. (<i>n</i> = 4). Effects of ICV IGFBP-3, HN or F6AHN during a hyperinsulinemic clamp (<i>n</i> = 6 each)on C) GIR, D) HGP and E) degree of suppression of HGP. *Significantly different from other experimental groups, <i>P</i><0.05.</p

Effects of peripheral administration of the potent HN analog, HNGF6A, on glucose metabolism.

<p>Effect of IV saline, F6AHN and HNGF6A (<i>n</i> = 6 each) on A) GIR, B) HGP, and C) glucose uptake. Values are means±SE. *Significantly different from other experimental groups, <i>P</i><0.05.</p

A decline in both plasma and tissue HN levels with aging is observed in rodents and humans.

<p>A) HN levels as assessed by ELISA, in young and old mice and B) across age in humans. C) Expression of RN in the hypothalamus and skeletal muscle of young and old rats. *Significantly different from other experimental groups, <i>P</i><0.05.</p

Effect of a single IV dose of HNGF6A on blood glucose levels in chronically catheterized, unstressed Zucker diabetic fatty (ZDF) rats over 4 hours.

<p>*Significantly different than saline controls, <i>P</i><0.05.</p