Bone marrow adipocytes resist lipolysis and remodeling in response to ß-adrenergic stimulation.

Bone marrow adipose tissue (BMAT) is preserved or increased in states of caloric restriction. Similarly, we found that BMAT in the tail vertebrae, but not the red marrow in the tibia, resists loss of neutral lipid with acute, 48-hour fasting in rats. The mechanisms underlying this phenomenon and its seemingly distinct regulation from peripheral white adipose tissue (WAT) remain unknown. To test the role of ß-adrenergic stimulation, a major regulator of adipose tissue lipolysis, we examined the responses of BMAT to ß-adrenergic agonists. Relative to inguinal WAT, BMAT had reduced phosphorylation of hormone sensitive lipase (HSL) after treatment with pan-ß-adrenergic agonist isoproterenol. Phosphorylation of HSL in response to ß3-adrenergic agonist CL316,243 was decreased by an additional ~90% (distal tibia BMAT) or could not be detected (tail vertebrae). Ex vivo, adrenergic stimulation of lipolysis in purified BMAT adipocytes was also substantially less than iWAT adipocytes and had site-specific properties. Specifically, regulated bone marrow adipocytes (rBMAs) from proximal tibia and femur underwent lipolysis in response to both CL316,243 and forskolin, while constitutive BMAs from the tail responded only to forskolin. This occurred independently of changes in gene expression of ß-adrenergic receptors, which were similar between adipocytes from iWAT and BMAT, and could not be explained by defective coupling of ß-adrenergic receptors to lipolytic machinery through caveolin 1. Specifically, we found that whereas caveolin 1 was necessary to mediate maximal stimulation of lipolysis in iWAT, overexpression of caveolin 1 was insufficient to rescue impaired BMAT signaling. Lastly, we tested the ability of BMAT to respond to 72-hour treatment with CL316,243 in vivo. This was sufficient to cause beiging of iWAT adipocytes and a decrease in iWAT adipocyte cell size. By contrast, adipocyte size in the tail BMAT and distal tibia remained unchanged. However, within the distal femur, we identified a subpopulation of BMAT adipocytes that underwent lipid droplet remodeling. This response was more pronounced in females than in males and resembled lipolysis-induced lipid partitioning rather than traditional beiging. In summary, BMAT has the capacity to respond to ß-adrenergic stimuli, however, its responses are muted and BMAT generally resists lipid hydrolysis and remodeling relative to iWAT. This resistance is more pronounced in distal regions of the skeleton where the BMAT adipocytes are larger with little intervening hematopoiesis, suggesting that there may be a role for both cell-autonomous and microenvironmental determinants. Resistance to ß-adrenergic stimuli further separates BMAT from known regulators of energy partitioning and contributes to our understanding of why BMAT is preserved in states of fasting and caloric restriction.

Brain GLUT4 Knockout Mice Have Impaired Glucose Tolerance, Decreased Insulin Sensitivity, and Impaired Hypoglycemic Counterregulation.

GLUT4 in muscle and adipose tissue is important in maintaining glucose homeostasis. However, the role of insulin-responsive GLUT4 in the central nervous system has not been well characterized. To assess its importance, a selective knockout of brain GLUT4 (BG4KO) was generated by crossing Nestin-Cre mice with GLUT4-floxed mice. BG4KO mice had a 99% reduction in GLUT4 protein expression throughout the brain. Despite normal feeding and fasting glycemia, BG4KO mice were glucose intolerant, demonstrated hepatic insulin resistance, and had reduced glucose uptake in the brain. In response to hypoglycemia, BG4KO mice had impaired glucose sensing, noted by impaired epinephrine and glucagon responses and impaired c-fos activation in the hypothalamic paraventricular nucleus. Moreover, in vitro glucose sensing of glucose-inhibitory neurons from the ventromedial hypothalamus was impaired in BG4KO mice. In summary, BG4KO mice are glucose intolerant, insulin resistant, and have impaired glucose sensing, indicating a critical role for brain GLUT4 in sensing and responding to changes in blood glucose.

Increased Circulating Adiponectin in Response to Thiazolidinediones: Investigating the Role of Bone Marrow Adipose Tissue.

BACKGROUND: Bone marrow adipose tissue (MAT) contributes to increased circulating adiponectin, an insulin-sensitizing hormone, during caloric restriction (CR), but whether this occurs in other contexts remains unknown. The antidiabetic thiazolidinediones (TZDs) also promote MAT expansion and hyperadiponectinemia, even without increasing adiponectin expression in white adipose tissue (WAT). OBJECTIVES: To test the hypothesis that MAT expansion contributes to TZD-associated hyperadiponectinemia, we investigated the effects of rosiglitazone, a prototypical TZD, in wild-type (WT) or Ocn-Wnt10b mice. The latter resist MAT expansion during CR, leading us to postulate that they would also resist this effect of rosiglitazone. DESIGN: Male and female WT or Ocn-Wnt10b mice (C57BL/6J) were treated with or without rosiglitazone for 2, 4, or 8 weeks, up to 30 weeks of age. MAT content was assessed by osmium tetroxide staining and adipocyte marker expression. Circulating adiponectin was determined by ELISA. RESULTS: In WT mice, rosiglitazone caused hyperadiponectinemia and MAT expansion. Compared to WT mice, Ocn-Wnt10b mice had significantly less MAT in distal tibiae and sometimes in proximal tibiae; however, interpretation was complicated by the leakage of osmium tetroxide from ruptures in some tibiae, highlighting an important technical consideration for osmium-based MAT analysis. Despite decreased MAT in Ocn-Wnt10b mice, circulating adiponectin was generally similar between WT and Ocn-Wnt10b mice; however, in females receiving rosiglitazone for 4 weeks, hyperadiponectinemia was significantly blunted in Ocn-Wnt10b compared to WT mice. Notably, this was also the only group in which tibial adiponectin expression was lower than in WT mice, suggesting a close association between MAT adiponectin production and circulating adiponectin. However, rosiglitazone significantly increased adiponectin protein expression in WAT, suggesting that WAT contributes to hyperadiponectinemia in this context. Finally, rosiglitazone upregulated uncoupling protein 1 in brown adipose tissue (BAT), but this protein was undetectable in tibiae, suggesting that MAT is unlikely to share thermogenic properties of BAT. CONCLUSION: TZD-induced hyperadiponectinemia is closely associated with increased adiponectin production in MAT but is not prevented by the partial loss of MAT that occurs in Ocn-Wnt10b mice. Thus, more robust loss-of-MAT models are required for future studies to better establish MAT's elusive functions, both on an endocrine level and beyond.

Expansion of Bone Marrow Adipose Tissue During Caloric Restriction Is Associated With Increased Circulating Glucocorticoids and Not With Hypoleptinemia.

Bone marrow adipose tissue (MAT) accounts for up to 70% of bone marrow volume in healthy adults and increases further in clinical conditions of altered skeletal or metabolic function. Perhaps most strikingly, and in stark contrast to white adipose tissue, MAT has been found to increase during caloric restriction (CR) in humans and many other species. Hypoleptinemia may drive MAT expansion during CR but this has not been demonstrated conclusively. Indeed, MAT formation and function are poorly understood; hence, the physiological and pathological roles of MAT remain elusive. We recently revealed that MAT contributes to hyperadiponectinemia and systemic adaptations to CR. To further these observations, we have now performed CR studies in rabbits to determine whether CR affects adiponectin production by MAT. Moderate or extensive CR decreased bone mass, white adipose tissue mass, and circulating leptin but, surprisingly, did not cause hyperadiponectinemia or MAT expansion. Although this unexpected finding limited our subsequent MAT characterization, it demonstrates that during CR, bone loss can occur independently of MAT expansion; increased MAT may be required for hyperadiponectinemia; and hypoleptinemia is not sufficient for MAT expansion. We further investigated this relationship in mice. In females, CR increased MAT without decreasing circulating leptin, suggesting that hypoleptinemia is also not necessary for MAT expansion. Finally, circulating glucocorticoids increased during CR in mice but not rabbits, suggesting that glucocorticoids might drive MAT expansion during CR. These observations provide insights into the causes and consequences of CR-associated MAT expansion, knowledge with potential relevance to health and disease.

Bone marrow adipose tissue is an endocrine organ that contributes to increased circulating adiponectin during caloric restriction.

The adipocyte-derived hormone adiponectin promotes metabolic and cardiovascular health. Circulating adiponectin increases in lean states such as caloric restriction (CR), but the reasons for this paradox remain unclear. Unlike white adipose tissue (WAT), bone marrow adipose tissue (MAT) increases during CR, and both MAT and serum adiponectin increase in many other clinical conditions. Thus, we investigated whether MAT contributes to circulating adiponectin. We find that adiponectin secretion is greater from MAT than WAT. Notably, specific inhibition of MAT formation in mice results in decreased circulating adiponectin during CR despite unaltered adiponectin expression in WAT. Inhibiting MAT formation also alters skeletal muscle adaptation to CR, suggesting that MAT exerts systemic effects. Finally, we reveal that both MAT and serum adiponectin increase during cancer therapy in humans. These observations identify MAT as an endocrine organ that contributes significantly to increased serum adiponectin during CR and perhaps in other adverse states.

Wnt6, Wnt10a and Wnt10b inhibit adipogenesis and stimulate osteoblastogenesis through a ß-catenin-dependent mechanism.

Wnt10b is an established regulator of mesenchymal stem cell (MSC) fate that inhibits adipogenesis and stimulates osteoblastogenesis, thereby impacting bone mass in vivo. However, downstream mechanisms through which Wnt10b exerts these effects are poorly understood. Moreover, whether other endogenous Wnt ligands also modulate MSC fate remains to be fully addressed. In this study, we identify Wnt6 and Wnt10a as additional Wnt family members that, like Wnt10b, are downregulated during development of white adipocytes in vivo and in vitro, suggesting that Wnt6 and/or Wnt10a may also inhibit adipogenesis. To assess the relative activities of Wnt6, Wnt10a and Wnt10b to regulate mesenchymal cell fate, we used gain- and loss-of function approaches in bipotential ST2 cells and in 3T3-L1 preadipocytes. Enforced expression of Wnt10a stabilizes ß-catenin, suppresses adipogenesis and stimulates osteoblastogenesis to a similar extent as Wnt10b, whereas stable expression of Wnt6 has a weaker effect on these processes than Wnt10a or Wnt10b. In contrast, knockdown of endogenous Wnt6 is associated with greater preadipocyte differentiation and impaired osteoblastogenesis than knockdown of Wnt10a or Wnt10b, suggesting that, among these Wnt ligands, Wnt6 is the most potent endogenous regulator of MSC fate. Finally, we show that knockdown of ß-catenin completely prevents the inhibition of adipogenesis and stimulation of osteoblast differentiation by Wnt6, Wnt10a or Wnt10b. Potential mechanisms whereby Wnts regulate fate of MSCs downstream of ß-catenin are also investigated. In conclusion, this study identifies Wnt10a and Wnt6 as additional regulators of MSC fate and demonstrates that mechanisms downstream of ß-catenin are required for Wnt6, Wnt10a and Wnt10b to influence differentiation of mesenchymal precursors.

Pharmacologic amelioration of severe hypoglycemia-induced neuronal damage.

Hypoglycemia is a common complication for insulin treated people with diabetes. Severe hypoglycemia, which occurs in the setting of excess or ill-timed insulin administration, has been shown to cause brain damage. Previous pre-clinical studies have shown that memantine (an N-methyl-d-aspartate receptor antagonist) and erythropoietin can be neuroprotective in other models of brain injury. We hypothesized that these agents might also be neuroprotective in response to severe hypoglycemia-induced brain damage. To test this hypothesis, 9-week old, awake, male Sprague-Dawley rats underwent hyperinsulinemic (0.2 U kg(-1)min(-1)) hypoglycemic clamps to induce severe hypoglycemia (blood glucose 10-15 mg/dl for 90 min). Animals were randomized into control (vehicle) or pharmacological treatments (memantine or erythropoietin). One week after severe hypoglycemia, neuronal damage was assessed by Fluoro-Jade B and hematoxylin and eosin staining of brain sections. Treatment with both memantine and erythropoietin significantly decreased severe hypoglycemia-induced neuronal damage in the cortex by 35% and 39%, respectively (both p<0.05 vs. controls). These findings demonstrate that memantine and erythropoietin provide a protective effect against severe hypoglycemia-induced neuronal damage.

Recurrent moderate hypoglycemia ameliorates brain damage and cognitive dysfunction induced by severe hypoglycemia.

OBJECTIVE: Although intensive glycemic control achieved with insulin therapy increases the incidence of both moderate and severe hypoglycemia, clinical reports of cognitive impairment due to severe hypoglycemia have been highly variable. It was hypothesized that recurrent moderate hypoglycemia preconditions the brain and protects against damage caused by severe hypoglycemia. RESEARCH DESIGN AND METHODS: Nine-week-old male Sprague-Dawley rats were subjected to either 3 consecutive days of recurrent moderate (25-40 mg/dl) hypoglycemia (RH) or saline injections. On the fourth day, rats were subjected to a hyperinsulinemic (0.2 units x kg(-1) x min(-1)) severe hypoglycemic ( approximately 11 mg/dl) clamp for 60 or 90 min. Neuronal damage was subsequently assessed by hematoxylin-eosin and Fluoro-Jade B staining. The functional significance of severe hypoglycemia-induced brain damage was evaluated by motor and cognitive testing. RESULTS: Severe hypoglycemia induced brain damage and striking deficits in spatial learning and memory. Rats subjected to recurrent moderate hypoglycemia had 62-74% less brain cell death and were protected from most of these cognitive disturbances. CONCLUSIONS: Antecedent recurrent moderate hypoglycemia preconditioned the brain and markedly limited both the extent of severe hypoglycemia-induced neuronal damage and associated cognitive impairment. In conclusion, changes brought about by recurrent moderate hypoglycemia can be viewed, paradoxically, as providing a beneficial adaptive response in that there is mitigation against severe hypoglycemia-induced brain damage and cognitive dysfunction.

Central but not systemic lipid infusion augments the counterregulatory response to hypoglycemia.

This study tests the hypothesis that lipids could act as an alternative fuel source in the brain during insulin-induced hypoglycemia. Male Sprague-Dawley rats were subjected to hyperinsulinemic (5 mU.kg(-1).min(-1)) hypoglycemic (approximately 50 mg/dl) clamps. In protocol 1, intralipid (IL), a fat emulsion, was infused intravenously to prevent the fall in free fatty acid levels that occurs in response to hyperinsulinemic hypoglycemia. Intravenous lipid infusion did not alter the counterregulatory responses to hypoglycemia. To test whether IL could have central effects in mediating the counterregulatory response to hypoglycemia, in protocol 2 the brains of precannulated rats were intracerebroventricularly (icv) infused with IL or artificial cerebrospinal fluid (aCSF) as control. Unexpectedly, the epinephrine and glucagon response to hypoglycemia was significantly augmented with icv IL infusion. To determine whether central IL infusion could restore defective counterregulation, in protocol 3 rats were made recurrently hypoglycemic (RH) for 3 days and on the 4th day underwent hyperinsulinemic hypoglycemic clamps with icv IL or aCSF infusion. RH rats had the expected impaired epinephrine response to hypoglycemia, and icv IL infusion again significantly augmented the epinephrine response in RH rats to normal. With regard to our experimental model of hypoglycemic counterregulation, we conclude that 1) systemic lipid infusion did not alter the counterregulatory response to hypoglycemia, 2) the icv infusion of lipids markedly increased CSF FFA levels and paradoxically augmented the epinephrine and glucagon responses, and 3) the blunted sympathoadrenal response in recurrently hypoglycemic rats was completely normalized with the icv lipid infusion. It is concluded that, in the setting of insulin-induced hypoglycemia, increased brain lipids can enhance the sympathoadrenal response.

Diabetes increases brain damage caused by severe hypoglycemia.

Insulin-induced severe hypoglycemia causes brain damage. The hypothesis to be tested was that diabetes portends to more extensive brain tissue damage following an episode of severe hypoglycemia. Nine-week-old male streptozotocin-diabetic (DIAB; n = 10) or vehicle-injected control (CONT; n = 7) Sprague-Dawley rats were subjected to hyperinsulinemic (0.2 U.kg(-1).min(-1)) severe hypoglycemic (10-15 mg/dl) clamps while awake and unrestrained. Groups were precisely matched for depth and duration (1 h) of severe hypoglycemia (CONT 11 +/- 0.5 and DIAB 12 +/- 0.2 mg/dl, P = not significant). During severe hypoglycemia, an equal number of episodes of seizure-like activity were noted in both groups. One week later, histological analysis demonstrated extensive neuronal damage in regions of the hippocampus, especially in the dentate gyrus and CA1 regions and less so in the CA3 region (P < 0.05), although total hippocampal damage was not different between groups. However, in the cortex, DIAB rats had significantly (2.3-fold) more dead neurons than CONT rats (P < 0.05). There was a strong correlation between neuronal damage and the occurrence of seizure-like activity (r(2) > 0.9). Separate studies conducted in groups of diabetic (n = 5) and nondiabetic (n = 5) rats not exposed to severe hypoglycemia showed no brain damage. In summary, under the conditions studied, severe hypoglycemia causes brain damage in the cortex and regions within the hippocampus, and the extent of damage is closely correlated to the presence of seizure-like activity in nonanesthetized rats. It is concluded that, in response to insulin-induced severe hypoglycemia, diabetes uniquely increases the vulnerability of specific brain areas to neuronal damage.