Issue published May 15, 2002

W e have derived from normal human, mouse, and rat postnatal bone marrow primitive, multipotent adult progenitor cells (MAPCs) that can differentiate into most mesodermal cells and neuroectodermal cells in vitro and into all embryonic lineages in vivo. Here, we show that MAPCs can also differentiate into hepatocyte-like cells in vitro. Human, mouse, and rat MAPCs, cultured on Matrigel with FGF-4 and HGF, differentiated into epithelioid cells that expressed hepatocyte nuclear factor-3β (HNF-3β), GATA4, cytokeratin 19 (CK19), transthyretin, and α-fetoprotein by day 7, and expressed CK18, HNF-4, and HNF-1α on days 14–28. Virtually all human, as well as a majority of rodent cells stained positive for albumin and CK18 on day 21; 5% (rodent) to 25% (human) cells were binucleated by day 21. These cells also acquired functional characteristics of hepatocytes: they secreted urea and albumin, had phenobarbital-inducible cytochrome p450, could take up LDL, and stored glycogen. MAPCs, which can be expanded in vitro and maintained in an undifferentiated state for more than 100 population doublings, can thus differentiate into cells with morphological, phenotypic, and functional characteristics of hepatocytes. MAPCs may therefore be an ideal cell for in vivo therapies for liver disorders or for use in bioartificial liver devices.

A diponectin, an adipocyte-derived hormone, was recently shown to have potential therapeutic applications in diabetes and obesity because of its influence on glucose and lipid metabolism. We found that brown fat in normal human bone marrow contains this protein and used marrow-derived preadipocyte lines and long-term cultures to explore potential roles in hematopoiesis. Recombinant adiponectin blocked fat cell formation in long-term bone marrow cultures and inhibited the differentiation of cloned stromal preadipocytes. Adiponectin also caused elevated expression of cyclooxygenase-2 (COX-2) by these stromal cells and induced release of prostaglandin E₂ (PGE₂). The COX-2 inhibitor Dup-697 prevented the inhibitory action of adiponectin on preadipocyte differentiation, suggesting involvement of stromal cell–derived prostanoids. Furthermore, adiponectin failed to block fat cell generation when bone marrow cells were derived from B6,129S^Ptgs2tm1Jed (COX-2^+/–) mice. These observations show that preadipocytes represent direct targets for adiponectin action, establishing a paracrine negative feedback loop for fat regulation. They also link adiponectin to the COX-2–dependent PGs that are critical in this process.

D espite intense interest in pathways that generate reactive nitrogen species, the physiologically relevant mechanisms for inflammatory tissue injury remain poorly understood. One possible mediator is myeloperoxidase, a major constituent of neutrophils, monocytes, and some populations of macrophages. The enzyme uses hydrogen peroxide and nitrite to generate 3-nitrotyrosine in vitro. To determine whether myeloperoxidase produces nitrating intermediates in vivo, we used isotope dilution gas chromatography/mass spectrometry to quantify 3-nitrotyrosine in two models of peritoneal inflammation: mice infected with Klebsiella pneumoniae and mice subjected to cecal ligation and puncture. Both models developed an intense neutrophil inflammatory response, and the inflammatory fluid contained markedly elevated levels of 3-chlorotyrosine, a marker of myeloperoxidase action. In striking contrast, 3-nitrotyrosine levels rose only in the mice infected with K. pneumoniae. Levels of total nitrite and nitrate were 20-fold higher in mice injected with K. pneumoniae than in mice subjected to cecal ligation and puncture. Levels of 3-nitrotyrosine failed to increase in mice infected with K. pneumoniae that lacked functional myeloperoxidase. Our observations provide strong evidence that myeloperoxidase generates reactive nitrogen species in vivo and that it operates in this fashion only when nitrite and nitrate become available.

R ecent studies have implicated fatty acid-dependent activation of the serine kinase IKKβ, which plays a key role in tissue inflammation, in the pathogenesis of insulin resistance. High doses of salicylates have recently been shown to inhibit IKKβ activity and might therefore ameliorate insulin resistance and improve glucose tolerance in patients with type 2 diabetes. To test this hypothesis, we studied nine type 2 diabetic subjects before and after 2 weeks of treatment with aspirin (∼7 g/d). Subjects underwent mixed-meal tolerance tests and hyperinsulinemic-euglycemic clamps with [6,6-²H₂]glucose to assess glucose turnover before and after treatment. High-dose aspirin treatment resulted in a ∼25% reduction in fasting plasma glucose, associated with a ∼15% reduction in total cholesterol and C-reactive protein, a ∼50% reduction in triglycerides, and a ∼30% reduction in insulin clearance, despite no change in body weight. During a mixed-meal tolerance test, the areas under the curve for plasma glucose and fatty acid levels decreased by ∼20% and ∼50%, respectively. Aspirin treatment also resulted in a ∼20% reduction in basal rates of hepatic glucose production and a ∼20% improvement in insulin-stimulated peripheral glucose uptake under matched plasma insulin concentrations during the clamp. In conclusion, these data support the hypothesis that IKKβ represents a new target for treating type 2 diabetes mellitus.

B ecker syndrome, a recessive nondystrophic myotonia caused by mutations in the chloride channel 1 gene (CLCN1), is characterized by delayed muscle relaxation after contraction. The ADR (arrested development of righting response) mouse is an animal model for Becker syndrome. Skeletal muscles from ADR myotonic animals show an increased number of oxidative fibers with a lack of glycolytic fibers as well as signs of muscle hypertrophy. Through breeding ADR myotonic mice with mice harboring a MEF2-dependent reporter gene, we found that the transcriptional activity of MEF2 was dramatically enhanced in myotonic muscles. Post-translational induction of MEF2 transcriptional activity correlated with the activation of p38 MAPK and did not affect MEF2 DNA-binding affinity. Expression of class II histone deacetylases (HDACs), which repress MEF2-dependent gene expression, was significantly reduced in skeletal muscles from myotonic mice. These findings suggest that the combined effects of class II HDAC deficiency and p38 MAPK activation lead to potent upregulation of MEF2 transcriptional activity, which contributes to the long-term changes in gene expression and fiber-type transformation observed in myotonic skeletal muscles. These findings provide new molecular targets for potential treatment of congenital myotonia.

T o understand the relationship between host antigen-presenting cells (APCs) and donor T cells in initiating graft-versus-host disease (GVHD), we followed the fate of host dendritic cells (DCs) in irradiated C57BL/6 (B6) recipient mice and the interaction of these cells with minor histocompatibility antigen- (miHA-) mismatched CD8⁺ T cells from C3H.SW donors. Host CD11c⁺ DCs were rapidly activated and aggregated in the T cell areas of the spleen within 6 hours of lethal irradiation. By 5 days after irradiation, <1% of host DCs were detectable, but the activated donor CD8⁺ T cells had already undergone as many as seven divisions. Thus, proliferation of donor CD8⁺ T cells preceded the disappearance of host DCs. When C3H.SW donor CD8⁺ T cells were primed in vivo in irradiated B6 mice or ex vivo by host CD11c⁺ DCs for 24–36 hours, they were able to proliferate and differentiate into IFN-γ–producing cells in β₂-microglobulin–deficient (β₂m^–/–) B6 recipients and to mediate acute GVHD in β₂m^–/– → B6 chimeric mice. These results indicate that, although host DCs disappear rapidly after allogeneic bone marrow transplantation, they prime donor T cells before their disappearance and play a critical role in triggering donor CD8⁺ T cell–mediated GVHD.

L ipodystrophy is a rare disorder that is characterized by selective loss of subcutaneous and visceral fat and is associated with hypertriglyceridemia, hepatomegaly, and disordered glucose metabolism. It has recently been shown that chronic leptin treatment ameliorates these abnormalities. Here we show that chronic leptin treatment improves insulin-stimulated hepatic and peripheral glucose metabolism in severely insulin-resistant lipodystrophic patients. This improvement in insulin action was associated with a marked reduction in hepatic and muscle triglyceride content. These data suggest that leptin may represent an important new therapy to reverse the severe hepatic and muscle insulin resistance and associated hepatic steatosis in patients with lipodystrophy.

T oll-like receptor 2 (TLR2) and TLR4 play important roles in the early innate immune response to microbial challenge. To clarify the functional roles of TLRs 2 and 4 in mast cells, we examined bone marrow–derived mast cells (BMMCs) from TLR2 or TLR4 gene-targeted mice. Peptidoglycan (PGN) from Staphylococcus aureus stimulated mast cells in a TLR2-dependent manner to produce TNF-α, IL-4, IL-5, IL-6, and IL-13, but not IL-1β. In contrast, LPS from Escherichia coli stimulated mast cells in a TLR4-dependent manner to produce TNF-α, IL-1β, IL-6, and IL-13, but not IL-4 nor IL-5. Furthermore, TLR2- but not TLR4-dependent mast cell stimulation resulted in mast cell degranulation and Ca²⁺ mobilization. In a mast cell–dependent model of acute sepsis, TLR4 deficiency of BMMCs in mice resulted in significantly higher mortality because of defective neutrophil recruitment and production of proinflammatory cytokines in the peritoneal cavity. Intradermal injection of PGN led to increased vasodilatation and inflammation through TLR2-dependent activation of mast cells in the skin. Taken together, these results suggest that direct activation of mast cells via TLR2 or TLR4 by respective microligands contributes to innate and allergic immune responses.

G protein–coupled receptors (GPCRs) play a key role in regulating bone remodeling. Whether GPCRs exert anabolic or catabolic osseous effects may be determined by the rate of receptor desensitization in osteoblasts. Receptor desensitization is largely mediated by direct phosphorylation of GPCR proteins by a family of enzymes termed GPCR kinases (GRKs). We have selectively manipulated GRK activity in osteoblasts in vitro and in vivo by overexpressing a GRK inhibitor. We found that expression of a GRK inhibitor enhanced parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor-stimulated cAMP generation and inhibited agonist-induced phosphorylation of this receptor in cell culture systems, consistent with attenuation of receptor desensitization. To determine the effect of GRK inhibition on bone formation in vivo, we targeted the expression of a GRK inhibitor to mature osteoblasts using the mouse osteocalcin gene 2 (OG2) promoter. Transgenic mice demonstrated enhanced bone remodeling as well as enhanced urinary excretion of the osteoclastic activity marker dexoypyridinoline. Both osteoprotegrin and OPG ligand mRNA levels were altered in calvaria of transgenic mice in a pattern that would promote osteoclast activation. The predominant effect of the transgene, however, was anabolic, as evidenced by an increase in bone density and trabecular bone volume in the transgenic mice compared with nontransgenic littermate controls.

L eukotrienes are lipid mediators that evoke primarily proinflammatory responses by activating receptors present on virtually all cells. The production of leukotrienes is tightly regulated, and expression of 5-lipoxygenase, the enzyme required for the first step in leukotriene synthesis, is generally restricted to leukocytes. Arachidonic acid released from the cell membrane of activated leukocytes is rapidly converted to LTA₄ by 5-lipoxygenase. LTA₄ is further metabolized to either LTC₄ or LTB₄ by the enzyme LTC₄ synthase or LTA₄ hydrolase, respectively. Unlike 5-lipoxygenase, these enzymes are expressed in most tissues. This observation previously has led to the suggestion that LTA₄ produced by leukocytes may, in some cases, be delivered to other cell types before being converted into LTC₄ or LTB₄. While in vitro studies indicate that this process, termed transcellular biosynthesis, can lead to the production of leukotrienes, it has not been possible to determine the significance of this pathway in vivo. Using a series of bone marrow chimeras generated from 5-lipoxygenase– and LTA₄ hydrolase–deficient mice, we show here that transcellular biosynthesis contributes to the production of leukotrienes in vivo and that leukotrienes produced by this pathway are sufficient to contribute significantly to the physiological changes that characterize an ongoing inflammatory response.

D eficiency of the membrane protein FAT/CD36 causes a marked defect in fatty acid uptake by various tissues and is genetically linked to insulin resistance in rats and humans. Here, we examined insulin responsiveness of CD36^–/– mice. When fed a diet high in complex carbohydrates and low (5%) in fat, these animals cleared glucose faster than the wild-type. In vivo, uptake of 2-fluorodeoxyglucose by muscle was increased severalfold, and in vitro, insulin responsiveness of glycogenesis by the soleus was enhanced. Null mice had lower glycogen levels in muscle and liver, lower muscle triglyceride levels, and increased liver triglyceride content—all findings consistent with increased insulin-sensitivity. However, when the chow diet was switched to one high in fructose, CD36^–/– mice but not wild-type mice developed marked glucose intolerance, hyperinsulinemia, and decreased muscle glucose uptake. High-fat diets impaired glucose tolerance equally in both groups, although CD36 deficiency helped moderate insulin-responsive muscle glucose oxidation. In conclusion, CD36 deficiency enhances insulin responsiveness on a high-starch, low-fat diet. It predisposes to insulin resistance induced by high fructose and partially protects from that induced by high-fat diets. In humans, CD36 deficiency may be an important factor in the metabolic adaptation to diet and in susceptibility to some forms of diet-induced pathology.