Macrophage Fcγ receptors (FcγRs) mediate the uptake and destruction of antibody-coated viruses, bacteria, and parasites. We examined FcγR signaling and phagocytic function in bone marrow–derived macrophages from mutant mice lacking the major Src family kinases expressed in these cells, Hck, Fgr, and Lyn. Many FcγR-induced functional responses and signaling events were diminished or delayed in these macrophages, including immunoglobulin (Ig)G-coated erythrocyte phagocytosis, respiratory burst, actin cup formation, and activation of Syk, phosphatidylinositol 3-kinase, and extracellular signal–regulated kinases 1 and 2. Significant reduction of IgG-dependent phagocytosis was not seen in hck − /−fgr − /− or lyn − /− cells, although the single mutant lyn − /− macrophages did manifest signaling defects. Thus, Src family kinases clearly have roles in two events leading to FcγR-mediated phagocytosis, one involving initiation of actin polymerization and the second involving activation of Syk and subsequent internalization. Since FcγR-mediated phagocytosis did occur at modest levels in a delayed fashion in triple mutant macrophages, these Src family kinases are not absolutely required for uptake of IgG-opsonized particles.

Cancellous bone decreases and bone marrow fat content increases with age. Osteoblasts and adipocytes are derived from a common precursor, and growth hormone (GH), a key hormone in integration of energy metabolism, regulates the differentiation and function of both cell lineages. Since an age-related decline in GH is associated with bone loss, we investigated the relationship between GH and bone marrow adiposity in hypophysectomized (HYPOX) rats and in mice with defects in GH signaling. HYPOX dramatically reduced body weight gain, bone growth and mineralizing perimeter, serum insulin-like growth factor 1 (IGF-1) levels, and mRNA levels for IGF-1 in liver and bone. Despite reduced body mass and adipocyte precursor pool size, HYPOX resulted in a dramatic increase in bone lipid levels, as reflected by increased bone marrow adiposity and bone triglyceride and cholesterol content. GH replacement normalized bone marrow adiposity and precursor pool size, as well as mineralizing perimeter in HYPOX rats. In contrast, 17β -estradiol, IGF-1, thyroxine, and cortisone were ineffective. Parathyroid hormone (PTH) reversed the inhibitory effects of HYPOX on mineralizing perimeter but had no effect on adiposity. Finally, bone marrow adiposity was increased in mice deficient in GH and IGF-1 but not in mice deficient in serum IGF-1. Taken together, our findings indicate that the reciprocal changes in bone and fat mass in GH signaling-deficient rodents are not directly coupled with one another. Rather, GH enhances adipocyte as well as osteoblast precursor pool size. However, GH increases osteoblast differentiation while suppressing bone marrow lipid accumulation. © 2010 American Society for Bone and Mineral Research

Receptors for the Fc portion of immunoglobulin (Ig)G (FcγR) mediate phagocytosis of IgG-opsonized particles by a process that can be divided into four major steps: receptor–ligand binding, pseudopod extension, internalization, and lysosomal fusion. We have expressed single classes of FcγR in COS fibroblasts to examine the structural determinants necessary to complete the four steps of phagocytosis. Using phase contrast, fluorescence, confocal, and electron microscopy we have demonstrated that FcγR-expressing COS cells can phagocytose in a manner similar to that of professional phagocytes. We have further analyzed the capacity of the three classes of FcγR to phagocytose, placing special emphasis on the FcγRIA–γ chain complex, which allowed us to examine independently the roles of the ligand-binding unit (FcγRIA) and the signaling unit (γ chain). We found that receptor complexes containing a conserved tyrosine activation motif (ITAM), as found in the cytoplasmic domain of FcγRIIA and in the γ chain associated with FcγRIA and FcγRIIIA, readily internalized target particles. In contrast, FcγRIA alone, having no ITAM, was unable to internalize target particles efficiently, but did mediate pseudopod extension. Cotransfection of γ chain with FcγRIA restored the ability of the receptor to internalize target particles. A mutant FcγRIA in which the cytoplasmic domain had been deleted was also capable of mediating pseudopod extension, showing that neither the γ chain nor the cytoplasmic domain of FcγRIA were required for this step. Cytochalasin D, an inhibitor of actin polymerization, blocked particle internalization by all FcγR, but did not block pseudopod extension. Staining the FcγRIA COS cells for F-actin and for tyrosine phosphoproteins, we found that actin did not polymerize during FcγRIA-mediated pseudopod extension, nor were tyrosine kinases activated. Our data suggest that pseudopod extension and internalization are functionally distinct steps mediated through different pathways.

Scope: We previously showed that two hydrogenated xanthohumol (XN) derivatives, α,βdihydro-XN (DXN) and tetrahydro-XN (TXN), improved parameters of metabolic syndrome (MetS), a critical risk factor of cardiovascular disease (CVD) and type 2 diabetes, in a dietinduced obese murine model. We hypothesized that improvements in obesity and MetS are linked to changes in the composition of the gut microbiota, bile acid metabolism, intestinal barrier function and inflammation. Methods and results: To test this hypothesis, we sequenced 16S rRNA genes and measured bile acids in fecal samples from C57BL/6J mice fed a high-fat diet (HFD) or HFD containing XN, DXN or TXN. We measured the expression of genes associated with epithelial barrier function, inflammation, and bile acid metabolism, in the colon, white adipose tissue (WAT), and liver, respectively. Administration of XN derivatives decreased intestinal microbiota diversity and abundance, specifically Bacteroidetes and Tenericutes, altered bile acid metabolism, and reduced inflammation. In WAT, TXN supplementation decreased pro-inflammatory gene expression by suppressing macrophage infiltration. Transkingdom network analysis connected changes in the microbiota to improvements in MetS in the host. Conclusion: Changes in the gut microbiota and bile acid metabolism may explain, in part, the improvements in obesity and MetS associated with administration of XN and its derivatives.