👤 Alexis Varin

Glucose-dependent insulinotropic polypeptide (GIP) conveys information from ingested nutrients to peripheral tissues, signaling energy availability. The GIP Receptor (GIPR) is also expressed in the bone marrow, notably in cells of the myeloid lineage. However, the importance of gain and loss of GIPR signaling for diverse hematopoietic responses remains unclear. We assessed the expression of the Gipr in bone marrow (BM) lineages and examined functional roles for the GIPR in control of hematopoiesis. Bone marrow responses were studied in (i) mice fed regular or energy-rich diets, (ii) mice treated with hematopoietic stressors including acute 5-fluorouracil (5-FU), pamsaccharide (LPS), and Pam3CysSerLys4 (Pam3CSK4), with or without pharmacological administration of a GIPR agonist, and (iii) mice with global (Gipr Gipr is expressed within T cells, myeloid cells, and myeloid precursors; however, these cell populations were not different in peripheral blood, spleen, or BM of Gipr These studies identify a functional gut hormone-BM axis positioned for the transduction of signals linking nutrient availability to the control of TLR and Notch genes regulating hematopoiesis. Nevertheless, stimulation or loss of GIPR signaling has minimal impact on basal hematopoiesis or the physiological response to hematopoietic stress. Show less

Glucose-dependent insulinotropic polypeptide (GIP) is secreted from the gut in response to nutrient ingestion and promotes meal-dependent insulin secretion and lipid metabolism. Loss or attenuation of GIP receptor (GIPR) action leads to resistance to diet-induced obesity through incompletely understood mechanisms. The GIPR is expressed in white adipose tissue; however, its putative role in brown adipose tissue (BAT) has not been explored. We investigated the role of the GIPR in BAT cells in vitro and in BAT-specific (Gipr The mouse Gipr gene is expressed in BAT, and GIP directly increased Il6 mRNA and IL-6 secretion in BAT cells. Additionally, levels of thermogenic, lipid and inflammation mRNA transcripts were altered in BAT cells transfected with Gipr siRNA. Body weight gain, energy expenditure, and glucose and insulin tolerance were normal in HFD-fed Gipr The BAT GIPR is linked to the control of metabolic gene expression, fuel utilization, and oxygen consumption. However, the selective loss of the GIPR within BAT is insufficient to recapitulate the findings of decreased weight gain and resistance to obesity arising in experimental models with systemic disruption of GIP action. Show less

β1- and β2-adrenergic receptors (β-ARs) produce different acute contractile effects on the heart partly because they impact on different cytosolic pools of cAMP-dependent protein kinase (PKA). They also exert different effects on gene expression but the underlying mechanisms remain unknown. The aim of this study was to understand the mechanisms by which β1- and β2-ARs regulate nuclear PKA activity in cardiomyocytes. We used cytoplasmic and nuclear targeted biosensors to examine cAMP signals and PKA activity in adult rat ventricular myocytes upon selective β1- or β2-ARs stimulation. Both β1- and β2-AR stimulation increased cAMP and activated PKA in the cytoplasm. Although the two receptors also increased cAMP in the nucleus, only β1-ARs increased nuclear PKA activity and up-regulated the PKA target gene and pro-apoptotic factor, inducible cAMP early repressor (ICER). Inhibition of phosphodiesterase (PDE)4, but not Gi, PDE3, GRK2 nor caveolae disruption disclosed nuclear PKA activation and ICER induction by β2-ARs. Both nuclear and cytoplasmic PKI prevented nuclear PKA activation and ICER induction by β1-ARs, indicating that PKA activation outside the nucleus is required for subsequent nuclear PKA activation and ICER mRNA expression. Cytoplasmic PKI also blocked ICER induction by β2-AR stimulation (with concomitant PDE4 inhibition). However, in this case nuclear PKI decreased ICER up-regulation by only 30%, indicating that other mechanisms are involved. Down-regulation of mAKAPβ partially inhibited nuclear PKA activation upon β1-AR stimulation, and drastically decreased nuclear PKA activation upon β2-AR stimulation in the presence of PDE4 inhibition. β1- and β2-ARs differentially regulate nuclear PKA activity and ICER expression in cardiomyocytes. PDE4 insulates a mAKAPβ-targeted PKA pool at the nuclear envelope that prevents nuclear PKA activation upon β2-AR stimulation. Show less

Liver X Receptors (LXRs) α and β are oxysterol-activated nuclear receptors involved in the control of lipid metabolism and inflammation. Pharmacological activation of LXR is promising in the treatment of atherosclerosis since it can promote cholesterol efflux from macrophages and prevent foam cell formation. However, the development of LXR agonists has been limited by undesirable side-effects such as hepatic steatosis mediated by LXRα activation. Therefore, it has been proposed that targeting LXRα activators to extrahepatic tissues or using LXRβ-specific activators could be used as alternative strategies. It is not clear whether these molecules will retain the full atheroprotective potential of non-selective agonists. Our aim was therefore to determine the contribution of LXRα and LXRβ to the control of cholesterol efflux in human macrophages. LXRα and/or LXRβ expression was suppressed by small interfering RNAs in human primary macrophages treated or not with synthetic LXRα/β dual agonists T0901317 and GW3965. We observed that LXRβ silencing had no detectable impact on the expression of LXR-target genes such as ABCA1 and ABCG1. Moreover it did not affect cholesterol efflux. In contrast, LXRα silencing reduced the response of these LXR-target genes to LXR agonist and inhibited cholesterol efflux to ApoA-I, HDL2 or to endogenous ApoE. Importantly, no differences were observed between LXRα and LXRα/β knockdown conditions. Altogether, our data demonstrate that LXRβ activation is unable to maintain maximal cholesterol efflux capacities in human primary macrophages when LXRα expression is impaired. In contrast to earlier mouse studies, LXRα levels appear as a limiting factor for macrophage cholesterol efflux in humans. Show less

The biochemical and biological properties of 4β-hydroxycholesterol and of its isomer, 4α-hydroxycholesterol, are not well known. So, we determined the ability of 4α- and 4β-hydroxycholesterol to react with LXRα and LXRβ, and we characterized the activities of these oxysterols on oligodendrocytes which are myelin synthesizing cells. The effects of 4α- and 4β-hydroxycholesterol were studied on 158N murine oligodendrocytes to assess their activities on cell growth and viability, oxidative and inflammatory status. To this end different parameters were used: cell counting with trypan blue; identification of dead cells and cell cycle analysis with propidium iodide; evaluation of mitochondrial depolarization, lysosomal membrane integrity, actin depolimerization, nuclear morphology, and superoxide anion production after staining with JC-1, acridine orange, rhodamine-phalloidin, Hoechst 33342, and dihydroethidium, respectively; evaluation of ultrastructural changes by transmission electron microscopy, and cytokine quantification with a cytometric bead array. Only 4β-hydroxycholesterol is a LXRα and β agonist. No cytotoxic effects were found with 4α-hydroxycholesterol except a slight inhibition of cell growth at elevated concentrations. At high concentrations, 4β-hydroxycholesterol was not only able to inhibit cell growth, but also to induce cell death associated with a loss of mitochondrial transmembrane potential, dysfunctions of lysosomal membrane integrity, and superoxide anion overproduction. These side effects were lower than those observed with 7-ketocholesterol and 25-hydroxycholesterol used as positive controls. On oligodendrocyte murine primary cultures, only lysosomal membrane integrity was slightly affected under treatment with 4α- and 4β-hydroxycholesterol. So, 4α- and 4β-hydroxycholesterol have different biological activities. Their ability to induce cytotoxic effects on oligodendrocytes can be considered as weak comparatively to 7-ketocholesterol and 25-hydroxycholesterol. Show less