A low-molecular-weight receptor tyrosine kinase inhibitor, 1-(6,7-dihydro-5H-benzo(6,7)cyclohepta(1,2-c)pyridazin-3-yl)-N3-((7-pyrrolidin-1-yl)-6,7,8,9-tetrahydro-5H-benzo(7)annulene-2-yl)-1H-1,2,4-triazole-3,5-diamine (R428) with high affinity and selectivity for the growth arrest-specific protein 6 (GAS6) receptor Axl was used to study a potential role of GAS6 signaling in adiposity. In vitro, R428 caused a concentration-dependent inhibition of preadipocyte differentiation into mature adipocytes, as evidenced by reduced lipid uptake. Inhibition of Axl-mediated signaling was confirmed by reduced levels of phospho-Akt activity. In vivo, oral administration of R428 for 5 weeks to mice kept on a high-fat diet resulted in significantly reduced weight gain and subcutaneous and gonadal fat mass. This was associated with marked adipocyte hypotrophy, enhanced macrophage infiltration, and apoptosis. Thus, affecting GAS6 signaling through receptor antagonism using a low-molecular-weight Axl antagonist impairs adipocyte differentiation and reduces adipose tissue development in a murine model of nutritionally induced obesity.
The TAM receptor protein tyrosine kinases, Tyro3 (also called Rse, Sky, Brt, Tif, Dtk, Etk-2), Axl (also called Ark, Ufo, Tyro7), and Mer (also called Eyk, Nyk, Tyro12) play pivotal roles in innate immunity (Lemke and Rothlin, 2008). These receptors contain two Ig-like domains and dual fibronectin type III repeats in the extracellular region, as well as a cytoplasmic kinase domain. The ligands for TAM receptors are growth arrest-specific protein 6) (GAS6) and protein S (Hafizi and Dahlbäck, 2006). The high-affinity interaction of GAS6 with Axl triggers antiapoptotic signals, resulting in improved cell survival. In addition, Axl-mediated signaling has been implicated in tumor invasion, migration, angiogenesis, proliferation, and adhesion (Li et al., 2009). GAS6 signaling stimulates cellular responses including activation of phosphatidylinositol 3-kinase, Akt, Erk, and p38 mitogen-activated protein kinase cascades, the nuclear factor-κB pathway, and signal transducer and activator of transcription signaling. GAS6 is expressed during the clonal expansion of postconfluent 3T3-L1 preadipocytes (Shugart et al., 1995) and during differentiation of 3T3-F442A preadipocytes into mature adipocytes (Maquoi et al., 2005). We have shown that adipose tissue development is impaired in GAS6-deficient mice (Maquoi et al., 2005). Whereas GAS6, Tyro3, and Mer are highly expressed in mature murine adipocytes, Axl is expressed primarily in the stromal-vascular cell fraction, including preadipocytes (Maquoi et al., 2005). Furthermore, Axl expression was found to be down-regulated during in vitro porcine adipocyte differentiation (Suzuki et al., 2008). Protein array revealed higher Axl levels in subcutaneous (SC) adipose tissue of obese human subjects compared with lean controls (Skopková et al., 2007).
In the present study, we have evaluated a potential role of the GAS6 receptors in in vitro preadipocyte differentiation and in vivo adipose tissue formation, by using a low-molecular-weight inhibitor, 1-(6,7-dihydro-5H-benzo(6,7)cyclohepta(1,2-c)pyridazin-3-yl)-N3-((7-pyrrolidin-1-yl)-6,7,8,9-tetrahydro-5H-benzo(7)annulene-2-yl)-1H-1,2,4-triazole-3,5-diamine (R428), which exhibits high activity against Axl signaling (EC50/IC50 of 14 nM). Its activity is limited to the tyrosine kinase subfamily, with 50- and >100-fold higher selectivity for Axl than for Mer and Tyro3 (Holland et al., 2010).
Materials and Methods
Culture and Differentiation of Embryonic Stem Cells.
Embryonic stem (ES) cells were derived from the C57BL/6 mouse strain. They are feeder-dependent and were maintained in a pluripotent undifferentiated state in vitro by using TX-WES medium (Thrombogenics, Leuven, Belgium) (Schoonjans et al., 2003). Differentiation into adipocytes was induced essentially as described previously (Dani et al., 1997). Therefore, ES cells were harvested and resuspended at a cell density of 5 × 105 cells/ml in Glasgow/BHK 21 medium (Invitrogen, Paisley, UK) containing 0.23% sodium bicarbonate, 1× minimal essential medium essential amino acids, 2 mM glutamine, 100 μM 2-mercaptoethanol, and 10% fetal calf serum (HyClone Laboratories, Logan, UT) (differentiation medium). Microdrops of 20 μl were positioned for the hanging drop culture (day 0) and allowed to grow for 2 days and differentiate into embryoid bodies (EBs) (Desbaillets et al., 2000). The EBs were then transferred into bacteriological plates (day 2) and maintained for 3 days in suspension in differentiation medium only or supplemented with 10 nM all-trans retinoic acid, which activates early-stage adipogenesis (Bost et al., 2002). After another 2 days in suspension in differentiation medium only, EBs were plated in gelatin-coated plates in differentiation medium only or supplemented with 85 nM insulin, 2 nM tri-iodothyronine, and 1 μM rosiglitazone-maleate (day 7). Medium was changed every 2 days. At regular time points during the differentiation process, cell lysates were taken for RNA extraction. After 30 days, the extent of adipocyte differentiation was estimated by staining with Oil Red O. In brief, cells were washed with phosphate-buffered saline (PBS) and fixed for 5 min with a 1.5% glutaraldehyde solution in PBS. Cells were washed with PBS, stained for 2 h with a 0.2% Oil Red O solution, washed, and kept in tissue culture water. The Oil Red O stained lipid droplets were first characterized by light microscopy, then extracted in isopropanol and quantified from the absorbance at 490 nm.
Culture and Differentiation of Preadipocytes.
3T3-F442A murine preadipocytes (Green and Kekinde, 1976) were cultured and differentiated as described previously (Scroyen et al., 2010). On day 12, the cultures were stained for triglycerides with Oil Red O as described above. Experiments were performed with the addition of R428 (50 nM–1 μM) or DMSO as control.
Axl Cell-Based Assay.
3T3-F442A preadipocytes were seeded in Dulbecco's modified Eagle's medium with 0.5% bovine calf serum iron-supplemented in 96-well plates. After 24-h starvation, 1 μM R428 (diluted in DMSO) or DMSO control was added to the cells for 1 h. Cells were stimulated for 5 min at 37°C with preclustered, prewarmed antibody mixture [37.5 μg/ml mouse anti-Axl (R&D Systems, Minneapolis, MN) plus 100 μg/ml goat anti-mouse IgG (Jackson ImmunoResearch Laboratories Inc., West Grove PA) in starvation medium, mixed at 4°C for 2 h]. Cells were fixed in 4% formaldehyde, quenched in 3% H2O2/0.1% sodium azide in wash buffer (0.1% Triton X-100 in PBS), blocked (in 5% bovine serum albumin), and incubated overnight at 4°C with diluted antiphospho-Akt Ser473 in blocking buffer under gentle shaking. After removal of the primary antibody, cells were incubated with diluted goat anti-rabbit-horseradish peroxidase (Jackson ImmunoResearch Laboratories Inc.), and developed with tetramethylbenzidine substrate (optical density at 450 nm) (R&D Systems), after stopping the reaction with 2 M H2SO4.
mRNA Expression Analysis.
Total DNA-free RNA was extracted from cells using the RNA Easy kit (QIAGEN, Valencia, CA), and the concentration was determined using the RiboGreen RNA quantification kit (Invitrogen, Carlsbad, CA).
The expression of GAS6 and its receptors was monitored by quantitative real-time PCR, using specific primers and probes (gene expression assays, Mm00490389_m1 for GAS6, Mm00627285_m1 for Axl, Mm00444547_m1 for Tyro3 and Mm00434922_m1 for Mer; Applied Biosystems, Foster City, CA). Reverse transcription reactions were performed from 50 ng of total RNA at 48°C for 60 min using the Taqman Reverse Transcription Kit supplemented with 5 μM random hexamers (Applied Biosystems). Quantitative real-time PCR was performed in the ABI 7500 fast sequence detector using Taqman Fast Universal PCR Master Mix (Applied Biosystems). The data were normalized to the expression of β-actin, as housekeeping gene. Analysis of the CT values was performed with the ΔΔCT method using 7500 Fast System SDS software (Applied Biosystems).
Male wild-type C57BL/6 mice were generated in the Katholieke Universiteit Leuven animal facility. Mice were kept in individual microisolation cages on a 12-h day/night cycle at 20 to 22°C and fed a high-fat diet (HFD; Harlan Teklad, Zeist, The Netherlands; 42% kcal as fat, caloric value 20.1 kJ/g). Water was always available at libitum. At the age of 5 weeks, mice were kept on the HFD for 3 weeks, and at the age of 8 weeks, one group (n = 12) was started on R428 at a dose of 75 mg/kg/day by oral gavage, and a second group (n = 12) was given 25 mg/kg, twice daily. Concentrations were adjusted to allow administration in the same volume for different doses. R428 (Holland et al., 2010) was a kind gift from Dr. S. Holland of Rigel Inc. (South San Francisco, CA). Food intake was monitored daily, and control groups were started receiving the same volume of vehicle (0.5% hydroxypropylmethylcellulose and 0.1% Tween 80) as the R428-treated groups, given as a single administration (n = 10) or twice daily (n = 11). The control groups were pair-fed compared with the treated groups. The study was continued for 5 weeks and monitored as described below.
Body weight was measured daily, and body temperature was measured at weekly intervals using a rectal probe (TR-100; Fine Science Tools, Foster City, CA). Physical activity (voluntary wheel running) at night was monitored in cages equipped with a turning wheel linked to a computer to register full turns/12 h (7:00 PM-7:00 AM). At the end of the experiments, after overnight fasting, mice were euthanized by intraperitoneal injection of 60 mg/kg pentobarbital (Nembutal; Abbott Laboratories, Abbott Park, IL). Blood was collected via the retro-orbital sinus on trisodium citrate (final concentration, 0.01 M); plasma was prepared by centrifugation at 4°C at 4000g for 5 min and stored at −80°C. Intra-abdominal (gonadal, GON) and inguinal SC fat pads were removed and weighed; portions were snap-frozen in liquid nitrogen for RNA extraction, and paraffin sections (10 μm) were prepared for histology and immunohistochemistry. Other organs including kidneys, lungs, spleen, pancreas, liver, heart, and brain were also removed and weighed.
All animal experiments were approved by the local ethical committee (Katholieke Universiteit Leuven P03112) and performed in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals (1996).
Analysis of Adipose Tissues.
The size and density of adipocytes or blood vessels in the adipose tissues were determined by staining with hematoxylin/eosin under standard conditions or with Bandeiraea simplicifolia lectin, followed by signal amplification with the Tyramide Signal Amplification Cyanine System (PerkinElmer Life and Analytical Sciences, Waltham, MA), as described previously (Laitinen, 1987; Van Hul and Lijnen, 2008). Blood vessel density was normalized to the adipocyte number. Analysis was performed using a Zeiss Axioplan 2 Imaging microscope with AxioVision rel. 4.6 software (Carl Zeiss GmbH, Jena, Germany).
Macrophage infiltration in adipose tissues was quantitated after staining with anti-Mac-3 antibody (BD Biosciences Pharmingen, San Diego, CA) and expressed as a percentage of the total section area. Apoptosis was monitored by staining with rabbit anti-active mouse caspase 3 (R&D Systems) and expressed in percentage of cells.
Blood glucose concentrations were measured using Glucocard strips (Menarini Diagnostics, Firenze, Italy). Other metabolic parameters and liver enzymes, including triglycerides, total cholesterol, HDL cholesterol, low-density lipoprotein cholesterol, alkaline phosphatase, AST, and alanine aminotransferase were determined using standard laboratory assays. Insulin (Mercodia, Uppsala, Sweden) and leptin (R&D Systems Europe, Lille, France) levels were measured using commercial enzyme-linked immunosorbent assays and PAI-1 antigen with a specific homemade enzyme-linked immunosorbent assay (Declerck et al., 1995). Blood cell analysis, including white blood cells, neutrophils, lymphocytes, monocytes, eosinophils, basophils, red blood cells, platelet count, hemoglobin, and hematocrit levels, was performed on a Cell-Dyn 3500R (Abbott Diagnostics, Abbott Park, IL).
To monitor the expression of GAS6 and its receptors, adipose tissues were homogenized using lysing matrix tubes (Qbiogene Inc., Irvine, CA) in a Hybaid RiboLyser (Thermo Fisher Scientific, Walham, MA). Total DNA-free RNA was extracted, and quantitative reverse transcription-PCR was performed as described above, using glyceraldehyde 3-phosphate dehydrogenase as housekeeping gene.
For statistical analysis data were first averaged per mouse and are given as means ± S.E.M. for the number of animals studied. Statistical significance between groups was evaluated by nonparametric Mann-Whitney U test or two-way ANOVA (only where specifically indicated). Correlations were examined using the nonparametric Spearman's rank correlation coefficient. Values of p < 0.05 are considered statistically significant.
Expression of GAS6 and Its Receptors at Different Stages of Adipogenesis
To monitor the expression of GAS6 and its receptors at different stages of adipogenesis, we measured mRNA levels during differentiation of ES cells and 3T3-F442A preadipocytes into mature adipocytes.
During differentiation of ES cells, expression of GAS6, Axl, Mer, and Tyro3 increased with time. At day 30 of differentiation, significantly higher expression of GAS6, Axl, Mer, and Tyro3 was observed in adipose EBs compared with control EBs (Fig. 1, left). Oil Red O staining revealed the presence of lipid droplets in approximately 80% of adipose EB outgrowths compared with 15% for control EBs (not shown).
As 3T3-F442A preadipocytes reached confluence, GAS6 mRNA levels increased progressively. In treatment with the differentiation medium (day 4), GAS6 mRNA declined sharply to reach the level of preconfluent cultures. A similar expression pattern was observed for Mer, whereas mRNA levels for Axl and Tyro3 declined during differentiation (Fig. 1, right). Oil Red O staining (OD 490 nm of 0.80–1.20 compared with 0.10–0.20 at the start) confirmed effective differentiation (not shown).
Effect of the Axl Antagonist R428 on In Vitro Differentiation of 3T3-F442A Preadipocytes
Addition of R428 (50 nM-1 μM) resulted in a dose-dependent inhibition of differentiation of 3T3-F442A preadipocytes into mature adipocytes, as indicated by less Oil Red O-positive lipid droplets on day 12 in the R428-treated cells then in the control cells treated with DMSO. Oil Red O staining ranged between 84 and 35% of that of DMSO control at R428 concentrations between 50 nM and 1 μM (mean of duplicate determinations in one experiment; data not shown). Inhibition of Axl signaling by R428 in differentiating preadipocytes was confirmed by the Axl cell-based assay, yielding lower values (A450) for phospho-Akt activity upon treatment with 1 μM R428 (0.26 ± 0.02; n = 12) compared with medium control (0.51 ± 0.03; n = 8) or DMSO control (0.37 ± 0.02; n = 8).
Effect of the Axl Antagonist R428 on In Vivo Adipose Tissue Formation
Effect on Body and Fat Pad Weight and Composition.
To evaluate the effect of R428 treatment on ongoing development of obesity, 8-week-old male mice, previously kept on a HFD for 3 weeks, were given R428 as a single dose of 75 mg/kg/day or as two doses of 25 mg/kg/day. Control mice were kept on the same administration scheme with vehicle and pair-fed. In the group receiving two times 25 mg/kg/day R428 one mouse was killed after 2 weeks, and in the corresponding control group two mice were killed after 2 and 3 weeks (stopped eating).
At day 35, the last day of HFD feeding, the body weight in both groups treated with R428 was significantly lower than in the corresponding vehicle-treated groups (Table 1). Compared with the start of the experiment, body weights at the end were significantly increased in both vehicle-treated groups (p = 0.018 and 0.003 for once and twice administration, respectively), but not in R428-treated groups (p = 0.42 and 0.43 for once and twice administration, respectively). Thus, the weight gain over 5 weeks (Fig. 2) was significantly lower in the R428-treated group compared with vehicle-treated groups: 0.79 ± 0.36 g versus 3.1 ± 0.59 g (p = 0.015) for the 75 mg/kg dose and 0.34 ± 0.45 g versus 4.1 ± 0.61 g (p = 0.0004) for the two times 25 mg/kg dose. Food intake in the different groups before the start of the administrations ranged between 3.0 ± 0.11 and 3.1 ± 0.06 g/day. The food intake during pair-feeding was not significantly different between groups (Table 1), suggesting a reduced feeding efficiency (weight gain normalized to caloric intake) in the R428-treated mice. Body temperature remained constant at 37 to 38°C over the 5-week experimental period and was not different between groups (not shown).
Voluntary wheel running at night was comparable for mice treated with two times with 25 mg/kg of R428 or vehicle (4280 ± 790 versus 6190 ± 1680 turns/night; p = 0.32), whereas for mice treated with one dose of 75 mg/kg, this seemed somewhat reduced compared with vehicle (5410 ± 770 versus 9320 ± 1910 turns/night; p = 0.07). The isolated SC and GON fat mass was significantly lower in both R428-treated groups compared with the corresponding vehicle-treated groups (Table 1). Liver weights were significantly reduced in both R428-treated groups, whereas spleen weight was somewhat reduced and pancreas weight was somewhat enhanced in the 75 mg/kg/day R428 group (Table 1). Statistical analysis by two-way ANOVA confirmed a significant lowering effect of treatment (R428 versus placebo) on body weight at the end (with and without fasting) and on SC and GON fat mass (all p < 0.0001). Treatment also significantly reduced the weight of liver (p = 0.0005), left kidney (p = 0.02), and brain (p = 0.03). The administration scheme (once 75 mg/kg versus twice 25 mg/kg) only affected liver weight (p = 0.01).
Adipocyte size was significantly reduced in SC (p < 0.005) and GON (p < 0.01) fat pads of mice treated with two times 25 mg/kg of R428 and in SC fat pads (p < 0.05) of mice treated with 75 mg/kg compared with the corresponding vehicle-treated groups (Fig. 3 and Table 2). Blood vessel size was comparable for all groups. The blood vessel density in SC fat pads was significantly higher in both R428-treated groups (p < 0.01 and < 0.05) compared with the corresponding vehicle-treated groups. In contrast, we observed a lower blood vessel density in GON fat pads (p < 0.05) of mice treated with 75 mg/kg. Normalized blood vessel density was lower in both SC and GON fat pads of both R428-treated groups.
Further analysis revealed that macrophage infiltration was higher in SC and GON fat pads of mice treated with two times 25 mg/kg of R428, whereas in fat pads of mice treated with 75 mg/kg of R428 it was comparable with the corresponding vehicle-treated groups (Table 2). Apoptosis was somewhat enhanced in the SC and GON adipose tissues of mice treated with R428 compared with vehicle (Table 2). Two-way ANOVA confirmed a significant effect of treatment on adipocyte size (p < 0.0001 for SC; p = 0.03 for GON), adipocyte density (p < 0.0001 for SC; p = 0.001 for GON), blood vessel size (p = 0.02 for GON), blood vessel density (p = 0.0009 for SC), normalized blood vessel density (p = 0.03 for SC; p < 0.0001 for GON), and macrophage content (p = 0.04 for GON). The administration scheme had no effect on any of these parameters.
Expression of GAS6 and its receptors Axl, Tyro3, and Mer could be demonstrated in SC and GON adipose tissues of mice in all four groups. A statistically significant effect of R428 was observed only for expression of GAS6 (1.4-fold up-regulated; p = 0.03) and Mer (1.5-fold up-regulated; p = 0.04) in the GON fat of mice treated with the two times 25 mg/kg dose. Expression of Axl or Tyro3 in adipose tissues was not affected by R428 treatment (Table 3). Two-way ANOVA confirmed a significant enhancing effect of treatment on the expression of GAS6 (p = 0.005) and Mer (p = 0.004) in GON fat but no effects of the administration scheme.
Analysis of blood cell composition at the end of the experiment for both R428-treated groups compared with vehicle revealed somewhat lower total white blood cell counts (1.9 ± 0.43 × 103/μl versus 2.4 ± 0.23 × 103/μl for the two times 25 mg/kg dose, p = 0.06; 1.7 ± 0.29 × 103/μl versus 3.1 ± 0.52 × 103/μl for the 75 mg/kg dose; p = 0.04). This was associated with significantly enhanced content of neutrophils (40 ± 2.4% versus 16 ± 2.3% for the two times 25 mg/kg and 35 ± 2.7% versus 16 ± 2.3% for the 75 mg/kg dose; p < 0.0005 for both doses) and monocytes (5.7 ± 1.7% versus 1.6 ± 0.34% for the two times 25 mg/kg and 4.3 ± 0.81% versus 2.2 ± 0.44% for the 75 mg/kg dose; p < 0.05 for both doses), and reduced content of lymphocytes (49 ± 3.3% versus 79 ± 2.3% for the two times 25 mg/kg and 56 ± 3.2% versus 78 ± 3.2% for the 75 mg/kg dose; p < 0.001 for both doses). Other blood cell fractions were not significantly affected by R428 treatment (data not shown).
Effect on Plasma Metabolic Parameters.
At the dose of two times 25 mg/kg R428, levels of glucose, total cholesterol, HDL cholesterol, and triglycerides were reduced at the end of the experiment, whereas liver enzymes were not affected (Table 4). In contrast, at the dose of 75 mg/kg R428, metabolic parameters were not affected, whereas alkaline phosphatase and AST levels were enhanced. Insulin levels were not affected by R428 treatment, whereas PAI-1 levels were enhanced with both doses (p < 0.05). Plasma leptin levels were lower in R428-treated animals and correlated positively (all r > 0.87 and p < 0.0001) with SC and GON fat mass.
Two-way ANOVA showed a significant effect of treatment on levels of glucose (p = 0.03), PAI-1 (p = 0.0009), leptin (p < 0.0001), total cholesterol (p = 0.007), HDL cholesterol (p = 0.008), alkaline phosphatase (p = 0.02), and AST (p = 0.03). The administration scheme affected only alkaline phosphatase levels (p = 0.007).
Development of obesity arises from increased size of individual adipose cells caused by lipid accumulation and/or from an enhanced number of adipocytes upon differentiation of precursor cells into mature adipocytes under the appropriate nutritional and hormonal stimuli (Liu et al., 2003). Because GAS6 and its receptors Tyro 3, Axl, and Mer are expressed by adipocytes and GAS6-deficient mice develop less adipose tissue, we hypothesized that the GAS6 signaling pathway may play a role in adipogenesis and adipose tissue formation. First, we have confirmed expression and modulation of GAS6 and its receptors during differentiation of ES cells and preadipocytes into adipocytes, indicating their involvement already in the early stages of adipogenesis. To study their functional role we used a low-molecular-weight inhibitor (R428) of tyrosine kinase activity that was identified as an orally bioavailable inhibitor with high selectivity for Axl over the other receptors (Holland et al., 2010). The doses used in the present study are based on available pharmacokinetic data indicating a long plasma half-life in mice (4 h at 25 mg/kg and 13 h at 75 mg/kg given by oral gavage), effective tissue distribution, and high steady-state plasma drug concentrations. Thus, prolonged dosing at only 25 mg/kg b.i.d. generated a steady-state R428 concentration sufficient to block Axl signaling in the circulation (Holland et al., 2010).
We have confirmed that R428 inhibits Axl signaling in (pre)adipocytes, as monitored by decreased levels of phospho-Akt. Furthermore, R428 had the potential to impair differentiation of preadipocytes into mature adipocytes, as evidenced by decreased lipid uptake. This is compatible with the concept that enhanced GAS6 levels are associated with the transient phase of growth arrest that precedes the phase of clonal expansion (Maquoi et al., 2005).
In the present study we have also shown the potential of R428 to impair adipose tissue development in mice pair-fed a HFD. This is associated with marked adipocyte hypotrophy in SC and GON adipose tissues. The weight of other organs was overall comparable with the exception of a reduced liver weight at both doses of R428. In the study by Holland et al. (2010) prolonged dosing with R428 at 25 mg/kg b.i.d. for up to 80 days in mice kept on normal chow was not associated with significant weight loss. With HFD feeding in our study, however, the feeding efficiency of the mice (weight gain normalized to caloric intake) was significantly lower upon R428 treatment. A limitation of the present study is that no comprehensive energy balance has been made; we monitored only food intake, physical activity (voluntary wheel running), and body temperature.
At the dose of two times 25 mg/kg R428, reduced levels of total cholesterol, HDL cholesterol, and triglycerides were observed compared with vehicle. Glucose levels were comparable with those of the 75 mg/kg group but were lower than in the corresponding vehicle group in which some animals had somewhat enhanced glucose levels. It is conceivable that these reduced levels were caused by enhanced clearance, but the mechanism remains unclear, because Axl signaling has been implicated in many biological phenomena (Li et al., 2009).
We observed enhanced macrophage content in SC and GON adipose tissues of mice treated with the two times 25 mg/kg dose but not with the 75 mg/kg dose. It was shown previously that the Tyro3 family receptors limit macrophage activation (Lemke and Lu, 2003). The observation that higher degrees of cellular apoptosis were observed in the R428-treated adipose tissues is compatible with the notion that the GAS6 signaling pathway promotes cell survival. Because of marked variability between samples, these differences are, however, not statistically significant. Increased macrophage influx in adipose tissues may serve to help clear debris of apoptotic cells.
Previous studies have revealed that R428 has antiangiogenic potential in breast cancer models (Holland et al., 2010), which is in line with the reported proangiogenic function of Axl (Li et al., 2009). It was also reported (Gallicchio et al., 2005) that Axl stimulation by GAS6 results in the inhibition of the ligand-dependent activation of vascular endothelial growth factor receptor 2 and the consequent activation of an angiogenic program in vascular endothelial cells. In our study, GAS6 receptor antagonism had no effect on blood vessel size in the adipose tissues, whereas blood vessel density was somewhat enhanced in SC fat. This may be related to the fact that vascular endothelial growth factor receptor 2 expression in adipose tissues is not markedly affected by high-fat feeding (Voros et al., 2005). However, when the blood vessel density was normalized to the adipocyte number [because the number and/or size of adipocytes may affect blood vessel density (Cao et al., 2001)], a lower normalized density was observed in fat tissues treated with R428.
The two different administration schemes used in our study (75 or two times 25 mg/kg/day) may be associated with different peak levels and/or steady-state concentrations of R428, resulting in differential effects on some parameters. Overall, however, statistical analysis by two-way ANOVA confirmed a beneficial effect on adiposity and metabolic parameters of the treatment (R428 versus vehicle), whereas the administration scheme had no effect on body and adipose tissue weight or composition and affected only liver weight and alkaline phosphatase levels.
Thus, the GAS6 receptor antagonist R428 has the potential to impair preadipocyte differentiation and reduce adipose tissue development in vivo, associated with marked adipocyte hypotrophy. Although R428 is a highly specific Axl antagonist, it cannot be excluded that at the doses achieved in vivo it also blocks Tyro 3 and/or Mer and may affect other pathways. Overall, however, affecting GAS6 signaling through the TAM receptor family seems to have the potential to impair adiposity.
Participated in research design: Lijnen and Scroyen.
Conducted experiments: Christiaens and Scroyen.
Performed data analysis: Lijnen, Christiaens, and Scroyen.
Wrote or contributed to the writing of the manuscript: Lijnen, Christiaens, and Scroyen.
Other: Lijnen acquired funding for the research.
We thank L. Frederix, S. Helsen, C. Vranckx, B. Van Hoef, and A. De Wolf for skillful technical assistance.
This study was supported by the Interuniversity Attraction Poles [Grant P6/30]. The Center for Molecular and Vascular Biology is supported by the Programmafinanciering Katholieke Universiteit Leuven [Grant PF/10/014].
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
- growth arrest-specific protein 6
- embryoid body
- embryonic stem
- high-fat diet
- analysis of variance
- phosphate-buffered saline
- dimethyl sulfoxide
- polymerase chain reaction
- high-density lipoprotein
- aspartate aminotransferase
- plasminogen activator inhibitor 1
- Received December 8, 2010.
- Accepted January 31, 2011.
- Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics