GM-CSF induces expression of soluble VEGF receptor-1 from human monocytes and inhibits angiogenesis in mice

Abstract

GM-CSF promotes homeostasis of myeloid cells. We report that GM-CSF upregulates mRNA and protein production of the soluble form of membrane bound VEGF receptor-1 (sVEGFR-1) in human monocytes. This sVEGFR-1 was biologically active, as cell-free supernatants from GM-CSF-stimulated monocytes blocked detection of endogenously expressed VEGF and inhibited endothelial cell migration and tube formation, even in the presence of exogenous rhVEGF. VEGF activity was recovered by neutralizing sVEGFR-1. To determine whether these events were important in vivo, Matrigel plugs were incubated with rhVEGF, rhGM-CSF, or rhGM-CSF/rhVEGF and injected into mice. Plugs containing GM-CSF or GM-CSF/VEGF had less endothelial cell invasion than plugs containing rhVEGF and were similar to plugs incubated with PBS alone. Neutralizing antibodies specific for sVEGFR-1 injected in these plugs reversed the effects of GM-CSF or GM-CSF/VEGF, while an isogenic antibody did not. Thus, GM-CSF and monocytes play a vital role in angiogenesis through the regulation of VEGF and sVEGFR-1.

Figure 1.

Proposed Model for the Inhibition of Pathological Angiogenesis This figure was based on models published by Scalerandi et al. (2002), Heuser et al. (1984), Shweiki et al. (1995), and Clarijs et al. (2003).

Figure 2.

The Reduction in VEGF Detection from GM-CSF-Stimulated Monocytes Was Not Due to Sequestration of VEGF within the Cells (A) Monocytes were left nonstimulated (white) or were stimulated with GM-CSF (100 ng/ml) (black) for 24 and 48 hr, and cell-free supernatants were subjected to VEGF ELISA. There was less VEGF detected by ELISA per monocyte in the GM-CSF-stimulated samples compared to nonstimulated samples (*p < 0.01 and **p < 0.001). These data represent the mean ± SEM calculated from three independent experiments. (B) Monocytes were left untreated (white) or were stimulated with GM-CSF (100 ng/ml) (black) for 2 and 24 hr. Cell lysates were assayed for VEGF by ELISA. At 24 hr, there is significantly less VEGF sequestered within monocytes due to GM-CSF treatment relative to untreated samples (*p < 0.05). These data represent the mean ± SEM calculated from four independent experiments. (C) Monocytes were left nonstimulated (NS) or were stimulated with M-CSF (100 ng/ml) (M-CSF) or GM-CSF (100 ng/ml) (GM-CSF) and were labeled with ³⁵S-Methionine/Cysteine. VEGF was purified from the cell-free supernatants by using heparin-agarose beads and separated on a SDS-PAGE gel, and densitometry was performed by using a phosphorimager. The VEGF band on the PAGE gel was clarified by detection of the band at the predicted size and was compared to the band at the same size purified from the supernatants of M-CSF-stimulated monocytes, used as a positive control. The photograph is representative of three independent experiments.

Figure 3.

Recombinant sVEGFR-1 Masks VEGF from ELISA Detection (A) Monocytes were left untreated (NS) or were treated with GM-CSF (100 ng/ml) (GM) for 24 hr. Supernatants were collected and subjected to VEGF ELISA. 450 pg/ml recombinant VEGF (rhVEGF added) was incubated with these supernatants at 37°C for 30 min, and supernatants were subjected to VEGF ELISA (NS sups + rhVEGF) and (GM sups + rhVEGF). There are significant differences in rhVEGF concentrations in both GM-CSF-stimulated samples compared to nonstimulated samples (*p < 0.001), but there is no difference between the GM-CSF-stimulated samples (GM sups) alone versus GM-CSF-stimulated supernatants supplemented with rhVEGF (GM sups + rhVEGF) (p > 0.85). These data represent the mean ± SEM calculated from three independent experiments. (B) Increasing concentrations of recombinant sVEGFR-1 (1, 8, and 16 ng/ml) were incubated with 600 pg/ml rhVEGF, and VEGF ELISA was performed to investigate the ability of sVEGFR-1 to mask VEGF from antigenic detection of the ELISA. There is a significant dose-dependent decrease in the detection of rhVEGF due to the presence of sVEGFR-1 (*p < 0.01 versus VEGF + sVEGFR-1 [1 ng/ml]; **p < 0.001 versus VEGF added; ***p < 0.001 versus VEGF + sVEGFR-1 [1 ng/ml]). These data represent the mean ± SEM calculated from three independent experiments.

Figure 4.

GM-CSF Augments sVEGFR-1 mRNA Transcription and Protein Production While Concomitantly Reducing the Detection of VEGF by ELISA (A) Monocytes were left nonstimulated or were stimulated with M-CSF (100 ng/ml) or GM-CSF (100 ng/ml) for 24 hr, and the supernatants were subjected to sVEGFR-1 ELISA. GM-CSF-stimulated supernatants have significantly increased levels of sVEGFR-1 compared to M-CSF-stimulated or nonstimulated controls (*p < 0.001). These data represent the mean ± SEM calculated from seven independent monocyte donors. (B) Monocytes were left nonstimulated (white) or were stimulated with GM-CSF (100 ng/ml) (black) for 4, 16, 24, 48, or 72 hr, and total cellular RNA was subjected to real-time PCR. sVEGFR-1 mRNA levels were significantly higher at both 24 and 48 hr versus nonstimulated samples at the same time points (*p < 0.01 and **p < 0.001). These data represent the mean ± SEM calculated from three independent experiments. (C) Monocytes were left nonstimulated or were stimulated with GM-CSF (100 ng/ml) for 24 hr. Supernatants (black) and cell lysates (white) were subjected to sVEGFR-1 ELISA. There is significantly more sVEGFR-1 released into the supernatant than what remains within the cells (*p < 0.001). These data represent the mean ± SEM calculated from six individual monocyte donors. (D) Monocytes were left nonstimulated or were stimulated with 0.1, 1, 10, or 100 ng/ml GM-CSF for 48 hr, and supernatants were subjected to sVEGFR-1 ELISA. There is a significant increase in sVEGFR-1 in both the 10 ng/ml and 100 ng/ml GM-CSF-treated samples compared to the 0, 0.1, and 1 ng/ml GM-CSF-stimulated samples (*p < 0.01 versus 0, 0.1, and 1 ng/ml GM-CSF-treated samples; **p < 0.05 versus 0, 0.1, 1, and 10 ng/ml GM-CSF-treated samples). These data represent the mean ± the SEM from three independent monocyte donors. (E) Supernatants from the GM-CSF dose-dependent trials in (D) were concomitantly assayed by VEGF ELISA to analyze the concentration of GM-CSF that reduced VEGF antigenic detection by VEGF ELISA. GM-CSF at both 10 ng/ml and 100 ng/ml stimulates significant reduction of VEGF in a dose-dependent manner compared to 0, 0.1, and 1 ng/ml GM-CSF-stimulated samples (*p < 0.01 versus 0, 0.1, and 1 ng/ml GM-CSF-treated samples). These data represent the mean ± SEM from three independent monocyte donors. (F) Monocytes were stimulated with GM-CSF (100 ng/ml) and were incubated for 24 hr. Supernatants were incubated with an antibody specific for sVEGFR-1 (sVR1 Ab) (2.5 μg/ml) or an isogenic IgG control antibody (IgG Ab) (2.5 μg/ml) for 30 min at 37°C and were then removed by using protein G beads. Supernatants were subjected to VEGF ELISA ([GM sups + sVR1 Ab] and [GM sups + IgG Ab]) for the presence of VEGF. rhVEGF (900 pg/ml) (rhVEGF Added) was incubated with some samples ([rhVEGF Added], [GM sups + sVR1 Ab + rhVEGF], and [GM sups + IgG Ab + rhVEGF]) for 30 min and subjected to VEGF ELISA. Supernatants from GM-CSF-stimulated monocytes incubated with the antibody for sVEGFR-1 allowed rhVEGF detection by ELISA, while the supernatants from GM-CSF-stimulated monocytes incubated with the isogenic IgG antibody did not (*p < 0.001 for VEGF detection versus GM sups + IgG Ab sample and no significant difference compared to rhVEGF Added sample). These data represent the mean ± SEM calculated from three individual monocyte donors.

Figure 5.

Supernatants from rhGM-CSF-Stimulated Monocytes Inhibit Angiogenesis Effects In Vitro (A) HUVECs were cultured in Matrigel as follows: HUVECs with EBM media (1 ml) (HUVECs); cells + rhVEGF (2.5 ng/ml) (VEGF); cells + 1 ml supernatants from rhGM-CSF (100 ng/ml)-stimulated monocytes + VEGF (2.5 ng/ml) (GM-CSF sups + VEGF); cells + 1 ml supernatants from rhGM-CSF (100 ng/ml)-stimulated monocytes + VEGF (2.5 ng/ml) + α-sVEGFR-1 neutralizing antibodies (2 μg/ml) (GM-CSF sups + VEGF + anti-VEGFR-1 Ab); or cells + 1 ml supernatants from rhGM-CSF (100 ng/ml)-stimulated monocytes + VEGF (2.5 ng/ml) + IgG₁ isotype antibodies (2 μg/ml) (GM-CSF sups + VEGF + IgG₁ isotype Ab). Pictures are representative of three independent trials. (B) Tubule branch points from HUVECs stimulated as indicated above were counted, and the sum of three different fields for each condition was averaged. Supernatants from monocytes stimulated with GM-CSF supernatants + rhVEGF (2.5 ng/ml) (GM-CSF sups + VEGF) for 24 hr significantly inhibited endothelial cells from forming tube branch points compared to the rhVEGF (2.5 ng/ml) control (VEGF), and inhibition was not statistically different from cells in EBM alone (HUVECs). GM-CSF supernatants incubated with rhVEGF (2.5 ng/ml) and neutralizing antibodies specific for sVEGFR-1 (2 μg/ml) (GM-CSF sups + VEGF + α-sVEGFR-1 Ab) rescued HUVEC tube formation and restored it to levels similar to rhVEGF control levels; these levels were significantly different from levels of GM-CSF supernatants incubated with rhVEGF (2.5 ng/ml) and IgG₁ isotype antibodies (2 μg/ml) (GM-CSF sups + VEGF + IgG₁ isotype Ab) (*p < 0.05 versus HUVECs, GM-CSF sups + VEGF, and GM-CSF sups + VEGF + IgG₁ isotype Ab). Error bars represent the mean ± SEM calculated from three independent studies. (C) HUVECs were grown on Matrigel as follows: HUVECs with EBM media (1 ml) (HUVECs); cells + rhVEGF (5 ng/ml) (VEGF); or cells + rhVEGF (5 ng/ml) + rh sVEGFR-1 (50 ng/ml) (VEGF+ recomb. sVEGFR-1). The circular photographs are shown at 40× magnification. Black boxes within the panels indicate areas at 200× magnification, which are shown below each panel. Arrows denote tubule formation. Photos are representative of two independent trials. (D) Quantification of tubule branch points from the photographs in (C). There are significantly more tubule branch points in the VEGF condition versus media alone and VEGF + recombinant sVEGFR-1 (p < 0.05 and p < 0.01, respectively).

Figure 6.

rhGM-CSF Treatment Inhibits Angiogenesis in a Matrigel Plug Assay in Mice Qualitative representation of angiogenesis in mice in response to Matrigel plugs treated with the following conditions: (A) Matrigel supplemented with PBS. (B) Matrigel supplemented with rhVEGF (10 ng/ml). (C) Matrigel supplemented with rhGM-CSF (10 ng/ml). (D) Matrigel supplemented with rhGM-CSF (10 ng/ml) + rhVEGF (10 ng/ml). (E) Matrigel supplemented with rhGM-CSF (100 ng/ml). (F) Matrigel supplemented with rhGM-CSF (100 ng/ml) + rhVEGF (10 ng/ml). (G) Matrigel supplemented with rhGM-CSF (100 ng/ml) + rhVEGF (10 ng/ml) + α-sVEGFR-1 antibodies (2 μg/ml). (H) Matrigel supplemented with rhGM-CSF (100 ng/ml) + rhVEGF (10 ng/ml) + IgG₁ isotype antibodies (2 μg/ml). (I) Relative count of CD31 (+) cells (black) and CD68 (+) cells (mononuclear cells) (white) that penetrate the Matrigel plugs in response to stimuli; cells were observed by using a 40× objective. There was significantly more CD31 (+) cells in Matrigel plugs treated with (B) rhVEGF (10 ng/ml) alone and (G) GM-CSF (100 ng/ml) + rhVEGF (10 ng/ml) + α-sVEGFR-1 antibodies (2 μg/ml) than in plugs treated with (E) GM-CSF (100 ng/ml), (F) GM-CSF (100 ng/ml) + rhVEGF (10 ng/ml), or (H) GM-CSF (100 ng/ml) + rhVEGF (10 ng/ml) + IgG isotype antibody (2 μg/ml) (*p < 0.05). (J) Total cell counts identified by H&E staining for cell nuclei that penetrated the Matrigel plugs in response to stimuli; cells were observed by using a 40× objective. (E) GM-CSF (100 ng/ml)-treated plugs displayed reduced blood vessel presence within the plugs compared to (B) rhVEGF (10 ng/ml) alone and those plugs containing (G) rhGM-CSF + VEGF + α-sVEGFR-1 neutralizing antibodies. The (E) GM-CSF (100 ng/ml)- and (C) GM-CSF (10 ng/ml)-treated plugs had less cells than plugs treated with (B) VEGF (10 ng/ml) or with (G) sVEGF-R Ab (*p < 0.003 versus the GM-CSF (10 ng/ml) + VEGF (10 ng/ml) sample, **p < 0.003 versus the GM-CSF [10 ng/ml] sample, and ***p < 0.001 versus each of the PBS, GM-CSF [100 ng/ml], and GM-CSF [100 ng/ml] + VEGF [10 ng/ml] samples). Values represent the number of total cell nuclei per 40× field. At least three mice were tested per group. Note: Large pictures are shown at 400× magnification, and insets are shown at 100× magnification for all samples. (K) C57BL/6 mice were sacrificed, and bone marrow from the femur was collected. After washing in RPMI-1640, the bone marrow was cultured and either left nonstimulated, stimulated with hGM-CSF (100 ng/ml), or stimulated with mGM-CSF (100 ng/ml). Every 2 days, the media was collected and fresh media and GM-CSF treatment was administered. After 10 days, the plates were washed with PBS and pictures taken (5 per well). (L) Cell counts represent the percentage of mGM-CSF activity (ability to induce murine bone marrow cell maturation) from Figure 7K. Human GM-CSF induces murine bone marrow cell maturation significantly more than PBS treatment (nonstimulated) (p < 0.05).

Figure 7.

Investigation of the Mechanism of VEGFR-1 Expression and sVEGFR-1 Secretion from Human Monocytes (A) A population of CD14 (+) cells isolated by flow cytometry by using α-human CD14-phycoerythrin antibodies. (B) Fresh CD14 (+) cells were analyzed for VEGFR-1 expression by using mouse α-human VEGFR-1-phycoerythrin (shaded) and mouse α-human IgG₁-phycoerythrin (open) antibodies. There is no significant difference between the isotype and VEGFR-1 antibody (p > 0.5). These data represent the mean ± SEM calculated from three independent monocyte donors. (C) Nonstimulated CD14 (+) cells were cultured for 24 hr and were analyzed for VEGFR-1 expression (shaded) and IgG₁ (open) compared to fresh cells (B). There is a significant difference between fresh and nonstimulated monocytes after 24 hr for VEGFR-1 expression (p < 0.05). These data represent the mean ± SEM calculated from three independent monocyte donors. (D) GM-CSF (100 ng/ml)-stimulated CD14 (+) cells were cultured for 24 hr and were analyzed for VEGFR-1 expression (shaded) and IgG₁ (open) compared to fresh cells (B). There is a significant difference in VEGFR-1 expression between fresh and GM-CSF-stimulated monocytes after 24 hr (p < 0.05), and there is no significant difference in VEGFR-1 expression between nonstimulated and GM-CSF-stimulated monocytes at 24 hr in culture (p > 0.2). These data represent the mean ± SEM calculated from three independent monocyte donors. (E) Monocytes were left untreated (NS), were left untreated with DMSO (NS + DMSO), or were treated with GM-CSF (100 ng/ml) (GM) in combination with inhibitors (AG490, LY294002, or SP600125) for 24 hr. Supernatants were subjected to sVEGFR-1 (black bars) and VEGF (white bars) ELISAs. AG490 (JAK2), LY294002 (PI3-kinase), and SP600125 (JNK) reduced the production of sVEGFR-1 (one closed circle, p < 0.05 versus GM-CSF + 50 μM AG490); two closed circles, p < 0.05 versus GM-CSF + 20 μM LY294002 one closed diamond, p < 0.03 versus GM-CSF + 80 μM SP600125; and two closed diamonds, p < 0.02 versus GM-CSF alone). Concomitantly, all three inhibitors rescued the detection of VEGF (one open circle, p < 0.05 versus GM-CSF + 20 μM LY294002; two open circles, p < 0.02 versus nonstimulated; one open diamond, p < 0.02 versus GM-CSF + 80 μM SP600125; and two open diamonds, p < 0.001 versus GM-CSF + 50 μM AG490). These data represent the mean ± SEM calculated from three independent monocyte donors.