Enhanced TGF-β Signaling Contributes to the Insulin-Induced Angiogenic Responses of Endothelial Cells

Abstract

Angiogenesis, the development of new blood vessels, is a key process in disease. We reported that insulin promotes translocation of transforming growth factor β (TGF-β) receptors to the plasma membrane of epithelial and fibroblast cells, thus enhancing TGF-β responsiveness. Since insulin promotes angiogenesis, we addressed whether increased autocrine TGF-β signaling participates in endothelial cell responses to insulin. We show that insulin enhances TGF-β responsiveness and autocrine TGF-β signaling in primary human endothelial cells, by inducing a rapid increase in cell surface TGF-β receptor levels. Autocrine TGF-β/Smad signaling contributed substantially to insulin-induced gene expression associated with angiogenesis, including TGF-β target genes encoding angiogenic mediators; was essential for endothelial cell migration; and participated in endothelial cell invasion and network formation. Blocking TGF-β signaling impaired insulin-induced microvessel outgrowth from neonatal aortic rings and modified insulin-stimulated blood vessel formation in zebrafish. We conclude that enhanced autocrine TGF-β signaling is integral to endothelial cell and angiogenic responses to insulin.

Keywords: Cell Biology; Functional Aspects of Cell Biology; Molecular Biology; Molecular Mechanism of Behavior.

Graphical abstract

Figure 1

Insulin Enhances TGF-β Receptor Abundance at the Plasma Membrane and Induces Smad2 and Smad3 Activation through Autocrine TGF-β Signaling (A and B) Immunoblot analyses of the TβRI and TβRII receptors in human umbilical vein endothelial cells (HUVECs), treated with 100 nM insulin, in the absence (A) or presence (B) of the Akt inhibitor AktVIII. The top two panels show the cell surface TβRI and TβRII receptors, affinity labeled by cell surface protein biotinylation, isolated by affinity adsorption to Neutravidin, and visualized by SDS-PAGE and immunoblotting. The lower panels show TβRI and TβRII in total cell lysates, with GAPDH as loading control. Inhibition of Akt activation prevents the insulin-induced increase in TβRI or TβRII abundance at the cell surface. Immunoblots are representative of three independent experiments. Control group (0) was without insulin for 540 min in starvation media. (C) HUVECs were treated with 100 nM insulin, with or without SB431542, for the indicated times, and Smad2 and Smad3 activation was assessed by immunoblotting for C-terminally phosphorylated Smad2 (pSmad2) or Smad3 (pSmad3). Control group (0) was without insulin for 360 min in starvation media. The changes in Smad2 or Smad3 phosphorylation are densitometrically shown on the right. (D) qRT-PCR of SMAD7 and SERPINE1 mRNA expressed by cells treated with or without insulin (Ins) in the presence or absence of SB431542 (SB) for 90 min and normalized against RPL19 mRNA. Error bars indicate standard error of means, based on four independent experiments. *p < 0.05. In panels A, B, and C: AP, affinity purification, IB, immunoblot.

Figure 2

Relative mRNA Levels of Selected Genes, Encoding Transcription Factors, Enzymes, Membrane Proteins, and Ligands or Secreted Proteins, Known to be Involved in Angiogenesis (A) mRNA expression of the indicated genes was measured using qRT-PCR, and values were normalized to RPL19 mRNA. Error bars indicate standard error of the means, based on three independent experiments. Inhibition of TGF-β signaling using SB431542 (SB) repressed the insulin-induced changes of the mRNA expression of many but not all genes. The statistical significance was determined by Wilcoxon test; *p < 0.05, **p < 0.0083. (B) Selected disease and Bio function analysis using IPA comparing different treatment groups. Values are activation Z scores. Z score represents the IPA regulation trend (Z score > 0: up-regulation [blue]; Z score < 0: down-regulation [red]).

Figure 3

Contribution of TGF-β Signaling to Insulin-Induced Cell Migration and Invasion (A–C) Migration of HUVECs measured in a monolayer wounding assay. Confluent monolayers were scratched with a pipette tip at time 0 and cells were allowed to migrate into the wounded area for 8 h, in the presence or absence of insulin with or without SB431542 (A and B) or the neutralizing anti-TGF-β antibody 1D11 (C). The migration distance is graphically presented. Error bars indicate standard error of the means of three experiments. The statistical significance was determined by Wilcoxon test; *p < 0.05, **p < 0.0083. (D and E) mRNA expression of RHOB, encoding a Rho-related GTP-binding protein, SNAI1 and SNAI2 (D), as well as CDH5, encoding VE-cadherin and FN, encoding fibronectin (E), after 90 min or 6 h insulin treatment, in the presence or absence of SB431542, determined by qRT-PCR and normalized against RPL19 mRNA. Statistical significance was determined by Wilcoxon test; *p < 0.05, **p < 0.0083. Ab, antibody. (F) Immunoblots of extracted HUVEC lysates treated for 48 h in the presence or absence of insulin with or without SB431542. Protein expression changes were assessed by immunoblotting for Snail2, N-cadherin, VE-cadherin, and fibronectin, with GAPDH as loading control. IB, immunoblot. (G) Invasion of HUVECs in Transwell assays in the presence or absence of insulin, with or without SB431542 or 1D11 antibody. HUVECs were treated with insulin with or without SB431542 or the anti-TGF-β neutralizing antibody 1D11 for 8 h, and the invaded cells at the bottom filter surface were DAPI-stained and counted. The microscopic fields show cells that invaded through the filter as black dots against a white background. The results, shown graphically, are averages of five random microscopic fields of three separate experiments, each conducted in duplicate. Scale bar, 100 μM. Statistical significance was determined by Wilcoxon test; *p < 0.05, **p < 0.0083. ns, non-significant. SB, SB431542. Ins, insulin.

Figure 4

Effects of Inhibiting TGF-β Signaling in Insulin-Induced Endothelial Network Formation (A) HUVECs were plated onto Matrigel with or without insulin in the absence or presence of SB431542. Representative images are shown after 6 h of treatment. (B and C) Quantitative analyses of numbers of nodes (red) and meshes (blue), relative to untreated controls, in experiments with or without SB431542 (B) or 1D11 antibody (C). Error bars represent mean ± SEM of three independent experiments. Statistical significance was determined by Wilcoxon test; *p < 0.05, **p < 0.0083, ***p < 0.0001. Scale bar, 500 μM.

Figure 5

Effects of Insulin and SB431542 on Intersomitic Vessels in Zebrafish Embryos (A) Scheme representing the treatment protocol. Tg(fli:gfp) zebrafish embryos were treated for 72 h. (B) Expression of pck1 mRNA was quantified by qRT-PCR. RNA was extracted from pool of larvae that had received insulin with DMSO or DMSO control for 48 h. (C) Altered blood vessels in insulin- and SB431542-treated embryos. Representative images show the trunk vasculature of 96 hpf tg(fli:EGFP) zebrafish larvae. The boxes mark the region shown at higher magnification in the lower right corner of the panel. (D) Branching quantification of zebrafish blood vessels at 96 hpf, as seen in C in control solution DMSO, SB431542, insulin, and insulin + SB431542. Bars represent mean ± SD. Zf, zebrafish. Ctrl, control.

Figure 6

Contribution of TGF-β Signaling in Insulin-Induced Mouse Aortic Ring Microvessel Sprouting (A) Scheme of the mouse aorta ring experiments. Segments of aorta ring from 6-day-old mice were embedded in Matrigel and incubated with insulin and/or SB431542 or a neutralizing anti-TGF-β antibody or control IgG for 96 h. (B) Microvessel sprouting from aortic ring segments cultured ex vivo for 96 h with or without insulin in the presence or absence of SB431542 or anti-TGF-β antibody or control IgG. The microvessel growth in response to insulin is blocked by SB431542 or anti-TGF-β antibody in a mouse aorta ring assay. Representative images are shown. (C) Quantitative analysis of the ex vivo microvessel growth, measured as surface area to which vessels are extended. Error bars show ±SD. The experiments were repeated twice. Ctrl, control.