Context Dependent Role of the CD36 - Thrombospondin - Histidine-Rich Glycoprotein Axis in Tumor Angiogenesis and Growth

James Scott Hale,Willi Jahnen-Dechent,Maksim Sinyuk,Roy Lee Silverstein,Justin Durla Lathia,Meizhang Li,Christina Lynn Addison

doi:10.1371/journal.pone.0040033

Abstract

The angiogenic switch is a promising therapeutic target in cancer. Work by our laboratory and others has described an important endogenous anti-angiogenic pathway mediated by interactions of CD36, a receptor on microvascular endothelial cells, with proteins containing thrombospondin (TSP) type I repeat domains (TSR). Recent studies revealed that circulating Histidine Rich Glycoprotein (HRG) inhibits the anti-angiogenic potential of the CD36-TSR pathway by functioning as a decoy receptor that binds and sequesters TSR proteins. As tumors of different origin display variable expression profiles of numerous targets, we hypothesized that the TSP-CD36-HRG axis regulates vascularization and growth in the tumor microenvironment in a context, or tumor type, dependent manner. Growth of Lewis Lung Carcinoma (LL2) and B16F1 Melanoma tumor cell implants in syngeneic wild type (WT), hrg, or cd36 null mice were used as a model to interrogate this signaling axis. LL2 tumor volumes were greater in cd36 null mice and smaller in hrg null mice compared to WT. Immunofluorescent staining showed increased vascularity in cd36 null vs. WT and WT vs. hrg null mice. No differences in tumor growth or vascularity were observed with B16F1 implants, consistent with lack of expression of TSP-1 in B16F1 cells. When TSR expression was induced in B16F1 cells by cDNA transfection, tumor growth and vascularity were similar to that seen with LL2 cells. These data show a role for CD36-mediated anti-angiogenic activity in the tumor microenvironment when TSR proteins are available and demonstrate that HRG modulates this activity. Further, they suggest a mechanism by which tumor microenvironments may regulate sensitivity to TSR containing proteins.

Highlights

Angiogenesis is the physiologic process by which new vessels sprout from the existing vasculature
Our group has shown that this important endogenous anti-angiogenic system can be dampened by Histidine Rich Glycoprotein (HRG), a protein with structural homology to CD36 that acts as a decoy for TSR
In vitro assays of microvascular endothelial cell migration, proliferation, and tube formation; and in vivo assays of angiogenesis in mouse corneal micropockets and implanted matrigel showed that addition of exogenous HRG blocks TSP-1, TSP-2 and vasculostatin binding to CD36 and thereby inhibits TSR-mediated vascular cell responses [9,10,26]

Summary

Introduction

Angiogenesis is the physiologic process by which new vessels sprout from the existing vasculature. With regard to microvascular endothelial cells, a specific region of CD36 known as the CLESH domain interacts with high affinity with TSR domains of at least three endogenous anti-angiogenic proteins - thrombospondins-1 and -2 and vasculostatin [8,9,10]. These interactions initiate a complex intracellular signaling cascade involving the Src family tyrosine kinase P59fyn and p38 mitogen-activated protein kinase (MAPK) resulting in direct activation of caspase 3 protease leading to induction of apoptosis [11]. We demonstrated that these effects depended on tumor cell secretion of TSR-containing protein

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Results

Discussion