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

<div><p>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), <em>hrg</em>, or <em>cd36</em> null mice were used as a model to interrogate this signaling axis. LL2 tumor volumes were greater in <em>cd36</em> null mice and smaller in <em>hrg</em> null mice compared to WT. Immunofluorescent staining showed increased vascularity in <em>cd36</em> null vs. WT and WT vs. <em>hrg</em> 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.</p>