ID proteins regulate diverse aspects of cancer progression and provide novel therapeutic opportunities

Abstract

The inhibitor of differentiation (ID) proteins are helix-loop-helix transcriptional repressors with established roles in stem cell self-renewal, lineage commitment, and niche interactions. While deregulated expression of ID proteins in cancer was identified more than a decade ago, emerging evidence has revealed a central role for ID proteins in neoplastic progression of multiple tumor types that often mirrors their function in physiological stem and progenitor cells. ID proteins are required for the maintenance of cancer stem cells, self-renewal, and proliferation in a range of malignancies. Furthermore, ID proteins promote metastatic dissemination through their role in remodeling the tumor microenvironment and by promoting tumor-associated endothelial progenitor cell proliferation and mobilization. Here, we discuss the latest findings in this area and the clinical opportunities that they provide.

Figure 1

Regulation of inhibitor of differentiation (ID) expression and their function in cancer biology. (a) ID proteins are sensitive to a diverse array of extracellular signals, including steroid hormones, growth factors, and members of the TGF-β superfamily. ID proteins are also downstream of well-established oncogenic pathways such as RAS-Egr1, MYC, and Src-PI3k as well as tumor suppressors RB p53 and KLF17. (b) ID proteins regulate cellular pathways that are essential to the development and progression of cancer. IDs regulate self-renewal and cell-cycle through a number of known stem and proliferation factors such as Notch, Sox2/4, LIF, cyclin genes and the CDK inhibitors p21^waf1 and p16^INK4A. In addition, IDs remodel the tumor microenvironment by inducing the expression of pro-angiogenic cytokines such as IL6 and CXCL1 which increase endothelial cell proliferation and migration and that might influence the biological properties of other cell types in the tumor microenvironment. ID proteins have also been shown to promote invasion by degrading the extracellular matrix through induction of several members of the maxtrix metalloproteinase (MMP) protein family such as MMP-2, MMP-9, and MMP-14. ID genes control a stem cell-intrinsic transcriptional program that preserves stem cell adhesion to the niche in neural stem cells and in glioma. ID proteins activate the Ras-related protein RAP1 by suppressing the GTPase activating protein RAP1GAP, thereby promoting adhesion of cells to a supportive endothelial niche.

Figure 2

A proposed conserved cellular mechanism for inhibitor of differentiation (ID) proteins in tumorigenesis and metastasis. (a) IDs control the glioma cancer stem cell niche in a cell autonomous as well as paracrine manner. TGF-β, expressed by endothelial cells, upregulates the expression of ID proteins in glioma stem cells, thereby maintaining glioma stem cell self renewal via RAP1-dependent adhesion to the niche. ID expression may also re-inforce the vascular niche by the induction of pro-angiogenic factors like IL6 and CXCL1. (b) ID proteins may play a similar role in coordinating an endothelial niche for disseminated breast cancer cells in the lung. Endothelial cells can enforce quiescence of disseminated breast cancer cells in the lung through the expression of thrombospondin-1 (TSP1). In contrast, activated or sprouting endothelial cells downregulate TSP1- and instead express TGF-β which promotes the escape of disseminated tumor cells from dormancy, in an ID-protein dependent manner. ID proteins in turn can activate the expression of proangiogenic factors CXCL1 and IL8 to drive endothelial activation and sprouting. Endothelial activation can be further promoted by the recruitment from the bone marrow to nascent metastatic sites of Id⁺ EPCs, which require ID1 for mobilization and proliferation. EPCs support angiogenesis by the expression of VEGF and through direct luminal incorporation. ID signaling in activated endothelial cells further represses TSP-1. (c) Targeting ID expression and their associated pathways in tumor cells and endothelial cells from bone marrow and tumor blood vessels will disrupt both cell-autonomous and cell-nonautonomous programs, which may eventually produce additive or even synergistic antitumor effects. VEGF, vascular endothelial growth factor.