Targeting the SLIT/ROBO pathway in tumor progression: molecular mechanisms and therapeutic perspectives

Abstract

The SLITs (SLIT1, SLIT2, and SLIT3) are a family of secreted proteins that mediate positional interactions between cells and their environment during development by signaling through ROBO receptors (ROBO1, ROBO2, ROBO3, and ROBO4). The SLIT/ROBO signaling pathway has been shown to participate in axonal repulsion, axon guidance, and neuronal migration in the nervous system and the formation of the vascular system. However, the role of the SLIT/ROBO pathway has not been thoroughly clarified in tumor development. The SLIT/ROBO pathway can produce both beneficial and detrimental effects in the growth of malignant cells. It has been confirmed that SLIT/ROBO play contradictory roles in tumorigenesis. Here, we discuss the tumor promotion and tumor suppression roles of the SLIT/ROBO pathway in tumor growth, angiogenesis, migration, and the tumor microenvironment. Understanding these roles will help us develop more effective cancer therapies.

Keywords: SLIT/ROBO pathway; migration; proliferation; tumor; tumor microenvironment.

Figure 1.

Structure of the SLIT/ROBO protein family. (a) Structure of SLIT protein, a secreted glycoprotein from the N terminus to C terminus, contains four leucine-rich repeats (LRRs, D1–D4), seven to nine epidermal growth factor (EGF) repeats, a laminin G-like module and a cysteine-rich knot. (b) The ROBO family contain five immunoglobulin (Ig) domains, three fibronectin type III modules in the extracellular region, one transmembrane region and one intracellular conserved cytoplasmic domain including CC0, CC1, CC2, and CC3. (c) SLIT D2 binds on its concave surface to the Ig1 domain of ROBO through electrostatic and hydrophobic contact regions such as the heparan sulfate proteoglycan (HSPG) syndecan.

Figure 2.

Mechanisms of the SLIT/ROBO pathway as an oncogene in cancer. The SLIT2/ROBO1 signal upregulates MMP-2 and MMP-9, thus promotes cell migration and invasion. SLIT2/ROBO1 signal recruits a ubiquitin ligase Hakai for E-cadherin ubiquitination and lysosomal degradation and thus promotes the epithelial–mesenchymal transition (EMT). The monoclonal antibody R5, which can interrupt the SLIT2/ROBO1 pathway, causes significantly suppressed cell growth and proliferation. ROBO3 promotes cancer cell growth, invasion and metastasis, which is associated with activated Wnt pathway components, β-catenin and GSK-3β, and other markers indicating the EMT, and miR-383 functions as a suppressor of ROBO3.

Figure 3.

Mechanisms of the SLIT/ROBO pathway as a tumor-suppressor gene in tumor progression. SLIT2/ROBO1 signal decreases the proliferative rate and increases the apoptotic rate through regulating Fas-FasL proteins and PI3K/AKT pathway. USP33 can inhibit the lysosomal degradation of ROBO1, and SLIT3 leads to strong inhibition of migration through downregulation of AP-1 activity and targeting vimentin, MMP2, and MMP9. SLIT2/ROBO1 signal prevents hepatocyte growth factor (HGF)-induced motile responses, reduces Cdc-42 activation, and stimulates Rac-1 activities and, thus, inhibits cell proliferation, survival, and motility. SLIT2/ROBO1 functions as negative regulators of CXCL12/CXCR4 pathway through inhibiting CXCL12-induced tyrosine phosphorylation of focal adhesion components such as RAFTK/Pyk2, FAK, and Paxillin.

Figure 4.

Mechanisms of the SLIT/ROBO pathway in tumor angiogenesis. SLIT2 and SLIT3 lead to bidirectional adjusting of angiogenesis. Through modulating the activity of ROBO1, ROBO2, ROBO4, Src, and vascular endothelial growth factor (VEGF) pathway, endothelial cell behaviors, including tube formation and filopodia formation, could be regulated.

Figure 5.

Mechanisms of the SLIT/ROBO pathway in tumor inflammation process. SLIT2 is involved in inflammation modulated by ROBO1, ROBO4, and AKT pathway. The downstream pathways of ROBO1 and ROBO4 including PI3K and nuclear factor (NF)-kB pathways involve in chemotaxis of inflammatory cells (T cells, Langerhans cells, neutrophils, and eosinophils) and angio-associated components (endothelial cells, platelets). SLIT3 regulates CXCL12/CXCR4 and RhoA in modulating inflammation, chiefly monitoring inflammatory migration and chemotaxis.