Selectins mediate leukocyte rolling and prime leukocytes for integrin-mediated leukocyte adhesion. However, neither the in vivo importance of nor the signaling pathway by which selectin-mediated integrin activation occurs has been determined. We report here that P-selectin-deficient mice manifested impaired leukocyte adhesion, which was 'rescued' by soluble P-selectin. Mechanistically, the cytoplasmic domain of P-selectin glycoprotein ligand 1 formed a constitutive complex with Nef-associated factor 1. After binding of P-selectin, Src kinases phosphorylated Nef-associated factor 1, which recruited the phosphoinositide-3-OH kinase p85-p110delta heterodimer and resulted in activation of leukocyte integrins. Inhibition of this signal-transduction pathway diminished the adhesion of leukocytes to capillary venules and suppressed peritoneal infiltration of leukocytes. Our data demonstrate the functional importance of this newly identified signaling pathway mediated by P-selectin glycoprotein ligand 1.

The Slit family of guidance cues binds to Roundabout (Robo) receptors and modulates cell migration. We report here that ectopic expression of Slit2 and Robo1 or recombinant Slit2 treatment of Robo1-expressing colorectal epithelial carcinoma cells recruited an ubiquitin ligase Hakai for E-cadherin (E-cad) ubiquitination and lysosomal degradation, epithelial-mesenchymal transition (EMT), and tumor growth and liver metastasis, which were rescued by knockdown of Hakai. In contrast, knockdown of endogenous Robo1 or specific blockade of Slit2 binding to Robo1 prevented E-cad degradation and reversed EMT, resulting in diminished tumor growth and liver metastasis. Ectopic expression of Robo1 also triggered a malignant transformation in Slit2-positive human embryonic kidney 293 cells. Importantly, the expression of Slit2 and Robo1 was significantly associated with an increased metastatic risk and poorer overall survival in colorectal carcinoma patients. We conclude that engagement of Robo1 by Slit2 induces malignant transformation through Hakai-mediated E-cad ubiquitination and lysosomal degradation during colorectal epithelial cell carcinogenesis.

Cancer research has been righteously and successfully focused on prevention, early detection and identification of specific molecular targets that distinguish the malignant cells from the neighboring benign cells1. However, a major clinical challenge concerns how we can reduce lethal tissue injury caused by intensive chemoradiotherapy during treatment of late-staged metastatic cancers. Here we tested whether induction of adult stem cells repairs chemoradiation-induced tissue injury and prolongs overall survival. We found that intestinal stem cells (ISCs)2 expressed Slit2 and its single-span transmembrane cell-surface receptor Roundabout 1 (Robo1)3,4. Partial genetic deletion of Robo1 decreased intestinal stem cells (ISCs) and caused villus hypotrophy, whereas Slit2 transgene increased ISCs and triggered villus hypertrophy. During lethal dosages of chemoradiation, administering a short pulse of R-spondin 1 (Rspo1; a Wnt agonist)5–14 plus Slit2 reduced ISC loss, mitigated gut impairment and protected animals from death, without concomitantly decreasing tumor sensitivity to chemotherapy. Rspo1 and Slit2 may thus act as therapeutic adjuvants to enhance host tolerance to aggressive chemoradiotherapy for eradicating metastatic cancers.