KAI1 suppresses HIF-1α and VEGF expression by blocking CDCP1-enhanced Src activation in prostate cancer

We used the human prostate cancer cell line PC3 which was devoid of KAI1 expression. Vector-transfected cells (PC3-GFP clone #8) and KAI1-expressing PC3 clones (PC3-KAI1 clone #5 and #6) were picked after stable transfection with KAI1 cDNA and selection in 800 μg/ml G418. Protein levels were assessed by immunoblotting and VEGF reporter gene activity was measured by assaying luciferase activitiy. We followed tumor growth in vivo and immunohistochemistry was performed for detection of HIF-1, CDCP1, and VHL protein level.We demonstrated that Hypoxia-inducible factor 1α (HIF-1α) and VEGF expression were significantly inhibited by restoration of KAI1 in PC3 cells. In response to KAI1 expression, CDCP1-enhanced Src activation was down-regulated and the level of von Hippel-Lindau (VHL) protein was significantly increased. In an in vivo xenograft model, KAI1 inhibited the expression of CDCP1 and HIF-1α.These novel observations may indicate that KAI1 exerts profound metastasis-suppressor activity in the tumor malignancy process via inhibition of CDCP1-mediated Src activation, followed by VHL-induced HIF-1α degradation and, ultimately, decreased VEGF expression.More than 20 metastasis-suppressor genes encoding products with specific metastasis-suppressing functions have been discovered [1]. KAI1 (CD82) was first identified as a prostate cancer metastasis suppressor through genetic screening [2]. Studies have shown that the expression of KAI1 is actually down-regulated in most metastatic cancers [3]. Consistent with this, reduced KAI1 expression is associated with malignant progression of human prostate cancer [4]. KAI1 is a member of the tetraspan transmembrane superfamily (TM4SF) of type III membrane proteins, specifically of the tetraspanin subgroup. It is ubiquitously expressed, especially in spleen, thymus, prostate and colon, and interacts with a large number of proteins, including integrins, epidermal growth factor receptor (EGFR), and other tetraspanins. KAI1 plays a r