β-Arrestin-2 Counters CXCR7-Mediated EGFR Transactivation and Proliferation

The atypical 7-transmembrane chemokine receptor, CXCR7, transactivates the EGFR leading to increased tumor growth in several tumor types. However, the molecular mechanism of CXCR7 ligand-independent EGFR transactivation is unknown. We used cDNA knock-in, RNAi and analysis of mitogenic signaling components in both normal prostate epithelial cells and prostate cancer cells to decipher the proliferation-inducing mechanism of the CXCR7-EGFR interaction. The data demonstrate that CXCR7-induced EGFR transactivation is independent of both the release of cryptic EGFR ligands (e.g., AREG/amphiregulin) and G-protein-coupled receptor signaling. An alternate signaling mechanism involving beta-arrestin-2 (ARRB2/beta-AR2) was examined by manipulating the levels of beta-AR2 and analyzing changes in LNCaP cell growth and phosphorylation of EGFR, ERK1/2, Src, and Akt. Depletion of beta-AR2 in LNCaP cells increased proliferation/colony formation and significantly increased activation of Src, phosphorylation of EGFR at Tyr-1110, and phosphorylation/activation of ERK1/2 compared with that with control shRNA. Moreover, beta-AR2 depletion downregulated the proliferation suppressor p21. Stimulation of beta-AR2-expressing cells with EGF resulted in rapid nuclear translocation of phosphorylated/activated EGFR. Downregulation of beta-AR2 enhanced this nuclear translocation. These results demonstrate that beta-AR2 is a negative regulator of CXCR7/Src/EGFR-mediated mitogenic signaling. Implications: This study reveals that beta-AR2 functions as a tumor suppressor, underscoring its clinical importance in regulating CXCR7/EGFR-mediated tumor cell proliferation.