A: Prostate cancer cells were seeded in 6 well plates. Twenty-four hours later, cells were treated with Geldanamycin (3 μM) for 1-24 hr, or were left untreated (NT). Protein extracts were subjected to western blotting. Actin served as a loading control. B: PC-3 cells were seeded in 6 well plates. Twenty-four hours later, cells were treated with 17-AAG (2 μM, 2 hr), proteasome inhibitor MG132 (10 µM, 2 hr) or were left untreated (NT). Protein extracts were collected and subjected to western blotting. Actin served as a loading control. C, D: PC-3 or LNCaP cells were treated with Geldanamycin as described above and nuclear or total protein extracts we collected and subjected to western blot analysis for detection of β-AR2 (C) and EGFR (D). AU: Arbitrary Units
ARTICLE ABSTRACT
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 β-arrestin-2 (ARRB2/β-AR2) was examined by manipulating the levels of β-AR2 and analyzing changes in LNCaP cell growth and phosphorylation of EGFR, ERK1/2, Src, and Akt. Depletion of β-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, β-AR2 depletion downregulated the proliferation suppressor p21. Stimulation of β-AR2–expressing cells with EGF resulted in rapid nuclear translocation of phosphorylated/activated EGFR. Downregulation of β-AR2 enhanced this nuclear translocation. These results demonstrate that β-AR2 is a negative regulator of CXCR7/Src/EGFR–mediated mitogenic signaling.Implications: This study reveals that β-AR2 functions as a tumor suppressor, underscoring its clinical importance in regulating CXCR7/EGFR–mediated tumor cell proliferation. Mol Cancer Res; 14(5); 493–503. ©2016 AACR.
