PDGFR activity is required for optimal CXCR4 signaling in SHH MB cells and GRK6 is downregulated and differentially expressed in MB. (A) Left panel: Daoy cells were starved and treated with or without 1 μg/ml anti-PDGFRβ blocking antibody for 24 h, then stimulated by 10 ng/ml PDGF-BB or 100 ng/ml CXCL12 (SDF-1α) for 15 min. Western blot of P-ERK shows PDGFR function is required for optimal CXCR4 signaling; Right panel: Quantitative analysis of Western blot shows CXCL12-induced P-ERK is significantly decreased by PDGFRβ blocking antibody (P < 0.05, lane 3, 1.53 ± 0.12 vs. lane 6, 1.09 ± 0.15, three independent experiments). (B) 29 human MB specimens were collected and used for microarray analysis. Average expression level of GRK6 mRNA is lowest among the detectable GRKs, (P < 0.01). (C) Microarray database of 9 human metastatic and 14 non-metastatic MB showed that the percentage of tumors with detectable GRK6 mRNA was notably decreased in metastatic MB (M+, 22%), compared to non-metastatic MB (M0, 43%). However, the percentage of tumors with detectable GRK5 mRNA was not appreciably different in M + (22%), vs. M0 (29%).