CD4 expression enhances CCR5-dependent ERK activation. (A) CHO-K1 cells were transfected with the plasmid for CCR5-Luc with or without the plasmid for CD4-YFP at concentrations, which correspond to the asterisk in Figure 4B. Cells were serum-starved overnight and then stimulated with MIP1β (5 × 10−8 M) for the indicated durations of time. The luciferase signal was measured to monitor the total amount of CCR5. Immunoblotting was performed with the indicated antibodies. Equal amounts of lysates were loaded in each lane and a representative immunoblot of 3 independent experiments is shown. Total ERK represents the loading control. (B) Quantification of phosphorylated ERK1-2 (normalized with total ERK) from 3 independent experiments. *, **, and *** indicate significant differences from untreated cells; ‡ and ‡‡‡, significant differences between cells expressing or not expressing CD4; * and ‡, P < .05; **, P < .01; *** and ‡‡‡, P < .001.
