Activation of p38 MAPK by gp120 treatment. Differentiated F11 cells were treated with 5 nM gp120 for 5, 15, and 30 min. The same volume of PBS, the diluent, was added as a control, and 0.5 M sorbitol treatment (5 min) was used as a positive control for p38 MAPK activation. To examine the contribution of CXCR4 activation on p38 MAPK activation, differentiated F11 cells were incubated with 2 µM AMD3100 for 1 hr prior to gp120 treatment. (a) A representative Western blot shows p38 MAPK activation, as shown by an antibody recognizing phosphorylated (active) p38 MAPK (p-p38). The H2 antibody for kinesin heavy chain (KHC) was used as a loading control. (b) Quantitation of p38 MAPK activation. Dark gray bars represent no AMD3100 pretreatment, and light gray bars represent AMD3100 pretreatment. The amount of phosphorylated p38 MAPK was significantly increased after 5 min of gp120 treatment compared with control (p < .05). This transient activation was abolished by pretreatment with AMD3100 (p < .0001) and was therefore dependent upon CXCR4 activation. However, AMD3100 treatment unmasked a CXCR4-independent activation of p38 MAPK after 30 min of treatment (p < .05). Data represent the mean ± SEM of the ratio of phospho-p38 MAPK to total p38 MAPK from 16 separate experiments. *p < .05. #p < .0001.