AML cell lines up-regulate CXCR4 within the first 2 – 10 hours of physiologic hypoxia. Up-regulation of transcription seems not to be involved as a major factor of increased expression. (A) Live microscopy (40×/1.5 NA) of OCI-AML3 cells after incubation with FITC-labeled CXCR4 antibody. Images show not only an increase of total CXCR4 as early as 3 hours after start of hypoxia, but also a distinct punctate pattern. (B) U937 cells show an identical behavior with rapid increase of CXCR4 and characteristic pattern. (C) Flow cytometry reveals increased CXCR4 as soon as 2 hours after initiation of hypoxia. (D) Real time RT-PCR for CXCR4 cDNA of OCI-AML3 and U937 cells. Transcriptional activity increases only 8 hours after hypoxia and only to a moderate extent (∼2-fold) in OCI-AML3, and not at all in U937. Graphs show mean and SD.