AbstractIntegrins are a family of cell surface heterodimers which mediate both cell‐cell and cell‐extracellular matrix interactions and affect cellular differentiation through their signal transduction capacity. Integrin expression is regulated during differentiation as well as by numerous growth factors and cytokines. We have analyzed the changes in p150,95 (CD11c/CD18 or αX/β2) and VLA‐4 (CD49d/CD29 or α4/β1) integrin subunits mRNA levels that take place during the myeloid differentiation of HL60 and U937 cells, and compared them to other integrins with similar functional activities. Northern blot analysis revealed that the monocytic differentiation of U937 and HL60 cells alters the αX and α4 mRNA steady‐state levels: αX mRNA is inducedde novowhereas α4 mRNA decreases to undetectable levels. Both changes were dependent on the activity of protein kinase C and were also observed upon granulocytic differentiation of HL60 cells. Parallel analysis of other integrin subunits mRNA (β1, α5, β7) demonstrated that the mRNA levels for the α subunits of the fibronectin receptors α4/β1 (VLA‐4) and α5/β1 (VLA‐5) are differentially regulated during the monocytic differentiation of myeloid cell lines, and suggested that myeloid cells express a heterodimer formed by the association of β7 with an integrin α subunit distinct from α4. Nuclear transcription assays and functional analysis of the αX and α4 promoter regions demonstrated that the transcription rate of the αX gene is considerably elevated after phorbol 12‐myristate 13‐acetate treatment of U937 cells, while that of α4 is almost unaffected, suggesting that posttranscriptional mechanisms are causing the extremely low α4 mRNA level