Interaction between α₅β₁ integrin and secreted fibronectin is involved in macrophage differentiation of human HL-60 myeloid leukemia cells

We examined the role of fibronectin (FN) and FN-binding integrins in macrophage differentiation. Increased FN and α₅β₁ integrin gene expression was observed in phorbol 12-myristate 13-acetate PMA-treated HL-60 cells and PMA- or macrophage-CSF-treated blood monocytes before the manifestation of macrophage markers. After treatment of HL-60 cells and monocytes, newly synthesized FN was released and deposited on the dishes. An HL-60 cell variant, HL-525, which is deficient in the protein kinase Cβ (PKC-β) and resistant to PMA-induced differentiation, failed to express FN after PMA treatment. Transfecting HL-525 cells with a PKC-β expression plasmid restored PMA-induced FN gene expression and macrophage differentiation. Untreated HL-525 cells (which have a high level of the α₅β₁ integrin) incubated on FN differentiated into macrophages. The percentage of cells having a macrophage phenotype induced by PMA in HL-60 cells, by FN in HL-525 cells, or by either PMA or macrophage-CSF in monocytes was reduced in the presence of mAbs to FN and α₅β₁ integrin. The integrin-signaling nonreceptor tyrosine kinase, p72(Syk), was activated in PMA-treated HL-60 and FN-treated HL-525 cells. We suggest that macrophage differentiation involves the activation of PKC-β and expression of extracellular matrix proteins such as FN and the corresponding integrins, α₅β₁ integrin in particular. The stimulated cells, through the integrins, attach to substrates by binding to the deposited FN. This attachment, in turn, may through integrin signaling activate nonreceptor tyrosine kinases, including p72(Syk), and later lead to expression of other genes involved in evoking the macrophage phenotype.