Impaired expression of integrin α-4 subunit in cultured mast cells derived from mutant mice of mi/mi genotype

The milocus encodes a member of the basic-helix-loop-helixleucine zipper protein family of transcription factors (hereafter called MITF). We have reported that expression of several genes was impaired in cultured mast cells (CMCs) of mi/mi mice due to a defective transactivation ability of mutant MITF (mi-MITF). Because attachment of mi/mi CMCs to fibreblasts is impaired, we examined the expression of integrin genes in mi/mi CMCs in the present study. Among the integrin genes examined, the expression of integrin α4 subunit was barely detectable in mi/mi CMCs, and the α4 protein was not detected by flow cytometry either. The specific adhesion to vascular cell adhesion molecule-1 (VCAM-1), the ligand for α4 subunit, was observed in +/+ CMCs but not in mi/miCMCs, indicating that the expression of integrin α4 subunit at a functional level did not occur in mi/mi CMCs. In the promoter region of the α4 subunit gene, there was a CACTTG motif to which normal MITF (+-MITF) bound. The coexpression of +-MITF but not of mi-MITF transactivated the promoter of the α4 subunit gene. The deletion or mutation of the CACTTG motif abolished the transactivation by +-MITF, suggesting that +-MITF directly transactivated the gene encoding α4 subunit of integrin.