Phorbol myristate acetate transactivates the avian β₃ integrin gene and induces α_vβ₃ integrin expression

1,25-Dihydroxyvitamin D₃ (1,25(OH)₂D₃) transactivates the avian β₃ integrin gene whose promoter contains at least two vitamin D response elements, one of which is in close proximity to a candidate AP1 site (TGACTCA). Since fos/jun and steroid hormones interact to regulate gene expression, we asked whether phorbol-12-myristate-13-acetate (PMA), which stimulates binding of fos/jun to AP1 sites, transactivates the avian β₃ integrin gene and, if so, does the phorbol ester modulate 1,25(OH)₂D₃ induction of the gene. We find the candidate AP1 sequence comigrates with the consensus AP1 sequence on electromobility shift assay when incubated with recombinant c-jun protein. Furthermore, PMA prompts expression of β₃ integrin mRNA in the avian monocytic line, HD11. The increase in message reflects transactivation of the β₃ gene and is mirrored by plasma membrane appearance of the integrin heterodimer α_vβ₃. Moreover, attesting to the functional significance of PMA-enhanced α_vβ₃ expression, cells treated with concentrations of the phorbol ester that induce the β₃ gene, spread extensively on plastic, an event blocked by an anti-α_v antibody and a peptide mimetic known to inhibit α_vβ₃-mediated cell attachment. Interestingly, co-addition of 1,25(OH)₂D₃ and PMA prompts greater expression of α_vβ₃ than when the cells are exposed to either agent alone and PMA enhances 1,25(OH)₂D₃-induced β₃ integrin mRNA expression. Thus, PMA and 1,25(OH)₂D₃ impact on the avian β₃ integrin gene independently and in combination.