Characterization of the human α9 integrin subunit gene: Promoter analysis and transcriptional regulation in ocular cells

- The promoter of the α9 gene has been cloned and its regulatory elements characterized.
- Sp1 binding predominates in the balance between Sp1 and NFI to ensure basal transcription of α9.
- The ligand of α9β1, tenascin-C, increases α9 transcription in corneal epithelial cells.
- Appearance of tenascin during corneal wound healing is coordinated with α9 gene transcription.

α9β1 is the most recent addition to the integrin family of membrane receptors and consequently remains the one that is the least characterized. To better understand how transcription of the human gene encoding the α9 subunit is regulated, we cloned the α9 promoter and characterized the regulatory elements that are required to ensure its transcription. Transfection of α9 promoter/CAT plasmids in primary cultured human corneal epithelial cells (HCECs) and uveal melanoma cell lines demonstrated the presence of both negative and positive regulatory elements along the α9 promoter and positioned the basal α9 promoter to within 118 bp from the α9 mRNA start site. In vitro DNaseI footprinting and in vivo ChIP analyses demonstrated the binding of the transcription factors Sp1, c-Myb and NFI to the most upstream α9 negative regulatory element. The transcription factors Sp1 and NFI were found to bind the basal α9 promoter individually but Sp1 binding clearly predominates when both transcription factors are present in the same extract. Suppression of Sp1 expression through RNAi also caused a dramatic reduction in the expression of the α9 gene. Most of all, addition of tenascin-C (TNC), the ligand of α9β1, to the tissue culture plates prior to seeding HCECs increased α9 transcription whereas it simultaneously decreased expression of the α5 integrin subunit gene. This dual regulatory action of TNC on the transcription of the α9 and α5 genes suggests that both these integrins must work together to appropriately regulate cell adhesion, migration and differentiation that are hallmarks of tissue wound healing.

Culturing human corneal epithelial cells (HCECs) on tenascin-C, the ligand of the α9β1 integrin, alters the ratio of the transcription factors that bind the α9 gene promoter to favor the positive regulatory influence of Sp1 and c-Myb, thereby increasing transcription of the α9 gene.496