Interaction between Sp1 and cell cycle regulatory proteins is important in transactivation of a differentiation-related gene.

The stratified squamous epithelium is a model system in which to define molecular mechanisms underlying the switch from proliferation to differentiation. This can be achieved through the functional dissection of keratin gene promoters. Having previously established the importance of keratin 4 in maintaining the differentiated phenotype in corneal epithelial cells, we investigated the role of Sp1-mediated transactivation of the keratin 4 promoter given the role of Sp1 in differentiation and cell cycle progression. Sp1 transactivation of the keratin 4 promoter was diminished in cyclin D1-overexpressing cells, which may be mediated through a newly described direct interaction between Sp1 and cyclin D1 and opposed by a complex between Sp1 and pRB.