Growth/cell cycle regulation of Sp1 phosphorylation

Sp1 sites can mediate growth/cell cycle induction of dihydrofolate reductas e in late G(1) (Jensen, D. E., Black, A. R. Swick, A. G., and Azizkhan, J. C. (1997) J. Cell. Biochem, 67, 24-31). To investigate mechanisms underlyin g this induction, effects of serum stimulation on regulation of Sp1 were ex amined. In Balb/c 3T3 cells, serum stimulation did not affect Sp1 synthesis or the relative binding of Sp1 family members to DNA; however, it did resu lt in a rapid, similar to 2-fold increase in Sp1 levels and an similar to 3 -fold increase in specific Sp1 phosphorylation in midG(1). In normal human diploid fibroblasts, serum stimulation also increased Sp1 phosphorylation i n mid-G(1) but did not affect Sp1 levels. Therefore, Sp1 phosphorylation is regulated in a growth/cell cycle-dependent manner which correlates tempora lly with induction of dihydrofolate reductase transcription. Further studie s revealed a kinase activity specifically associated with Sp1 in a growth-r egulated manner. This activity is distinct from purified kinases previously shown to phosphorylate Sp1 in vitro and phosphorylates Sp1 between amino a cids 612 and 678 in its C terminus, a region also phosphorylated in mid-G, in vivo. Therefore, this study indicates that phosphorylation of the C term inus of Sp1 may play a role in the cell cycle regulation of its transcripti onal activity.