Black A R, Jensen D, Lin S Y, Azizkhan J C
Department of Pharmacology and Therapeutics, Roswell Park Cancer Institute, Buffalo, New York 14263, USA.
J Biol Chem. 1999 Jan 15;274(3):1207-15. doi: 10.1074/jbc.274.3.1207.
Sp1 sites can mediate growth/cell cycle induction of dihydrofolate reductase in late G1 (Jensen, D. E., Black, A. R. Swick, A. G., and Azizkhan, J. C. (1997) J. Cell. Biochem. 67, 24-31). To investigate mechanisms underlying this induction, effects of serum stimulation on regulation of Sp1 were examined. 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 result in a rapid, approximately 2-fold increase in Sp1 levels and an approximately 3-fold increase in specific Sp1 phosphorylation in mid-G1. In normal human diploid fibroblasts, serum stimulation also increased Sp1 phosphorylation in mid-G1 but did not affect Sp1 levels. Therefore, Sp1 phosphorylation is regulated in a growth/cell cycle-dependent manner which correlates temporally with induction of dihydrofolate reductase transcription. Further studies revealed a kinase activity specifically associated with Sp1 in a growth-regulated manner. This activity is distinct from purified kinases previously shown to phosphorylate Sp1 in vitro and phosphorylates Sp1 between amino acids 612 and 678 in its C terminus, a region also phosphorylated in mid-G1 in vivo. Therefore, this study indicates that phosphorylation of the C terminus of Sp1 may play a role in the cell cycle regulation of its transcriptional activity.
Sp1位点可介导二氢叶酸还原酶在G1晚期的生长/细胞周期诱导（詹森，D.E.，布莱克，A.R.，斯威克，A.G.，和阿齐兹汗，J.C.（1997年）《细胞生物化学杂志》67卷，24 - 31页）。为了研究这种诱导作用的潜在机制，检测了血清刺激对Sp1调控的影响。在Balb/c 3T3细胞中，血清刺激不影响Sp1的合成或Sp1家族成员与DNA的相对结合；然而，它确实导致Sp1水平在G1中期迅速增加约2倍，特异性Sp1磷酸化增加约3倍。在正常人二倍体成纤维细胞中，血清刺激也增加了G1中期的Sp1磷酸化，但不影响Sp1水平。因此，Sp1磷酸化以生长/细胞周期依赖性方式受到调控，这在时间上与二氢叶酸还原酶转录的诱导相关。进一步的研究揭示了一种激酶活性，它以生长调节的方式与Sp1特异性相关。这种活性不同于先前在体外显示能使Sp1磷酸化的纯化激酶，并且在Sp1的C末端氨基酸612和678之间使其磷酸化，该区域在体内G1中期也被磷酸化。因此，这项研究表明Sp1 C末端的磷酸化可能在其转录活性的细胞周期调控中起作用。
