Sp1 Trans-Activation of Cell Cycle Regulated Promoters Is Selectively Repressed by Sp3

Abstract

The transcription factor Sp1 plays a key role in the activation of many cellular and viral gene promoters, including those that are regulated during the cell cycle. However, recent evidence indicates that Sp1 belongs to a larger family of factors which bind G/C box elements in order to either activate or repress transcription. Sp3, a member of this family, functions to repress transcriptional activation in two viral promoters, most likely by competing with Sp1 for GC box/Sp binding sites. However, the physiological role of Sp3 in the repression of endogenous cellular promoters has not been experimentally addressed. In the present study, we analyze the activity and binding of Sp3 on several eukaryotic promoters that contain G/C boxes and are known to be regulated during cellular proliferation and the cell cycle. Using antibodies specific for Sp1 and Sp3, we observe that both of these factors localize to the cell nucleus and have a similar, dispersed subnuclear distribution. Further, using gel mobility shift assays, we show that both Sp1 and Sp3 interact specifically with the histone H4 promoter. Transient cotransfections of Drosophila cells with Sp1 and Sp3 expression vectors and with the histone H4, thymidine kinase (TK), or dihydrofolate reductase (DHFR) promoters show that only the DHFR promoter, containing multiple functional GC boxes, displays Sp3 repression of Sp1 activation. In contrast, the single G/C boxes within the histone H4 or TK promoters, which confer transcriptional activation via Sp1 binding, are not responsive to repression by Sp3. Therefore, we demonstrate that the endogenous cellular DHFR promoter is selectively responsive to Sp3 repression. The data suggest that Sp3 may contribute to the control of proliferation- and/or cell-regulated promoters depending upon the context and/or number of functional Sp1 binding sites.

Identification of regulatory elements in the human adipose most abundant gene transcript-1 (apM-1) promoter: role of SP1/SP3 and TNF-? as regulatory pathways

“…The data obtained by our LRA's show that SP3 has inhibitory effects on apM-1 promoter activity due to the fact that simultaneous coexpression of SP3 with SP1 reduces promoter activity induced by expression of SP1 alone. These data concerning SP1 and SP3 interactions are in agreement with previous reports [40,41,42,47,48]. SP3 alone is not able to activate SP1-responsive promoters [48] and it fails to activate reporter constructs in cells lacking endogenous SP factors [40,47].…”

“…SP3 has inhibitory partial effects on those effects mediated by SP1 [40,41,42]. The data obtained by our LRA's show that SP3 has inhibitory effects on apM-1 promoter activity due to the fact that simultaneous coexpression of SP3 with SP1 reduces promoter activity induced by expression of SP1 alone.…”

“…SP1 and SP3 contain a similar DNA-binding domain with three zinc fingers and two glutamine-and serine/threonin-rich trans-activation domains [33,40]. SP3 comprises an additional inhibitory domain, leading to either activation or repression depending on the promoter and cellular context [40,41,42,43,44,45,46]. In this context, we reported, that the apM-1 promoter lacks a classical TATAA box, but contains a classical CCAAT box and has several transcription start sites [22], thus belonging to the group of class II promoters.…”

Sp1 as G1 cell cycle phase specific transcription factor in epithelial cells

“…It is possible that delta-Sp1 inhibits both Sp1 and Sp3 thus complicating the interpretation of our results. However, in many cases Sp3 antagonizes the activity of Sp1 (Hagen et al, 1994;Birnbaum et al, 1995;Kumar and Butler, 1999). Moreover, promoters which consist of several Sp1 sites are selectively repressed by Sp3 (Hagen et al, 1994;Birnbaum et al, 1995).…”

“…However, in many cases Sp3 antagonizes the activity of Sp1 (Hagen et al, 1994;Birnbaum et al, 1995;Kumar and Butler, 1999). Moreover, promoters which consist of several Sp1 sites are selectively repressed by Sp3 (Hagen et al, 1994;Birnbaum et al, 1995). In this context it is important to note that the cyclin D1 and the EGFR promoters contain multiple Sp1 binding sites (Motokura and Arnold, 1993;Johnson et al, 1988).…”