Overexpression of transcription factor Sp1 leads to gene expression perturbations and cell cycle inhibition.
BACKGROUND
The ubiquitous transcription factor Sp1 regulates the expression of a vast number of genes involved in many cellular functions ranging from differentiation to proliferation and apoptosis. Sp1 expression levels show a dramatic increase during transformation and this could play a critical role for tumour development or maintenance. Although Sp1 deregulation might be beneficial for tumour cells, its overexpression induces apoptosis of untransformed cells. Here we further characterised the functional and transcriptional responses of untransformed cells following Sp1 overexpression.


RESULTS
We made use of wild-type and DNA-binding-deficient Sp1 to demonstrate that the induction of apoptosis by Sp1 is dependent on its capacity to bind DNA. Genome-wide expression profiling identified genes involved in cancer, cell death and cell cycle as being enriched among differentially expressed genes following Sp1 overexpression. In silico search to determine the presence of Sp1 binding sites in the promoter region of modulated genes was conducted. Genes that contained Sp1 binding sites in their promoters were enriched among down-regulated genes. The endogenous sp1 gene is one of the most down-regulated suggesting a negative feedback loop induced by overexpressed Sp1. In contrast, genes containing Sp1 binding sites in their promoters were not enriched among up-regulated genes. These results suggest that the transcriptional response involves both direct Sp1-driven transcription and indirect mechanisms. Finally, we show that Sp1 overexpression led to a modified expression of G1/S transition regulatory genes such as the down-regulation of cyclin D2 and the up-regulation of cyclin G2 and cdkn2c/p18 expression. The biological significance of these modifications was confirmed by showing that the cells accumulated in the G1 phase of the cell cycle before the onset of apoptosis.


CONCLUSIONS
This study shows that the binding to DNA of overexpressed Sp1 induces an inhibition of cell cycle progression that precedes apoptosis and a transcriptional response targeting genes containing Sp1 binding sites in their promoter or not suggesting both direct Sp1-driven transcription and indirect mechanisms.
Deniaud
Emmanuelle
E
Inserm, U851, Lyon, France.
Baguet
Joël
J
Chalard
Roxane
R
Blanquier
Bariza
B
Brinza
Lilia
L
Meunier
Julien
J
Michallet
Marie-Cécile
MC
Laugraud
Aurélie
A
Ah-Soon
Claudette
C
Wierinckx
Anne
A
Castellazzi
Marc
M
Lachuer
Joël
J
Gautier
Christian
C
Marvel
Jacqueline
J
Leverrier
Yann
Y
D000818:Animals; D017209:Apoptosis; D001483:Base Sequence; D002453:Cell Cycle; D002461:Cell Line, Transformed; D049109:Cell Proliferation; D002471:Cell Transformation, Neoplastic; D003593:Cytoplasm / Q000378:metabolism; D005786:Gene Expression Regulation; D051379:Mice; D008954:Models, Biological; D008969:Molecular Sequence Data; D020411:Oligonucleotide Array Sequence Analysis; D011401:Promoter Regions, Genetic; D016329:Sp1 Transcription Factor / Q000378:metabolism*
D016329:Sp1 Transcription Factor