| Abstract |
The p73 gene possesses an extrinsic P1 promoter and an intrinsic P2 promoter,
resulting in synthesis of the full-length TAp73 and the N-terminal-truncated ΔNp73
isoforms, respectively. The ultimate effect of p73 in oncogenesis is thought to depend
on the apoptotic TAp73 to the antiapoptotic ΔNp73 isoforms’ ratio. The selective
activation of p73 promoters could trigger the expression of either the apoptotic or the
antiapoptotic p73 isoforms, thus shifting the ΔN/ΤΑ ratio towards an oncogenic or an
oncosuppression direction. Therefore, the epigenetic and transcription factors that
differentially activate P1 and P2 promoters are crucial deterninants of the role of p73
in cancer, since they can alter the ΔN/ΤΑ ratio. This study was aimed at identifying
novel epigenetic and transcription factors that affect both TAp73 and ΔNp73 isoform
synthesis in the context of lung cancer, where both TAp73 and ΔNp73 isoforms have
been previously found overexpressed. First, we investigated the DNA methylation
status of both promoters as a means of epigenetic transcriptional control of their
corresponding isoforms in 102 primary non-small cell lung carcinomas (NSCLCs).
We demonstrated that while P1 hypermethylation-associated reduction of TAp73
mRNA levels is rare, the P2 hypomethylation-associated overexpression of ΔNp73
mRNA is a frequent event, particularly among squamous cell carcinomas. P1
promoter remained essentially unmethylated in the majority of lung cancer
carcinomas. On the other hand, P2 hypomethylation strongly correlated with the
hypomethylation of LINE-1 transposon, a marker of global DNA hypomethylation,
indicating that ΔNp73 overexpression may be a passive consequence of global DNA
hypomethylation. Since overexpression of TAp73 isoforms could not be attributed to
change in the methylation status of P1 promoter, we plausibly hypothesized that it
could potentially be caused by deregulated activity of specific transcription factor(s) .
In the light of this notion, we then searched for novel transcription factors that could
activate P1 promoter and result to TAp73 overexpression in lung cancer. With the use
of bioinformatics tools, in vitro binding assays, and chromatin immunoprecipitation
analysis, a region extending -233 to -204 bps upstream of the transcription start site of
the human p73 P1 promoter, containing conserved Sp1 binding sites, was
characterized. Treatment of cells with Sp1 RNAi and Sp1 inhibitor functionally
suppresses TAp73 expression, indicating positive regulation of P1 by the Sp1 protein.
Notably Sp1 inhibition or silencing also reduces ΔΝp73 protein levels. Therefore, Sp1
directly regulates TAp73 transcription and affects ΔΝp73 levels in lung cancer.
ABSTRACT
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TAp73γ was shown to be the only TA isoform overexpressed in several lung cancer
cell lines and in 26 non-small cell lung cancers, consistent with Sp1 overexpression,
thereby questioning the apoptotic role of this specific p73 isoform in lung cancer.
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Μελέτη του ρόλου των ισόμορφων του γονιδίου ρ73 στον καρκίνο του πνεύμονα
