Sp1 Transactivation of the TCL1 Oncogene

Abstract

Cis-regions and trans-factors controlling TCL1 oncogene expression are not known. We identified the functional TCL1 promoter by mapping four transcriptional start sites 24 -30 bp downstream of a TATA box. A 424-bp fragment upstream of the major start site showed robust promoter activity comparable with SV40 in both TCL1 expressing and non-expressing cell lines. Additional constructs spanning 10 kb upstream and 20 kb downstream of the start site showed only modest increases in reporter activity indicating that TCL1 expression is primarily controlled by the promoter. Ten putative Sp1-binding sites were identified within 300 bp of the start site, and three of these specifically bound Sp1. A dosedependent transactivation of the TCL1 promoter with Sp1 addition in Sp1-negative Drosophila SL2 cells was observed, and mutation of the three identified Sp1-binding sites significantly repressed reporter gene expression in 293T cells, confirming a key role for Sp1 in activating the TCL1 promoter in vivo. In TCL1 silent cell lines, CpG DNA methylation was rarely observed at functional Sp1 sites, and methylation of a previously reported NotI restriction site was associated with dense CpG methylation rather than endogenous TCL1 gene silencing. Together, these results indicate that Sp1 mediates transactivation of the TCL1 core promoter and that TCL1 gene silencing is not dependent on mechanisms involving Sp1 and NotI site methylation.

TORC2 regulates germinal center repression of the TCL1 oncoprotein to promote B cell development and inhibit transformation

“…CREB shRNA validated a role for CREB in regulating TCL1, with knockdown to 25% of native levels resulting in robust repression of TCL1 promoter activity. Overall, the data indicate that CREB along with Sp1 supports robust basal activity of the TCL1 promoter (22).…”

“…The effect of CREB on TCL1 promoter activity was determined in HEK293T cells using 424 bp of the TCL1 promoter (Ϫ424luc), as done before (SI Fig. 9) (22). TCL1 is not expressed in HEK293T cells, although its promoter is highly active in transient reporter assays (22).…”

“…9) (22). TCL1 is not expressed in HEK293T cells, although its promoter is highly active in transient reporter assays (22). CREB transactivation is mediated by increased cAMP, which results in protein kinase A or PKC activation and phosphorylation of CREB on Ser-133 (pCREB-133) (26-28).…”

“…Impaired apoptosis from failed TCL1 repression in GC B cells connects TCL1 dysregulation in patient lymphomas (12,14) to a mechanism for increased transformation (16,18). Factors so far suggested to regulate TCL1, including Nur77 (19,20), miRNA-29 and miRNA-181 (21), Sp1 (22), and EBV infection (23), fail to adequately explain TCL1 repression in GC B cells and its continued aberrant expression in lymphomas (11).…”

Overexpression of Sp1 transcription factor induces apoptosis

“…This indicates that Sp1 overexpression or overactivation contributes to transformation. This effect could be achieved via the Sp1-dependent regulation of growth-promoting oncogenes such as IEX-1 and TCL1 (Im et al, 2002;French et al, 2003). Involvement of Sp1 in cancer cell growth regulation is also strongly suggested by the growth inhibitory effect of dominantnegative Sp1 or Sp1-decoy oligodeoxynucleotides (Chen et al, 2000;Ishibashi et al, 2000;Chae et al, 2004).…”