Regulation of constitutive expression of mouse PTEN by the 5′-untranslated region

Abstract

PTEN tumor suppressor serves as a major negative regulator of survival signaling mediated by PI3 kinase/ AKT/protein kinase B pathway, and is inactivated in various human tumors. Elucidation of mechanisms responsible for PTEN expression is important for providing insight into strategies to control the loss of PTEN expression in human cancers. Although recent studies suggested that p53 and Egr-1 can modulate induced PTEN expression, the mechanism responsible for ubiquitous constitutive expression of PTEN remains elusive. PTEN mRNA contains a highly conserved and GC-rich 5 0 -untranslated region (5 0 -UTR). Recently, it has been shown that the long 5 0 -UTR sequences of several growthregulated mRNAs contain promoters that can generate mRNAs with shorter 5 0 -UTRs. In this paper, we tested whether the 5 0 -UTR sequence of mouse PTEN contains a promoter that is responsible for constitutive expression of PTEN. We found that the long 5 0 -UTR sequence of mouse PTEN severely inhibits translation of PTEN and a heterologous gene firefly luciferase. Deletion of the most 5 0 -UTR sequence would enhance translation efficiency 100-fold. We also showed that the 5 0 -UTR sequence of mouse PTEN does not have an internal ribosome entry site (IRES) that can mediate cap-independent initiation of translation. Instead, we found that the 5 0 -UTR sequence of mouse PTEN contains a strong promoter that drives the production of a transcript with shorter 5 0 -UTRs, which can be translated with higher efficiency. This promoter was mapped to the region between À551 and À220 bases upstream of the translation start codon. Cotransfection analysis using Drosophila SL2 cells showed that Sp1 is one of the major transcription factors that can constitutively activate this promoter. Two endogenous PTEN transcripts with 5 0 -UTRs of 193 and 109 bases were found in DU145 and H226 cell lines. Based on these observations, we conclude that the PTEN expression may be regulated at both transcriptional and translational levels, and that the 5 0 -UTR sequence of PTEN contains a promoter that is responsible for constitutive PTEN expression.

The transcription factor Egr1 is a direct regulator of multiple tumor suppressors including TGFβ1, PTEN, p53, and fibronectin

“…In addition to the mouse system and human cells (MEFs, HT1080, 293T 20 ), consistent significant effects have been observed in other cells and tissues, [21][22][23][24][25] (Table 1). Moorehead et al 22 observed that Egr1 is induced by and required for IGF-II-dependent induction of PTEN in the mouse mammary gland, an effect that is absent in Egr1-null mice.…”

Tumours and tremors: how PTEN regulation underlies both

“…Interestingly, the function of the PTEN promoter and PTEN mRNA expression appear to be regulated by more than one transcription factor. For example, the genomic PTEN promoter sequence contains a GC-rich 5 0 untranslated region that can be activated by the transcription factor Sp1 to drive constitutive PTEN expression (Han et al, 2003). Another transcription factor, early growth response-1 (Egr-1), binds to the PTEN untranslated region and activates PTEN transcription during irradiation-induced signalling.…”

Increased PTEN expression due to transcriptional activation of PPARγ by Lovastatin and Rosiglitazone

“…Several groups have shown that PTEN transcription may be upregulated by Early growth response-1 (Egr-1), 14 p53 15 and Sp1. 16 In contrast, NF-jB has recently been shown to inhibit PTEN transcription by a yet undetermined mechanism. 17 In 2001, 2 putative binding sites for the transcription factor peroxisome proliferator-activated receptor-gamma (PPARg) were identified about 10 kb upstream of the minimal promoter region of PTEN.…”