Transcriptional activation of the insulin-like growth factor I receptor gene by the Kruppel-like factor 6 (KLF6) tumor suppressor protein: potential interactions between KLF6 and p53.
The IGF system plays an important role in prostate cancer initiation and progression. Most of the biological actions of IGF-I and IGF-II are mediated by activation of the IGF-I receptor (IGF-IR). Evidence accumulated in recent years indicates that acquisition of the malignant phenotype is initially IGF-IR dependent, but progression toward metastatic stages is usually associated with a decrease in IGF-IR levels. The Kruppel-like factor 6 (KLF6) is a zinc finger-containing transcription factor that was shown to be mutated in a significant portion of prostate and other types of cancer. To examine the potential regulation of IGF-IR gene expression by KLF6, we measured KLF6 levels in prostate-derived cell lines displaying different levels of IGF-IR. The results of Western analysis showed that KLF6 levels were higher in nontumorigenic P69 cells expressing high IGF-IR levels than in metastatic M12 cells containing reduced IGF-IR levels. Transient coexpression of wild-type, but not mutated, KLF6 together with an IGF-IR promoter-luciferase reporter plasmid resulted in an approximately 3.4-fold stimulation of IGF-IR promoter activity. Furthermore, KLF6 expression induced a significant increment in endogenous IGF-IR levels. Deletion analysis of the IGF-IR promoter revealed that a cluster of four GC boxes located between nucleotides -399 and -331 mediates a significant portion of the transactivating effect of KLF6. KLF6, although unable to stimulate IGF-IR promoter activity in Sp1-null Drosophila-derived Schneider cells, significantly enhanced the effect of Sp1. To assess the potential interactions between KLF6 and p53 in the regulation of IGF-IR gene expression, transfections were performed in the colorectal cancer cell line HCT116(+/+), which expresses p53, and its HCT116(-/-) derivative, which lacks p53. KLF6 exhibited an enhanced activity in p53-containing, compared with p53-null, cells. In addition, we were able to detect a physical interaction between KLF6 and p53. In summary, we have identified the IGF-IR gene as a novel downstream target for transcription factor KLF6. The regulation of IGF-IR gene expression by KLF6 may have significant implications in terms of cancer initiation and/or progression.
Rubinstein
Moran
M
Department of Clinical Biochemistry, Sackler School of Medicine, Tel Aviv University, Tel Aviv 69978, Israel.
Idelman
Gila
G
Plymate
Stephen R
SR
Narla
Goutham
G
Friedman
Scott L
SL
Werner
Haim
H
D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D006801:Humans; D000075723:Kruppel-Like Factor 6; D051741:Kruppel-Like Transcription Factors; D011401:Promoter Regions, Genetic; D011518:Proto-Oncogene Proteins; D017526:Receptor, IGF Type 1 / Q000235:genetics*; D015534:Trans-Activators / Q000502:physiology*; D015533:Transcriptional Activation; D016159:Tumor Suppressor Protein p53 / Q000502:physiology*
C105465:KLF6 protein, human; D000075723:Kruppel-Like Factor 6; D051741:Kruppel-Like Transcription Factors; D011518:Proto-Oncogene Proteins; D015534:Trans-Activators; D016159:Tumor Suppressor Protein p53; D017526:Receptor, IGF Type 1