PIN1 Is an E2F Target Gene Essential for Neu/Ras-Induced Transformation of Mammary Epithelial Cells

Abstract

Oncogenes Neu/HER2/ErbB2 and Ras can induce mammary tumorigenesis via upregulation of cyclin D1. One major regulatory mechanism in these oncogenic signaling pathways is phosphorylation of serines or threonines preceding proline (pSer/Thr-Pro). Interestingly, the pSer/Thr-Pro motifs in proteins exist in two completely distinct cis and trans conformations, whose conversion is catalyzed specifically by the essential prolyl isomerase Pin1. By isomerizing pSer/Thr-Pro bonds, Pin1 can regulate the conformation and function of certain phosphorylated proteins. We have previously shown that Pin1 is overexpressed in breast tumors and positively regulates cyclin D1 by transcriptional activation and posttranslational stabilization. Moreover, in Pin1 knockout mice, mammary epithelial cells fail to undergo massive proliferation during pregnancy, as is the case in cyclin D1 null mice. These results indicate that Pin1 is upregulated in breast cancer and may be involved in mammary tumors. However, the mechanism of Pin1 overexpression in cancer and its significance in cell transformation remain largely unknown. Here we demonstrate that PIN1 expression is mediated by the transcription factor E2F and enhanced by c-Neu and Ha-Ras via E2F. Furthermore, overexpression of Pin1 not only confers transforming properties on mammary epithelial cells but also enhances the transformed phenotypes of Neu/Ras-transformed mammary epithelial cells. In contrast, inhibition of Pin1 suppresses Neu-and Ras-induced transformed phenotypes, which can be fully rescued by overexpression of a constitutively active cyclin D1 mutant that is refractory to the Pin1 inhibition. Thus, Pin1 is an E2F target gene that is essential for the Neu/Ras-induced transformation of mammary epithelial cells through activation of cyclin D1.Phosphorylation of proteins on serine/threonine residues preceding proline (pSer/Thr-Pro) is a key regulatory mechanism for the control of cell proliferation and transformation (6,18,22,31). For example, oncogenic Neu/Ras signaling has shown to lead to activation of various Pro-directed protein kinases, which eventually enhance transcription of the cyclin D1 gene via multiple transcription factors, including E2F, cjun/AP-1, and ␤-catenin/T-cell factor (TCF) (1,3,17,26,28,47). In addition to transcriptional activation, cyclin D1 is regulated by posttranslational modifications. Phosphorylation of cyclin D1 on the Thr286-Pro site by glycogen synthase kinase 3␤ (GSK-3␤) enhances its nuclear export and subsequent degradation (2, 9, 10).Cyclin D1 has been shown to play a pivotal role in the development of cancer, especially breast cancer. Overexpression of cyclin D1 is found in 50% of patients with breast cancer (5, 15). Importantly, overexpression of cyclin D1, especially the mutant cyclin D1 T286A , can transform fibroblasts (2, 20). In contrast, inhibition of cyclin D1 expression causes growth arrest in tumor cells (4,11,26,45). Furthermore, transgenic overexpression of cyclin D1 in the mouse mammary gland leads to mammary hyperplasia and event...

Novel link between E2F and p53: proapoptotic cofactors of p53 are transcriptionally upregulated by E2F

“…One mechanism involves E2F-induced activation of genes whose protein products positively regulate p53 stability and transcriptional activity; this group is constantly growing and presently includes ARF, ATM, Chk2 and PIN1. [15][16][17][30][31][32][33][34][35][36] After its stabilization and activation, p53 can elicit a number of different cellular responses including, most notably, growth arrest and apoptosis. The E2F-regulated genes listed above, while contributing to p53 activation, do not explain how E2F1 directs p53 to favor apoptosis over growth arrest.…”

Some p53-binding proteins that can function as arbiters of life and death

“…Pin1 is also controlled by E2F1 and induced during cell cycle progression. 76,77 Moreover, phosphorylation events regulate both its nuclear localization and substrate binding capability (reviewed in Wulf et al 59 ), while recently its ubiquitin-mediated degradation has been found to be inhibited by mitotic signals through Polo-like Kinase1 (Plk1). 78 Plk1 is overexpressed in many tumors and this may contribute to the increased levels of Pin1 observed in several cancers.…”

Pin1 overexpression is associated with poor differentiation and survival in oral squamous cell carcinoma

“…Recent studies have demonstrated that Pin1 increased the transcription level of several oncogenes including cyclin D1 and c-myc as well as conferred anchorage-independent cell growth to normal mammary epithelial cells (16,34). Breast cancer for example, Pin1 protein overexpression can lead to cyclin D1 elevation and transformation of breast epithelial cells (16,34).…”

“…By using an antisense approach, Ryo et al, showed that down-regulation of Pin1 in HeLa cells led to a decrease in ß-catenin levels (15). In Pin1 knockout mice, lower ß-catenin levels were found in all tissues compared to those of wild-type mice (15,34). In addition, Pin1 has been demonstrated to bind directly to phosphorylated ß-catenin at pT246P motif in vitro and in vivo and block the interaction between ß-catenin and APC (15,34).…”

“…In Pin1 knockout mice, lower ß-catenin levels were found in all tissues compared to those of wild-type mice (15,34). In addition, Pin1 has been demonstrated to bind directly to phosphorylated ß-catenin at pT246P motif in vitro and in vivo and block the interaction between ß-catenin and APC (15,34). Moreover, Pin1-ß-catenin signaling has been shown to regulate cyclin D1 expression in human breast and liver cancers when Pin1 was overexpressed (16)(17)(18)(19)(20).…”