Involvement of polypyrimidine tract-binding protein (PTBP1) in maintaining breast cancer cell growth and malignant properties

Abstract

We have investigated some roles of splicing factor polypyrimidine tract-binding protein (PTBP1) in human breast cancer. We found that PTBP1 was upregulated in progressively transformed human mammary epithelial cells (HMECs), as well as in breast tumor cell lines compared with HMECs with finite growth potential and found that the level of PTBP1 correlated with the transformation state of HMECs. Knockdown of PTBP1 expression substantially inhibited tumor cell growth, colony formation in soft agar and in vitro invasiveness of breast cancer cell lines, a result similar to what we have reported in ovarian cancer. However, ectopic expression of PTBP1 (as a PTBP1–EGFP fusion protein) did not enhance the proliferation of immortalized HMEC. Rather, PTBP1 expression promoted anchorage-independent growth of an immortalized HMEC as assessed by increased colony formation in soft agar. In addition, we found that knockdown of PTBP1 expression led to upregulation of the expression of the M1 isoform of pyruvate kinase (PKM1) and increase of the ratio of PKM1 vs PKM2. PKM1 has been reported to promote oxidative phosphorylation and reduce tumorigenesis. Correspondingly, we observed increased oxygen consumption in PTBP1-knockdown breast cancer cells. Together, these results suggest that PTBP1 is associated with breast tumorigenesis and appears to be required for tumor cell growth and maintenance of transformed properties. PTBP1 exerts these effects, in part, by regulating the splicing of pyruvate kinase, and consequently alters glucose metabolism and contributes to the Warburg effect.

Splicing-factor alterations in cancers

“…Furthermore, PTB KD decreases breast and ovarian tumor cell proliferation, anchorage-independent growth, and invasiveness in vitro (He et al 2007(He et al , 2014, and is accompanied by an increased ratio of PKM1 versus PKM2 isoform and increased oxygen consumption (He et al 2014). PTB overexpression also enhances anchorage-dependent growth of immortalized human mammary epithelial cells (HMECs) (He et al 2014); however, overexpression of PTB alone is not sufficient to transform murine fibroblasts (Wang et al 2008). Together, these results suggest that PTB plays a role in breast tumorigenesis.…”

“…4). PTB expression levels are significantly up-regulated in breast, ovarian, glioblastoma, colorectal, and mucosal cancerous tissues compared with the corresponding normal tissues (Jin et al 2000;He et al 2004He et al , 2007He et al , 2014Takahashi et al 2015). PTB controls alternative splicing of target genes, such as pyruvate kinase PKM2 (Clower et al 2010), fibroblast growth factor receptor-1α-exon FGFR-1 (Jin et al 2000), and multidrug resistance protein 1 (MRP1), which contributes to the drug-resistance phenotype associated with many cancers (He et al 2004).…”

“…PTB is also overexpressed in most ovarian tumors, compared with matched normal tissue, and its overexpression is associated with an increased number of expressed MRP1 spliced isoforms (He et al 2004(He et al , 2007. Furthermore, PTB KD decreases breast and ovarian tumor cell proliferation, anchorage-independent growth, and invasiveness in vitro (He et al 2007(He et al , 2014, and is accompanied by an increased ratio of PKM1 versus PKM2 isoform and increased oxygen consumption (He et al 2014). PTB overexpression also enhances anchorage-dependent growth of immortalized human mammary epithelial cells (HMECs) (He et al 2014); however, overexpression of PTB alone is not sufficient to transform murine fibroblasts (Wang et al 2008).…”

A network-based analysis of colon cancer splicing changes reveals a tumorigenesis-favoring regulatory pathway emanating from ELK1

“…Intriguingly, PTBP1 is an alternative splicing factor previously shown to be overexpressed in brain tumors and implicated in their formation . It has also been suggested to be involved in breast cancer epithelial-mesenchymal transition (Shapiro et al 2011;He et al 2014). In the pathway we identified, ELK1 directly regulates the gene expression of PTBP1 and MYC, an oncogenic transcription factor that plays an important role in colon tumor development (Fearon 2011).…”

The role of splicing factors in deregulation of alternative splicing during oncogenesis and tumor progression

“…PTB1 and its variant nPTB are important regulators of neuronal development and their ratio changes during development. 116,117 hnRNP I was found to be overexpressed in ovarian, 82,118 brain 119 and breast cancers. 120 In brain cancer it was found to promote the skipping of an exon in FGF1, leading to the production of a high-affinity receptor.…”