We identified two splicing variants of the leucine-rich repeat Fli-I-interacting protein 2 (LRRFIP2) whose expression levels are tightly regulated by epithelial splicing regulatory protein 1 (ESRP1); the epithelial isoform inhibits liver metastasis of gastric cancer cells and the mesenchymal isoform promotes cancer metastasis.
Alternative splicing is a ubiquitous mechanism in complex organisms that can generate protein diversity. Different splicing events select and utilize alternative splice sites in pre-mRNA to generate multiple mRNA variants in a single gene1. These events are tightly regulated by several factors, and dysregulation has been observed in several diseases, including various types of cancer. ESRP1 is an epithelial cell-specific RNA binding protein that regulates alternative splicing of several genes involved in the epithelial-mesenchymal transition (EMT), which plays an important role in metastasis by reducing tumor motility and invasiveness2-4.
What do our findings show?
In our paper, we showed that the epithelial and mesenchymal isoform switches of LRRFIP2 regulated by ESRP1 correlate with the metastatic potential of gastric cancer cells. We found that the expression of two splicing variants of LRRFIP2 - epithelial (variant 2) and mesenchymal type (variant 3) - was directly regulated by ESRP1 as well as closely correlated with the expression of ESRP1 (Figure 1). Furthermore, even with only one small exon (exon 7) distinguishing the two variants, ectopic expression of the mesenchymal isoform of LRRFIP2 dramatically increased liver metastases in gastric cancer cells, whereas deletion of exon 7 leading to isoform transition from the mesenchymal form to the epithelial form resulted in marked inhibition of liver metastasis. To gain further insight into the molecular mechanisms behind this phenomenon, we identified coactivator-associated arginine methyltransferase 1 (CARM1) as a variant 2-specific binding partner and found that the epithelial LRRFIP2 isoform inhibits the oncogenic function of CARM1 through direct interaction. It is interesting to observe that this single exon acts as a constraint on the protein-protein interactions of the oncogenic isoform, allowing CARM1, another villain in the story, to continue to aid the transcription of other oncogenes downstream.
Future research will focus on three aspects; to elucidate the regulatory mechanisms of ESRP1 expression required to control the expression of two splicing variants of LRRFIP2, to explore the potential of LRRFIP2 isoforms as biomarkers for cancer metastasis and as prognostic markers for clinical outcomes in cancer patients, and to investigate whether LRRFIP2 isoforms can be used as therapeutic biomarkers for CARM1 inhibitors.
Now that we know what controls the evil side of proteins that lurk in the dark, at least for this specific protein with a dual personality, we can better answer the question asked in The strange case of Dr. Jekyll and Mr. Hyde. "Yeah, Henry Jekyll went to bed and woke Edward Hyde. How do I explain this? I asked myself. Then I got caught up in another panic and how do I deal with it?"
- Keren, H., Lev-Maor, G. & Ast, G. Alternative splicing and evolution: diversification, exon definition and function. Nat Rev Genet 11, 345-55 (2010).
- Warzecha, C.C., Sato, T.K., Nabet, B., Hogenesch, J.B. & Carstens, R.P. ESRP1 and ESRP2 are epithelial cell-type-specific regulators of FGFR2 splicing. Mol Cell 33, 591-601 (2009).
- Shapiro, I.M. et al. An EMT-driven alternative splicing program occurs in human breast cancer and modulates cellular phenotype. PLoS Genet 7, e1002218 (2011).
- Yao, D., Dai, C. & Peng, S. Mechanism of the mesenchymal-epithelial transition and its relationship with metastatic tumor formation. Mol Cancer Res 9, 1608-20 (2011).