CANCER Splicing Your Way to Invasive Growth

Activation of the RON receptor tyrosine kinase promotes "invasive growth," a process that resembles the epithelial-to-mesenchymal transition that occurs during embryonic development and tumor progression. A constitutively active alternatively spliced isoform that lacks exon 11, -Ron, confers invasive properties when overexpressed and has been associated with the metastatic phenotype. Ghigna et al. found that -Ron was the predominant isoform in a majority of breast and colon cancer specimens, whereas Ron transcripts containing exon 11 predominated in healthy tissue. The authors identified two regulatory elements in exon 12, an exonic splicing silencer (ESS) and an exonic splicing enhancer (ESE), by expressing minigenes with Ron cassettes containing exons 10, 11, and portions of 12 in KATOIII cells (which preferentially express -Ron) and comparing the pattern of splicing to that of endogenous Ron mRNA. When cloned into a heterologous pre-mRNA and expressed in KATOIII and T47D cells (which show low abundance of -Ron), the ability of the ESE to promote splicing paralleled the relative abundance of -Ron mRNA in the two cell lines. The abundance of the splicing factor SF2/ASF also paralleled the abundance of -Ron, and SF2/ASF overexpression increased splicing of a reporter containing the ESE (an effect that was abolished by mutation of an SF2/ASF binding sequence) as well as that of endogenous Ron. Moreover, SF2/ASF bound an RNA probe containing the wild-type but not the mutated enhancer. SF2/ASF overexpression promoted cell migration, an effect that was blocked by -Ron knockdown, whereas down-regulation of SF2/ASF inhibited cell motility. Thus, the authors suggest that SF2/ASF may promote cell motility and contribute to malignant transformation through its regulation of Ron splicing.

C. Ghigna, S. Giordano, H. Shen, F. Benvenuto, F. Castiglioni, P. M. Comoglio, M. R. Green, S. Riva, G. Biamonti, Cell motility is controlled by SF2/ASF through alternative splicing of the Ron protooncogene. Mol. Cell 20, 881-890 (2005). [PubMed]