Aberrant laminin beta3 isoforms downstream of EWS-ETS fusion genes in Ewing family tumors.

Ewing family tumors (EFTs) are associated with a chromosomal translocation resulting in a fusion of the amino-terminus of EWS with the DNA-binding domain of an ETS transcription factor. Although previous reports suggested that these chimeric proteins would act as aberrant transcription factors, their downstream targets have not been fully elucidated. To identify downstream targets of these EWS-ETS fusion proteins, we introduced EWS-ETS fusion constructs into a human fibrosarcoma cell line, HT-1080, by retroviral transduction. Here we report that the LAMB3 gene encoding the beta3 chain of basement membrane protein laminin-5 is induced to a significantly higher level in cells expressing EWS-ETSs than in cells expressing normal ETSs. Additionally through use of an antisense oligonucleotide for EWS-ERG in the W-ES EFT cell line, laminin beta3 protein was reduced coordinately with EWS-ERG fusion protein expression. Furthermore, we found small mRNAs were preferentially transcribed from the LAMB3 gene in EFT cell lines. Molecular cloning of the entire coding region shows that the alternative transcripts from different promoter(s) located within the intron 14, which encode small proteins, likely are major products of the LAMB3 gene in EFT cells. We show that the small isoforms conferred increased anchorage-independent proliferation to NIH3T3 cells. Together with previous studies showing that laminin-5 is involved in the invasive and malignant phenotype of several tumor types, our data suggest that the oncogenic effect of EWS-ETS may be mediated in part by upregulation of LAMB3 expression.