Cell-type-specific expression of alternatively spliced human fibronectin IIICS mRNAs

Fibronectin polypeptide diversity is generated to a large extent by alternative splicing of the fibronectin primary transcript at three sites: two extra domain exons encoding extra structural repeats and a region of nonhomologous sequence termed the type-III connecting segment (IIICS). A novel double primer extension assay was developed to identify and quantify simultaneously each of the five human IIICS mRNA splicing variants. Expression of the five IIICS variants was analyzed in a variety of human normal and tumor cell types as well as in human liver. Differences in IIICS expression patterns were observed among different cell types, among fibroblasts of different tissue origins, and between comparable normal and transformed cells. The most predominant cell-type-specific differences were in the abundance of the one IIICS- mRNA variant relative to the four IIICS+ variants. The percentage of O variant (IIICS-) mRNAs within the total fibronectin mRNA pool varied between 3 and 17% among tumor cells and between 7 and 46% among normal cells. The O variant composed 57% of the fibronectin mRNA in liver tissue, correlating with the previously described increased abundance of IIICS- polypeptide subunits in plasma fibronectin, compared with those in cellular fibronectins. Additional cell-type-specific changes among the expression levels of the four IIICS+ mRNA variants are consistent with a proposed model in which regulation of an alternative selection of a 3'splice site predominates over regulation of the selection of a 5' splice site in generating specific patterns of IIICS mRNA expression.