Objective. The angiogenic capability of colorectal carcinomas (CRC) is determined by alternative splicing of VEGF-A pre-mRNA. The intracellular protein TIA-1 alters posttranscriptional RNA processing and binds VEGF-A mRNA. We therefore tested the hypothesis that TIA-1 could regulate VEGF-A isoform expression in colorectal cancers.
Design. TIA-1 and VEGF-A isoform expression was investigated in colorectal cancers and cell lines. The effect of isoform specific knockdown and over-expression of TIA-1 on VEGF, tumour growth and angiogenesis in vivo was determined.
Results. An endogenous splice variant of TIA-1 encoding a truncated protein, short TIA-1 (shTIA-1) was expressed in CRC tissues and invasive K-Ras mutant colon cancer cells and tissues but not in adenoma cell lines. shTIA-1 was more highly expressed in CRC than in normal tissues and increased with tumour stage. Knockdown of shTIA-1 or overexpression of full length TIA-1 (flTIA-1) induced expression of VEGF-A165b. Whereas flTIA-1 selectively bound VEGF-A165 mRNA and increased translation of VEGF-A165b, shTIA-1 prevented this binding. In nude mice, xenografted colon cancer cells overexpressing flTIA-1 formed smaller, less vascular tumours than those expressing shTIA-1, and inhibited the effect of anti-VEGF antibodies.
Conclusions. These results indicate that alternative splicing of an RNA binding protein may regulate isoform specific expression of VEGF providing an added layer of complexity to the angiogenic profile of colorectal cancer and their resistance to anti-angiogenic therapy.