Differential expression of IGF-1 mRNA isoforms in colorectal carcinoma and normal colon tissue

Abstract

The insulin-like growth factor (IGF)-1 gene consists of 6 exons resulting in the expression of 6 variant forms of mRNA (IA, IB, IC, IIA, IIB and IIC) due to an alternative splicing. The mechanisms of IGF-1 gene splicing and the role of local expression manifested by IGF-1 mRNA variants in colorectal carcinoma (CRC) have not been extensively investigated. Therefore, the aim of our study was to analyse the expression of IGF-1 mRNA isoforms [A, B, C, P1 (class I) and P2 (class II)], as well as the protein expression in CRC and control samples isolated from 28 patients. The expression of Ki-67 was also analysed and clinical data were obtained. For this purpose, we used quantitative real-time PCR (qPCR) and immunocytochemistry. The expression of mRNAs coding for all splicing isoforms of IGF-1 was observed in every tissue sample studied, with a significantly lower expression noted in the CRC as compared to the control samples. The cytoplasmic expression of IGF-1 protein was found in 50% of the CRC and in ~40% of the non-tumor tissues; however, no significant quantitative inter-group differences were observed. The expression of the IGF-1 gene in the 2 groups of tissues was controlled by the P1 and P2 promoters in a similar manner. No significant differences were detected in the expression of the IGF-1 A and B isoforms; however, their expression was significantly higher compared to that of isoform C. No significant differences were observed between the expression of Ki-67 mRNA in the CRC and control tissue even though the expression of the Ki-67 protein was higher in the CRC compared to the control samples. Ki-67 protein expression was associated with the macroscopic and microscopic aspects of CRC. A significant positive correlation was found between the local production of total mRNA and isoform A and the expression of Ki-67 mRNA, although only in the non-tumor tissues. In CRC samples, the local expression of the total IGF-1 mRNA and all splicing isoforms of IGF-1 mRNA decreased as compared to the normal colon tissues, although however, with conservation of both gene promoter activities and with the continued principal splicing IGF-1 mRNA isoforms.