Induction of apoptosis by the 16-kDa amino-terminal fragment of the insulin-like growth factor binding protein 3 in human colonic carcinoma cells

Abstract

The insulin-like growth factor binding protein 3 (IGFBP-3) is the major circulating IGF binding protein, its function regulated by proteolytic cleavage. The fragments generated have recently been suggested to have IGF-independent biological activity. We have previously established that IGFBP-3 can potentiate apoptosis in colorectal epithelial cells, although its use as a therapeutic reagent may be limited by the fact that it is cleaved in the circulation. Therefore the aim of these experiments was to determine whether the 16-kDa proteolytic fragment (1-95IGFBP-3) would have IGF-independent pro-apoptotic activity in human colonic carcinoma derived cells. We report that the enforced expression of 1-95IGFBP-3 increased the induction of apoptosis by the naturally occurring short chain fatty acid sodium butyrate (NaBt) in the IGF non-responsive HT29 human colorectal carcinoma cell line. Furthermore, the addition of condition medium containing the secreted 1-95IGFBP-3 was as effective as the intact IGFBP-3 protein at potentiating apoptosis. Although not associated with changes in Bcl-2, Bcl-XL, Bax, Bad or Bak expression levels, we report that the expression of the pro-apoptotic 1-95IGFBP-3 fragment is associated with the inhibition of TNFalpha-induced NF-kappaB activity, similar to that reported for the full length IGFBP-3 protein. These results suggest that the 16-kDa 1-95IGFBP-3 fragment is as effective as an intact recombinant protein when used in combination with apoptosis inducing agents, and due to its relative stability in the circulation, it may be important for use as an adjuvant in the treatment of colorectal cancer.