Selective cytotoxicity of recombinant STXA1-GM-CSF protein in hematopoetic cancer cells

A-NGR fusion protein induces apoptosis in human cancer cells

The NGR peptide is one of the well-known peptides for targeting tumor cells. It has the ability to target aminopeptidase N (CD13) on tumor cells or the tumor vascular endothelium. In this study, the NGR peptide was used for targeting A subunit of the Shiga toxin to cancer cells. The cytotoxic effect of the A-NGR fusion protein was assessed on HT1080, U937, HT29 cancer cells and MRC-5 normal cells. For this purpose, cells were treated with different concentrations of A-NGR (0.5-40 µg/ml). The evaluation of cell viability was achieved by MTT assay. Apoptosis was determined by annexin-V/PI double staining flow cytometry. Alterations in the mRNA expression of apoptosis - related genes were assessed by real time RT- PCR. The results showed that A-NGR fusion protein effectively inhibited the growth of HT1080 and U937 cancer cells in comparison to negative control (PBS) but for CD13-negative HT-29 cancer cells, only at high concentrations of fusion protein was inhibited growth recorded. On the other hand, A-NGR had little cytotoxic effect on MRC-5 normal cells. The flow cytometry results showed that A-NGR induces apoptosis. Furthermore, the results of real time RT-PCR revealed that A-NGR significantly increases the mRNA expression of caspase 3 and caspase 9. Conclusively, A-NGR fusion protein has the ability of targeting CD13-positive cancer cells, the cytotoxic effect on CD13-positive cancer cells as well as has low cytotoxic effect on normal cells.

Shiga toxins

Shiga toxins are virulence factors produced by the bacteria Shigella dysenteriae and certain strains of Escherichia coli. There is currently no available treatment for disease caused by these toxin-producing bacteria, and understanding the biology of the Shiga toxins might be instrumental in addressing this issue. In target cells, the toxins efficiently inhibit protein synthesis by inactivating ribosomes, and they may induce signaling leading to apoptosis. To reach their cytoplasmic target, Shiga toxins are endocytosed and transported by a retrograde pathway to the endoplasmic reticulum, before the enzymatically active moiety is translocated to the cytosol. The toxins thereby serve as powerful tools to investigate mechanisms of intracellular transport. Although Shiga toxins are a serious threat to human health, the toxins may be exploited for medical purposes such as cancer therapy or imaging.

Attempts to express the A1-GMCSF immunotoxin in the baculovirus expression vector system

Immunotoxins are fusion proteins consisting of two elements, a targeting and a toxin moiety, and are designed for specific elimination of tumor cells. Previously we expressed a recombinant fusion protein consisting of the toxic fragment of Shiga toxin (A1) and GMCSF (A1-GMCSF) in Escherichia coli, and evaluated its cytotoxic properties in acute myeloid leukemia and colon carcinoma cell lines. In view of the specific cytotoxic effects of this immunotoxin, further detailed in-vitro and preclinical studies were undertaken. Large amounts of the recombinant protein of high purity and free of unwanted side products, such as lipopolysaccharides (LPS), were required. Since GMCSF is of mammalian origin and it requires proper disulfide bond formation, we intended to use the baculovirus expression vector system (BEVS) for the expression of the recombinant fusion protein. However, despite previous reports on the expression of several other immunotoxins by this system, the A1 derived fusion proteins revealed an inhibitory effect on baculoviral particle formation and even caused cell death in insect cells. This observation was further pursued and confirmed by the use of other baculoviral specific promoters. The salient features of this finding are described below.