[Cytotoxicity of the B subunit of diphtheria toxin to human histocytic lymphoma U937].

The B subunit of diphtheria toxin (DT) is responsible for interaction with receptor on the cell surface and translocation of the catalytically active A subunit across endosomal membrane into the cell cytosole. Receptor for DT and its B subunit is membrane-anchored precursor of heparin-binding epidermal growth factor-like growth factor (pro-HB-EGF), which under the action of metalloproteases turns into soluble form (sHB-EGF), which acts as a potent mitogen for different cell types. Since free B subunit of DT has no catalytic activity it is considered to be nontoxic. However its influence on the cells in vitro remains to be investigated. The aim of this study was to examine the influence of diphtheria toxin B subunit on viability of diphtheria-sensitive cells using B subunit recombinant analogues. It was shown that diphtheria toxin B subunit recombinant analogue at a concentration of 12.8 x 10(-7) M had a cytotoxic effect on the human histocytic lymphoma cell line U937, which expresses a large amount of sHB-EGF. Besides, the similar cytotoxic effect had a fusion protein which consisted of a B subunit and an enhanced green fluorescent protein (EGFP). However recombinant EGFP alone didnot influence the cell viability. Annexin-V-FITC/PI staining demonstrated that maximal cytotoxic effect had been elicited after 48 hours of cultivation. Cytotoxic test with trypan blue and propidium iodide staining excluded the direct influence of investigated proteins on the integrity of plasma membrane because of the ability of B subunit to pore formation. So, we offer a hypothesis that realization of cytotoxic effect of diphtheria toxin B subunit and its derivative on the U937 cell culture occurs via inhibition of mitogenic activity of sHB-EGF resulted in induction of apoptosis.