Effects of Botulinum Toxin Type D on Secretion of Tumor Necrosis Factor from Human Monocytes

Abstract

Botulinum toxins are potent neurotoxins which block the release of neurotransmitters. The effects of these toxins on hematopoietic cells, however, are unknown. Monocytes secrete a variety of polypeptide growth factors, including tumor necrosis factor (TNF). In the study reported here, the effects of botulinum toxin type D on the secretion of TNF from human monocytes were examined. The results demonstrate that botulinum toxin type D inhibits the release of TNF from monocytes activated by lipopolysaccharide (LPS) but not by 12-O-tetradecanoylphorbol-13-acetate. Botulinum toxin type D had no detectable effect on intracellular TNF levels in LPS-treated monocytes, indicating that the effects of this toxin involve the secretory process. This inhibitory effect of botulinum toxin type D on TNF secretion from LPS-treated monocytes was partially reversed by treatment with 12-O-tetradecanoylphorbol-13-acetate or introduction of guanosine 5′-[γ-thio]t-riphosphate into these cells. The results demonstrate that TNF secretion is regulated by at least two distinct guanine nucleotide-binding proteins, one responsible for the activation of phospholipase C and another which acts as a substrate for botulinum toxin type D. ADP-ribosylation of monocyte membranes by botulinum toxin type D demonstrated the presence of three substrates with M_rs of 45,000, 21,000, and 17,000. While the role of these substrates in exocytosis is unknown, the results suggest that the M_r 21,000 substrate is involved in a process other than TNF secretion.