Butyltins and calmodulin: which interaction?

Abstract

Tributyltin (TBT), widely used as an antifouling biocide, is the most abundant pesticide in coastal environments. One of its main toxic effects is immunosuppression in both vertebrates and invertebrates. At sublethal doses of TBT, phagocytes lose their ability to move towards and ingest foreign particles. For short-term cultures of haemocytes (60 min at 25 °C) of a marine invertebrate, the colonial ascidian Botryllus schlosseri, exposed to 10−5M TBT, we previously reported dose- and time-dependent impairment of yeast phagocytosis and changes in cell morphology related to cytoskeleton disorganization. These effects are Ca2+-dependent, since inactivation of Ca2+-adenosine triphosphatase and a sustained increase in cytosolic Ca2+ occurred. As TBT can antagonize the effect of chlorpromazine, a specific calmodulin (CaM) inhibitor, and the co-presence of exogenous CaM and TBT in the incubation medium resulted in the absence of effects, we hypothesized an interaction between TBT and CaM. TBT may remove endogenous CaM from cell proteins, thus inactivating them and causing alteration of Ca2+ homeostasis. With the aim of confirming the hypothesis of a direct TBT–CaM interaction, we first studied the effects of co-incubation of TBT with other exogenous proteins on restoring the ability of phagocyte morphology. Although bovine serum albumin was never able to restore cell morphology, the effect of human spectrin was similar to that described for CaM, suggesting a common non-specific mechanism of action based on the interaction of TBT with exposed hydrophobic pouches. We also analysed the conformational changes of pure CaM (10−5M) in the presence of various concentrations (10−4 to 10−3M) of TBT and its degradation products, dibutyltin (DBT) and monobutyltin (MBT), by circular dichroism. Results indicate the dose- and time-dependent interaction of TBT with CaM. This interaction is a non-covalent interaction, probably hydrophobic in nature, between the aliphatic chains of TBT and the hydrophobic regions of Ca2+-activated CaM. DBT and MBT turned out to be less active in inducing CaM conformational changes, without any significant differences between the two compounds

Biological activity studies on organotin(IV)n+ complexes and parent compounds

“…Cima and Ballarin revealed apoptosis induced by TBT in haemocytes of the ascidian Botryllus schlosseri [82]. The cellular death for apoptosis is an integrant part of embryonic development, with organized and regulated biochemical events, as intracellular signal transduction, ordered enzyme cascades and targeted cell deletion, in response to a variety of stimuli, when various structures no longer needed, must be removed [83].…”

Impact of triphenyltin acetate in microcosms simulating floodplain lakes. II. Comparison of species sensitivity distributions between laboratory and semi-field

“…Organotins have been reported to inhibit mitochondrial oxidative phosphorylation and consequently energy transfer, Ca 2+ homeostasis, protein and DNA synthesis in the cell (Chandra et al 1989;Girard et al 1997;Tiano et al 2003), to cause immunosuppression and premature apoptosis (programmed cell death) in both vertebrates and invertebrates (Stridh et al 1999;Cima et al 2002), and have photosynthesis inhibiting properties (Mooney and Patching 1995). This variety of fundamental processes are not immediately visible and may take time before effects can be observed.…”

Immunotoxicity in ascidians: Antifouling compounds alternative to organotins—IV. The case of zinc pyrithione

“…In contrast with TBT (Cima and Ballarin, 2000;Cima et al, 2002), a little effect was observed on Ca 2+ homeostasis because a decrement in Ca 2+ -ATPase activity occurred after exposure to the highest ZnP concentration but no increase of cytosolic Ca 2+ occurred as expected. No effect on the cytosolic Ca 2+ content was detected after exposure to ZnP with the exception of the highest concentration, which caused a significant decrease.…”

“…The results indicated an antagonist-like interaction on their effects on the cell spreading index, suggesting competition between the two substances for the same molecular receptor, most likely at the level of proteins of the cytoskeleton, associated with it, or acting as regulators. Because the direct interaction of TBT with calmodulin in the alteration of morphology of the cytoskeleton is a well-known mechanism of action (Cima and Ballarin, 2000;Cima et al, 2002), it is possible to hypothesise a similar interaction for ZnP involving a hydrophobic domain (pyrithiolate) and positive charges (on zinc). Phagocytosis can be inhibited by various mechanisms of action in which subcellular and molecular components are affected, resulting in cytoskeletal alterations with loss of focal adhesions.…”