The immunosuppressant tributyltin oxide blocks the mTOR pathway, like rapamycin, albeit by a different mechanism

Abstract

We treated the thymoma cell line (EL4) with two model immunosuppressants, rapamycin and tributyltin oxide (TBTO), and compared their effects on the expression levels of proteins that are downstream targets of mTOR kinase 1 (mammalian target of rapamycin, known also as mechanistic target of rapamycin): p70 ribosomal S6 kinase1 and 4E-binding protein 1, a repressor of the cap-binding protein eIF4E. In addition, we evaluated the levels of ribosomal protein S6, p-eIF4B, substrates of p70S6 kinase1, matrin 3 and ribonucleotide reductase, subunit RRM2. The levels of these proteins were evaluated in cell lysates by immunoblot. We found that both compounds inhibited the phosphorylation state of p70S6 kinase 1 and its substrates; however, TBTO, in contrast to rapamycin, reduced the level of the total p70S6k1. Besides, we detected a band with a molecular weight of c. 32 kDa only in the TBTO-treated lysates. This band was detected with a monoclonal antibody specific for S6k1, suggesting that this band might be a degradation product of the kinase. Further, TBTO and rapamycin differentially affected 4E-binding protein 1; the former compound stimulated its phosphorylation state whereas the latter inhibited it. The two immunosuppressants did not affect the level of ribonucleotide reductase, but TBTO downregulated matrin3, in agreement with a previous report, whereas rapamycin had no effect on the expression level of this latter protein. We conclude that TBTO inhibits, like rapamycin, the p70 S6 kinase 1 pathway, but with a different mechanism. However, in contrast to rapamycin, which inhibits the cap-dependent translation, TBTO increases the phosphorylation of 4E-binding protein1.

Tributyltin Oxide Exposure During in vitro Maturation Disrupts Oocyte Maturation and Subsequent Embryonic Developmental Competence in Pigs

“…Several studies have shown that harmful internal and external factors can decrease oocyte quality and in turn reduce embryonic developmental competence ( Liang et al, 2017b ; Nie et al, 2019 ; Sun et al, 2020 ). The negative effects of TBTO exposure on different types of cells in different species, especially germ cells, have also been reported ( Baken et al, 2007 ; Mochida et al, 2007 ; Osman and van Loveren, 2012 , 2014 ). Previous research has suggested that TBTO exposure impairs mouse oocyte quality by inducing mitochondrial dysfunction, oxidative stress and apoptosis, thus causing abnormal spindle organization and chromosome alignment and eventually resulting in fertility failure ( Yang et al, 2019 ).…”

“…As TBTO is a model immune poison, TBTO exposure can inhibit chemotaxis associated with the chemokine CXCL12, thereby affecting the migration of white blood cells ( Shao et al, 2016 ). In addition, TBTO exposure can reduce the immunotoxicity of interleukin 2 by blocking the mTOR pathway ( Osman and van Loveren, 2014 ). Research has shown that TBTO exposure affects proliferation and energy sensor pathways by downregulating MAPK, matrin-3 and the ribonucleotide reductase subunit RRM2, which are implicated in cell proliferation in mouse thymoma cells ( Osman and van Loveren, 2012 ).…”

Protein phosphorylation profiling identifies potential mechanisms for direct immunotoxicity

“…Our results are in line with the previously reported findings by Osman and van Loveren (2012) that TBTO down-regulated the expression level of phosphorylated p70S6K in the EL4 mouse thymoma line. In addition, recent results from Osman and van Loveren (2013) showed that, although TBTO dephosphorylated p70S6K, the phosphorylation of the other downstream effector of mTORC1, 4EBP1, was induced by TBTO, whereas rapamycin dephosphorylated this protein. This result also indicated that dephosphorylation of p70S6K by TBTO was independent of mTOR.…”

Tributyltin oxide exposure impairs mouse oocyte maturation and its possible mechanisms

“…Osman et al figured out TBTO inhibits EL4 cells proliferation by impairing protein synthesis about cell structure and cytoskeleton. To fully understand the underlying mechanism, microarray studies and phosphor proteomic analysis have been conducted and researchers figure out that TBTO affects mammalian target of rapamycin signaling pathway first, further influences energy pathways and cell proliferation . Except for the immunotoxicity effect, TBTO also causes adverse effects to vertebrate and invertebrate endocrine systems and endocrine disruption .…”

“…To fully understand the underlying mechanism, microarray studies and phosphor proteomic analysis have been conducted and researchers figure out that TBTO affects mammalian target of rapamycin signaling pathway first, further influences energy pathways and cell proliferation. 14 Except for the immunotoxicity effect, TBTO also causes adverse effects to vertebrate and invertebrate endocrine systems and endocrine disruption. 15 Otherwise, reactive oxygen species (ROS) in the liver of rats exposed to TBTO is increased significantly.…”