Tiron and trolox potentiate the autophagic cell death induced by magnolol analog Ery5 by activation of Bax in HL-60 cells

Abstract

This study describes the mechanism of trolox and tiron induced potentiation of cytotoxicity caused by Ery5, an analog of magnolol, in human myeloid leukemia HL-60 cells. Ery5 induced cytotoxicity in HL-60 cells by involving activation of bax and cleavage of caspase 3, which contributed towards activation of both apoptotic and autophagic pathways. Trolox and tiron, even at non-toxic concentrations, contributed to the cytotoxicity of Ery5 by activation of autophagic proteins like ATG7, ATG12 and LC3-II. Z-VAD-fmk mediated reduction in the cytotoxicity and expression of autophagic proteins, further suggested that autophagy induced by Ery5 is largely dependent upon caspases. Interestingly, Ery5 induced autophagy was accompanied by the downregulation of PI3K/AKT pathway whereas, trolox and tiron strongly enhanced this effect. In addition to that treatment of cells with Ery5, trolox and tiron individually, displayed a marked upregulation of Bax. The involvement of Bax in trolox and tiron induced enhancement of the cytotoxicity of Ery5 was confirmed, when siRNA induced silencing of Bax led to increased viability of the cells and exerted a strong inhibitory effect on LC3-II accumulation and p62 degradation in case of cells treated by the combination of Ery5 with trolox or tiron. Additionally, an important role of PARP in Ery5 mediated cell death has been suggested by PARP silencing experiments, however, potentiation of autophagic cytotoxicity by trolox and tiron did not seem to be dependent on PARP-1. Therefore, Bax seems to play a vital role in trolox and tiron mediated potentiation of autophagic cell death by Ery5 in HL-60 cells.

Ribosomal Protein Mutations Induce Autophagy through S6 Kinase Inhibition of the Insulin Pathway

“…As mentioned, the increase of LC3-II that we observe with western blotting analysis despite a clear block of autophagosome formation has been previously reported in cells treated with Trolox, but the results in this report are in the absence of any confocal or IF data showing LC3 localization [47]. Consistent with our results it has been reported that many other antioxidants block autophagy [65].…”

“…Moreover, when we treated RP-deficient zebrafish embryos with 10 mM Trolox overnight we observed a substantial decrease in the levels of S6 kinase phosphorylation in embryos carrying mutations in rpS7 or rpS3a , which in turn alleviated the inhibition of AKT phosphorylation activity (Figure 7D). Interestingly, the Trolox treatment while clearly blocking autophagosome formation in the GFP-LC3 HEK cells was unexpectedly coupled to an increase of LC3-II by western blotting analysis (Supporting Figure S2), the latter a feature of Trolox that has been previously reported [47]. However, in the same cells we also find that Trolox results in an increase of p62 expression and a decrease of phosphorylated S6 kinase, supporting the confocal results indicating that there is a block of autophagy due to a reduction of S6 kinase signaling (Supporting Figure S2).…”

Autophagy triggered by magnolol derivative negatively regulates angiogenesis

“…Magnolol is a natural compound that has been studied to have anticancer potential by showing antiproliferative activity against different cancer cell lines through the induction of apoptosis or autophagy. 29, 30 …”

Peneciraistin C induces caspase‐independent autophagic cell death through mitochondrial‐derived reactive oxygen species production in lung cancer cells

“…(13) However, when this process occurs in excess, autophagy itself becomes cytotoxic and eventually leads to autophagic cell death. (14) Thus, in addition to apoptotic cell death, the study on autophagic response is a prospective direction for the development of anticancer drugs. Reactive oxygen species (ROS) production, especially through the mitochondria, is important for the regulation for autophagy induction in cancer therapy.…”