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Dithiolethione compounds inhibit Akt signaling in human breast and lung cancer cells by increasing PP2A activity.

Switzer CH, Ridnour LA, Cheng RY, Sparatore A, Del Soldato P, Moody TW, Vitek MP, Roberts DD, Wink DA - Oncogene (2009)

Bottom Line: The effect of ACS-1 on Akt activation was not observed in the presence of the PP2A inhibitor okadaic acid.ACS-1 effects on PP2A activity were independent of ARE activation and cAMP formation.In addition to ACS-1, other dithiolethione compounds showed similar effects in reducing Akt activation, suggesting that this class of compounds may have other effects beyond chemoprevention.

View Article: PubMed Central - PubMed

Affiliation: Radiation Biology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA.

ABSTRACT
The chemopreventative effects of dithiolethione compounds are attributed to their activation of antioxidant response elements (AREs) by reacting with the Nrf2/Keap1 protein complex. In this study, we show antiproliferative effects of the dithiolethione compound ACS-1 in human cancer cell lines (A549 and MDA-MB-231) by increasing the activity of the tumor suppressor protein phoshatase 2A (PP2A). ACS-1 inhibited epidermal growth factor (EGF)-induced cellular proliferation in a concentration- and time-dependent manner. Akt activation, as determined by serine-473 phosphorylation, was inhibited by ACS-1 in cells stimulated with either EGF or fibronectin. Furthermore, ACS-1 inhibited mammalian target of rapamycin signaling and decreased c-myc protein levels. ACS-1 did not proximally alter EGF receptor or integrin signaling, but caused a concentration-dependent increase in PP2A activity. The effect of ACS-1 on Akt activation was not observed in the presence of the PP2A inhibitor okadaic acid. ACS-1 effects on PP2A activity were independent of ARE activation and cAMP formation. In addition to ACS-1, other dithiolethione compounds showed similar effects in reducing Akt activation, suggesting that this class of compounds may have other effects beyond chemoprevention.

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ACS-1 inhibits EGF activation of Akt and mTOR in MB231 and A549 cells. Serum starved cells were treated with ACS-1 for 2 hours and stimulated with EGF. (A) Western blot analysis of EGF signaling pathway activation in response to EGF and/or ACS-1. (B) Graphical representation of Akt activation in MB231 cells; ratios of P-Akt-(ser 473):total Akt are presented as percent of EGF control (mean ± SD). (C) Western blot analysis of ACS-1 effects on EGF signaling in A549 cells. (D) Graphical representation of Akt activation in A549 cells; ratios of P-Akt-(ser 473):total Akt are presented as percent of EGF control (mean ± SD). (E) Representative western blot analysis of P-mTOR-(ser 2448) in MB231 cells treated with ACS-1 and stimulated with EGF.
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Figure 3: ACS-1 inhibits EGF activation of Akt and mTOR in MB231 and A549 cells. Serum starved cells were treated with ACS-1 for 2 hours and stimulated with EGF. (A) Western blot analysis of EGF signaling pathway activation in response to EGF and/or ACS-1. (B) Graphical representation of Akt activation in MB231 cells; ratios of P-Akt-(ser 473):total Akt are presented as percent of EGF control (mean ± SD). (C) Western blot analysis of ACS-1 effects on EGF signaling in A549 cells. (D) Graphical representation of Akt activation in A549 cells; ratios of P-Akt-(ser 473):total Akt are presented as percent of EGF control (mean ± SD). (E) Representative western blot analysis of P-mTOR-(ser 2448) in MB231 cells treated with ACS-1 and stimulated with EGF.

Mentions: Because ACS-1 inhibited cellular proliferation in response to EGF stimulation, the effect of ACS-1 on epidermal growth factor receptor (EGFR) signaling was examined. Serum starved MB231 cells were treated with ACS-1 for 2 hr at 37°C followed by EGF stimulation for 2 minutes. ACS-1 did not effect EGFR activation as represented by tyrosine-992 phosphorylation when normalized to total EGFR (Figure 3A). EGFR phosphorylation mediates activation of the PI3K pathway and PIP3 formation. While enhanced PIP3 levels mediate increases in pPDK1-(serine 241) as well as pAkt-(thr 308), complete activation of Akt is dependent upon phosphorylation at serine 473, which is mediated by mTOR/RICTOR (Hresko and Mueckler, 2005). ACS-1 significantly suppressed EGF mediated Akt-(ser 473) phosphorylation (Figures 3A and 3B) at concentrations of 50 μM and higher but had little to no effect on pPDK1-(serine 241) (Figure 3A) or pAkt-(thr 308) (data not shown). Similar results were observed in the A549 cell line (Figure 3C). At concentrations consistent with the inhibition of Akt-(ser 273) phosphorylation, ACS-1 also inhibited EGF-induced mTOR activation in MB231 cells (Figure 3E), a downstream substrate of active Akt.


Dithiolethione compounds inhibit Akt signaling in human breast and lung cancer cells by increasing PP2A activity.

Switzer CH, Ridnour LA, Cheng RY, Sparatore A, Del Soldato P, Moody TW, Vitek MP, Roberts DD, Wink DA - Oncogene (2009)

ACS-1 inhibits EGF activation of Akt and mTOR in MB231 and A549 cells. Serum starved cells were treated with ACS-1 for 2 hours and stimulated with EGF. (A) Western blot analysis of EGF signaling pathway activation in response to EGF and/or ACS-1. (B) Graphical representation of Akt activation in MB231 cells; ratios of P-Akt-(ser 473):total Akt are presented as percent of EGF control (mean ± SD). (C) Western blot analysis of ACS-1 effects on EGF signaling in A549 cells. (D) Graphical representation of Akt activation in A549 cells; ratios of P-Akt-(ser 473):total Akt are presented as percent of EGF control (mean ± SD). (E) Representative western blot analysis of P-mTOR-(ser 2448) in MB231 cells treated with ACS-1 and stimulated with EGF.
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Figure 3: ACS-1 inhibits EGF activation of Akt and mTOR in MB231 and A549 cells. Serum starved cells were treated with ACS-1 for 2 hours and stimulated with EGF. (A) Western blot analysis of EGF signaling pathway activation in response to EGF and/or ACS-1. (B) Graphical representation of Akt activation in MB231 cells; ratios of P-Akt-(ser 473):total Akt are presented as percent of EGF control (mean ± SD). (C) Western blot analysis of ACS-1 effects on EGF signaling in A549 cells. (D) Graphical representation of Akt activation in A549 cells; ratios of P-Akt-(ser 473):total Akt are presented as percent of EGF control (mean ± SD). (E) Representative western blot analysis of P-mTOR-(ser 2448) in MB231 cells treated with ACS-1 and stimulated with EGF.
Mentions: Because ACS-1 inhibited cellular proliferation in response to EGF stimulation, the effect of ACS-1 on epidermal growth factor receptor (EGFR) signaling was examined. Serum starved MB231 cells were treated with ACS-1 for 2 hr at 37°C followed by EGF stimulation for 2 minutes. ACS-1 did not effect EGFR activation as represented by tyrosine-992 phosphorylation when normalized to total EGFR (Figure 3A). EGFR phosphorylation mediates activation of the PI3K pathway and PIP3 formation. While enhanced PIP3 levels mediate increases in pPDK1-(serine 241) as well as pAkt-(thr 308), complete activation of Akt is dependent upon phosphorylation at serine 473, which is mediated by mTOR/RICTOR (Hresko and Mueckler, 2005). ACS-1 significantly suppressed EGF mediated Akt-(ser 473) phosphorylation (Figures 3A and 3B) at concentrations of 50 μM and higher but had little to no effect on pPDK1-(serine 241) (Figure 3A) or pAkt-(thr 308) (data not shown). Similar results were observed in the A549 cell line (Figure 3C). At concentrations consistent with the inhibition of Akt-(ser 273) phosphorylation, ACS-1 also inhibited EGF-induced mTOR activation in MB231 cells (Figure 3E), a downstream substrate of active Akt.

Bottom Line: The effect of ACS-1 on Akt activation was not observed in the presence of the PP2A inhibitor okadaic acid.ACS-1 effects on PP2A activity were independent of ARE activation and cAMP formation.In addition to ACS-1, other dithiolethione compounds showed similar effects in reducing Akt activation, suggesting that this class of compounds may have other effects beyond chemoprevention.

View Article: PubMed Central - PubMed

Affiliation: Radiation Biology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA.

ABSTRACT
The chemopreventative effects of dithiolethione compounds are attributed to their activation of antioxidant response elements (AREs) by reacting with the Nrf2/Keap1 protein complex. In this study, we show antiproliferative effects of the dithiolethione compound ACS-1 in human cancer cell lines (A549 and MDA-MB-231) by increasing the activity of the tumor suppressor protein phoshatase 2A (PP2A). ACS-1 inhibited epidermal growth factor (EGF)-induced cellular proliferation in a concentration- and time-dependent manner. Akt activation, as determined by serine-473 phosphorylation, was inhibited by ACS-1 in cells stimulated with either EGF or fibronectin. Furthermore, ACS-1 inhibited mammalian target of rapamycin signaling and decreased c-myc protein levels. ACS-1 did not proximally alter EGF receptor or integrin signaling, but caused a concentration-dependent increase in PP2A activity. The effect of ACS-1 on Akt activation was not observed in the presence of the PP2A inhibitor okadaic acid. ACS-1 effects on PP2A activity were independent of ARE activation and cAMP formation. In addition to ACS-1, other dithiolethione compounds showed similar effects in reducing Akt activation, suggesting that this class of compounds may have other effects beyond chemoprevention.

Show MeSH
Related in: MedlinePlus