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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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Okadaic acid abates the effect of ACS-1 on Akt activation. (A) Western blot and (B) densitometry analysis of Akt-(ser 473) phosphorylation in A549 cells incubated with ACS-1 and/or Okadaic acid for 2 hours and stimulated with EGF for 30 minutes. (C) ACS-1 increases PP2A activity as determined by total c-myc protein expression in MB231 cells treated with ACS-1 for 2 hours. C-myc protein levels are not altered in the presence of okadaic acid (OA). (D) Serum starved MB231 cells were treated with ACS-1 for 2 hours stimulated with EGF and lysed. PP2A (subunit C) was immunoprecipitated from whole cell extracts, washed with assay buffer and PP2A phosphatase activity measured. (E) ACS-1 does not affect the level of PP2A expression. Serum starved MB231 cells were treated with ACS-1 for 2 hours and stimulated with EGF for 30 minutes. PP2A and actin protein levels were determined by Western blot analysis.
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Figure 5: Okadaic acid abates the effect of ACS-1 on Akt activation. (A) Western blot and (B) densitometry analysis of Akt-(ser 473) phosphorylation in A549 cells incubated with ACS-1 and/or Okadaic acid for 2 hours and stimulated with EGF for 30 minutes. (C) ACS-1 increases PP2A activity as determined by total c-myc protein expression in MB231 cells treated with ACS-1 for 2 hours. C-myc protein levels are not altered in the presence of okadaic acid (OA). (D) Serum starved MB231 cells were treated with ACS-1 for 2 hours stimulated with EGF and lysed. PP2A (subunit C) was immunoprecipitated from whole cell extracts, washed with assay buffer and PP2A phosphatase activity measured. (E) ACS-1 does not affect the level of PP2A expression. Serum starved MB231 cells were treated with ACS-1 for 2 hours and stimulated with EGF for 30 minutes. PP2A and actin protein levels were determined by Western blot analysis.

Mentions: Since ACS-1 suppressed Akt phosphorylation without proximally affecting EGFR or integrin signaling, we surmised that ACS-1 might impact a downstream target specific to Akt regulation. A major endogenous negative regulator of Akt signaling is PP2A (Andjelkovic et al., 1996; Resjö et al., 2002; Rocher et al., 2007; Sato et al., 2000; Ugi et al., 2004). To examine a potential role of ACS-1 in targeting PP2A, serum starved A549 or MB231 cells were treated with ACS-1 and okadaic acid (50 nM), an inhibitor of PP2A, for 2 hr and then stimulated with EGF. Figures 5A and 5B demonstrate inhibitory effects of okadaic acid on ACS-1 mediated suppression of pAkt-(ser 473) in EGF stimulated cells, indicating that PP2A may be a critical target of the anti-proliferative properties of ACS-1.


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)

Okadaic acid abates the effect of ACS-1 on Akt activation. (A) Western blot and (B) densitometry analysis of Akt-(ser 473) phosphorylation in A549 cells incubated with ACS-1 and/or Okadaic acid for 2 hours and stimulated with EGF for 30 minutes. (C) ACS-1 increases PP2A activity as determined by total c-myc protein expression in MB231 cells treated with ACS-1 for 2 hours. C-myc protein levels are not altered in the presence of okadaic acid (OA). (D) Serum starved MB231 cells were treated with ACS-1 for 2 hours stimulated with EGF and lysed. PP2A (subunit C) was immunoprecipitated from whole cell extracts, washed with assay buffer and PP2A phosphatase activity measured. (E) ACS-1 does not affect the level of PP2A expression. Serum starved MB231 cells were treated with ACS-1 for 2 hours and stimulated with EGF for 30 minutes. PP2A and actin protein levels were determined by Western blot analysis.
© Copyright Policy
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC3472634&req=5

Figure 5: Okadaic acid abates the effect of ACS-1 on Akt activation. (A) Western blot and (B) densitometry analysis of Akt-(ser 473) phosphorylation in A549 cells incubated with ACS-1 and/or Okadaic acid for 2 hours and stimulated with EGF for 30 minutes. (C) ACS-1 increases PP2A activity as determined by total c-myc protein expression in MB231 cells treated with ACS-1 for 2 hours. C-myc protein levels are not altered in the presence of okadaic acid (OA). (D) Serum starved MB231 cells were treated with ACS-1 for 2 hours stimulated with EGF and lysed. PP2A (subunit C) was immunoprecipitated from whole cell extracts, washed with assay buffer and PP2A phosphatase activity measured. (E) ACS-1 does not affect the level of PP2A expression. Serum starved MB231 cells were treated with ACS-1 for 2 hours and stimulated with EGF for 30 minutes. PP2A and actin protein levels were determined by Western blot analysis.
Mentions: Since ACS-1 suppressed Akt phosphorylation without proximally affecting EGFR or integrin signaling, we surmised that ACS-1 might impact a downstream target specific to Akt regulation. A major endogenous negative regulator of Akt signaling is PP2A (Andjelkovic et al., 1996; Resjö et al., 2002; Rocher et al., 2007; Sato et al., 2000; Ugi et al., 2004). To examine a potential role of ACS-1 in targeting PP2A, serum starved A549 or MB231 cells were treated with ACS-1 and okadaic acid (50 nM), an inhibitor of PP2A, for 2 hr and then stimulated with EGF. Figures 5A and 5B demonstrate inhibitory effects of okadaic acid on ACS-1 mediated suppression of pAkt-(ser 473) in EGF stimulated cells, indicating that PP2A may be a critical target of the anti-proliferative properties of ACS-1.

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