Targeting SET/I(2)PP2A oncoprotein functions as a multi-pathway strategy for cancer therapy.

The SET oncoprotein participates in cancer progression by affecting multiple cellular processes, inhibiting the tumor suppressor protein phosphatase 2A (PP2A), and inhibiting the metastasis suppressor nm23-H1. On the basis of these multiple activities, we hypothesized that targeted inhibition of SET would have multiple discrete and measurable effects on cancer cells. Here, the effects of inhibiting SET oncoprotein function on intracellular signaling and proliferation of human cancer cell lines was investigated. We observed the effects of COG112, a novel SET interacting peptide, on PP2A activity, Akt signaling, nm23-H1 activity and cellular migration/invasion in human U87 glioblastoma and MDA-MB-231 breast adenocarcinoma cancer cell lines. We found that COG112 interacted with SET protein and inhibited the association between SET and PP2A catalytic subunit (PP2A-c) and nm23-H1. The interaction between COG112 and SET caused PP2A phosphatase and nm23-H1 exonuclease activities to increase. COG112-mediated increases in PP2A activity resulted in the inhibition of Akt signaling and cellular proliferation. Additionally, COG112 inhibited SET association with Ras-related C(3) botulinum toxin substrate 1 (Rac1), leading to decreased cellular migration and invasion. COG112 treatment releases the SET-mediated inhibition of the tumor suppressor PP2A, as well as the metastasis suppressor nm23-H1. These results establish SET as a novel molecular target and that the inhibition of SET may have beneficial effects in cancer chemotherapy.

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

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.

Mechanisms of nitrogen oxide-mediated disruption of metalloprotein function: an examination of the copper-responsive yeast transcription factor Ace1.

Nitric oxide (NO) has been found to inhibit the copper-responsive yeast transcription factor Ace1 in an oxygen-dependent manner. However, the mechanism responsible for NO-dependent inhibition of Ace1 remains unestablished. In the present study, the chemical interaction of nitrogen oxide species with Ace1 was examined using a yeast reporter system. Exposure of yeast to various nitrogen oxides, under a variety of conditions, revealed that the oxygen-dependent inhibition of Ace1 is due to the reaction of NO with O(2). The nitrosating nitrogen oxide species N(2)O(3) is likely to be the disrupter of Ace1 activity. Considering the similarity of metal-thiolate ligation in Ace1 with other mammalian metalloproteins such as metallothionein, metal chaperones, and zinc-finger proteins, these results help to understand the biochemical interactions of NO with those mammalian metalloproteins.

The role of oxygen and reduced oxygen species in nitric oxide-mediated cytotoxicity: studies in the yeast Saccharomyces cerevisiae model system.

The cytotoxicity of nitric oxide (NO) is well established, yet the mechanism(s) of its cytotoxicity is (are) still undefined and a matter of significant interest and speculation. Many of the previously proposed mechanisms for NO-mediated cytotoxicity involve interactions between NO and molecular oxygen (O(2)) and/or O(2)-derived species such as O(-)(2) and H(2)O(2). The yeast Saccharomyces cerevisiae represents a useful model system for evaluating the role of O(2) and O(2)-derived species in NO-mediated cytotoxicity. This study examines the contribution of O(2) and O(2)-derived species to NO-mediated cytotoxicity in the yeast S. cerevisiae. NO-mediated cytotoxicity was determined to be O(2)-dependent. However, this O(2) dependence was only minimally due to the generation of O(2)-derived species such as O(-)(2) and/or H(2)O(2).

Effects of nitric oxide on the copper-responsive transcription factor Ace1 in Saccharomyces cerevisiae: cytotoxic and cytoprotective actions of nitric oxide.

Previous studies indicate that nitric oxide (NO) can serve as a regulator/disrupter of metal-metabolizing systems in cells and, indeed, this function may represent an important physiological and/or pathophysiological role for NO. In order to address possible mechanisms of this aspect of NO biology, the effect of NO on copper metabolism and toxicity in the yeast Saccharomyces cerevisiae was examined. Exposure of S. cerevisiae to NO resulted in an alteration of the activity of the copper-responsive transcription factor Acel. Low concentrations of the NO donor DEA/NO were found to slightly enhance copper-mediated activation of Acel. Since Acel regulates the expression of genes responsible for the protection of S. cerevisiae from metal toxicity, the effect of NO on the toxicity of copper toward S. cerevisiae was also examined. Interestingly, low concentrations of NO were also found to protect S. cerevisiae against the toxicity of copper. The effect of NO at high concentrations was, however, opposite. High concentrations of DEA/NO inhibited copper-mediated Acel activity. Correspondingly, high concentrations of DEA/NO (1 mM) dramatically enhanced copper toxicity. An intermediate concentration of DEA/NO (0.5 mM) exhibited a dual effect, enhancing toxicity at lower copper concentrations (<0.5 mM) and protecting at higher (> or =0.5 mM) copper concentrations. Thus, it is proposed that the ability of NO to both protect against (at low concentrations) and enhance (at high concentration) copper toxicity in S. cerevisiae is, at least partially, a result of its effect on Acel. The results of this study have implications for the role of NO as a mediator of metal metabolism.

The interaction of nitric oxide (NO) with the yeast transcription factor Ace1: A model system for NO-protein thiol interactions with implications to metal metabolism.

Nitric oxide (NO) was found to inhibit the copper-dependent induction of the yeast CUP1 gene. This effect is attributable to an inhibition of the copper-responsive CUP1 transcriptional activator Ace1. A mechanism is proposed whereby the metal binding thiols of Ace1 are chemically modified via NO- and O(2)-dependent chemistry, thereby diminishing the ability of Ace1 to bind and respond to copper. Moreover, it is proposed that demetallated Ace1 is proteolytically degraded in the cell, resulting in a prolonged inhibition of copper-dependent CUP1 induction. These findings indicate that NO may serve as a disrupter of yeast copper metabolism. More importantly, considering the similarity of Ace1 to other mammalian metal-binding proteins, this work lends support to the hypothesis that NO may regulate/disrupt metal homeostasis under both normal physiological and pathophysiological circumstances.