Activation of tumor suppressor protein PP2A inhibits KRAS-driven tumor growth

Targeted cancer therapies, which act on specific cancer-associated molecular targets, are predominantly inhibitors of oncogenic kinases. While these drugs have achieved some clinical success, the inactivation of kinase signaling via stimulation of endogenous phosphatases has received minimal attention as an alternative targeted approach. Here, we have demonstrated that activation of the tumor suppressor protein phosphatase 2A (PP2A), a negative regulator of multiple oncogenic signaling proteins, is a promising therapeutic approach for the treatment of cancers. Our group previously developed a series of orally bioavailable small molecule activators of PP2A, termed SMAPs. We now report that SMAP treatment inhibited the growth of KRAS-mutant lung cancers in mouse xenografts and transgenic models. Mechanistically, we found that SMAPs act by binding to the PP2A Aα scaffold subunit to drive conformational changes in PP2A. These results show that PP2A can be activated in cancer cells to inhibit proliferation. Our strategy of reactivating endogenous PP2A may be applicable to the treatment of other diseases and represents an advancement toward the development of small molecule activators of tumor suppressor proteins

Zinc homeostasis and signaling in health and diseases: Zinc signaling

The essential trace element zinc (Zn) is widely required in cellular functions, and abnormal Zn homeostasis causes a variety of health problems that include growth retardation, immunodeficiency, hypogonadism, and neuronal and sensory dysfunctions. Zn homeostasis is regulated through Zn transporters, permeable channels, and metallothioneins. Recent studies highlight Zn’s dynamic activity and its role as a signaling mediator. Zn acts as an intracellular signaling molecule, capable of communicating between cells, converting extracellular stimuli to intracellular signals, and controlling intracellular events. We have proposed that intracellular Zn signaling falls into two classes, early and late Zn signaling. This review addresses recent findings regarding Zn signaling and its role in physiological processes and pathogenesis

Prion protein facilitates uptake of zinc into neuronal cells

Zinc is released into the synaptic cleft upon exocytotic stimuli, although the mechanism for its reuptake into neurons is unresolved. Here we show that the cellular prion protein enhances the uptake of zinc into neuronal cells. This prion-protein-mediated zinc influx requires the octapeptide repeats and amino-terminal polybasic region in the prion protein, but not its endocytosis. Selective antagonists of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors block the prion protein-mediated zinc uptake, and the prion protein co-immunoprecipitates with both GluA1 and GluA2 AMPA receptor subunits. Zinc-sensitive intracellular tyrosine phosphatase activity is decreased in cells expressing prion protein and increased in the brains of prion-protein-null mice, providing evidence of a physiological consequence of this process. Prion protein-mediated zinc uptake is ablated in cells expressing familial associated mutants of the protein and in prion-infected cells. These data suggest that alterations in the cellular prion protein-mediated zinc uptake may contribute to neurodegeneration in prion and other neurodegenerative diseases

Proper chromatin condensation and maintenance of histone H3 phosphorylation during mouse oocyte meiosis requires protein phosphatase activity

We have shown okadaic acid (OA) and calyculin-A (CLA) inhibition of mouse oocyte phosphoprotein phosphatase 1 (PPP1C) and/or phosphoprotein phosphatase 2A (PPP2CA) results in aberrant chromatin condensation, as evidenced by the inability to resolve bivalents. Phosphorylation of histone H3 at specific residues is thought to regulate chromatin condensation. Therefore, we examined changes in histone H3 phosphorylation during oocyte meiosis and the potential regulation by protein PPPs. Western blot and immunocytochemical analysis revealed histone H3 phosphorylation changed during mouse oocyte meiosis, with changes in chromatin condensation. Germinal vesicle-intact (GV-intact; 0 h) oocytes had no phospho-Ser10 but did have phospho-Ser28 histone H3. Oocytes that had undergone germinal vesicle breakdown (GVBD; 2 h) and progressed to metaphase I (MI; 7 h) and MII (16 h) had phosphorylated Ser10 and Ser28 histone H3 associated with condensed chromatin. To determine whether OA-induced aberrations in chromatin condensation were due to alterations in levels of histone H3 phosphorylation, we assessed phosphorylation of Ser10 and Ser28 residues following PPP inhibition. Oocytes treated with OA (1 mu M) displayed increased phosphorylation of histone H3 at both Ser10 and Ser28 compared with controls. To begin to elucidate which OA-sensitive PPP is responsible for regulating chromatin condensation and histone H3 phosphorylation, we examined spatial and temporal localization of OA-sensitive PPPs, PPP1 C, and PPP2CA. PPPC2A did not localize to condensed chromatin, whereas PPP1 beta (PPP1 CB) associated with condensing chromatin in GVBD, MI, and MII oocytes. Additionally, Western blot and immunocytochemistry confirmed presence of the PPP1C regulatory inhibitor subunit 2 (PPP1R2) in oocytes at condensed chromatin during meiosis and indicated a change in PPP1R2 phosphorylation. Inhibition of oocyte glycogen synthase kinase 3 (GSK3) appeared to regulate phosphorylation of PPP1R2. Furthermore, inhibition of GSK3 resulted in aberrant oocyte bivalent formation similar to that observed following PPP inhibition. These data suggest that PPP1CB is the OA/CLA-sensitive PPP that regulates oocyte chromatin condensation through regulation of histone H3 phosphorylation. Furthermore, GSK3 inhibition results in aberrant chromatin condensation and appears to regulate phosphorylation of PPP1R2

Inactivation of PP2A by a recurrent mutation drives resistance to MEK inhibitors

The serine/threonine Protein Phosphatase 2A (PP2A) functions as a tumor suppressor by negatively regulating multiple oncogenic signaling pathways. The canonical PP2A holoenzyme comprises a scaffolding subunit (PP2A Aα/β), which serves as the platform for binding of both the catalytic C subunit and one regulatory B subunit. Somatic heterozygous missense mutations in PPP2R1A, the gene encoding the PP2A Aα scaffolding subunit, have been identified across multiple cancer types, but the effects of the most commonly mutated residue, Arg-183, on PP2A function have yet to be fully elucidated. In this study, we used a series of cellular and in vivo models and discovered that the most frequent Aα R183W mutation formed alternative holoenzymes by binding of different PP2A regulatory subunits compared with wild-type Aα, suggesting a rededication of PP2A functions. Unlike wild-type Aα, which suppressed tumorigenesis, the R183W mutant failed to suppress tumor growth in vivo through activation of the MAPK pathway in RAS-mutant transformed cells. Furthermore, cells expressing R183W were less sensitive to MEK inhibitors. Taken together, our results demonstrate that the R183W mutation in PP2A Aα scaffold abrogates the tumor suppressive actions of PP2A, thereby potentiating oncogenic signaling and reducing drug sensitivity of RAS-mutant cells