ABSTRACT
Lysine specific demethylase 1 (LSD1, also known as KDM1) is a histone modifying enzyme that regulates the expression of many genes important in cancer progression and proliferation. It is present in various transcriptional complexes including those containing the estrogen receptor (ER). Indeed, inhibition of LSD1 activity and or expression has been shown to attenuate estrogen signaling in breast cancer cells in vitro, implicating this protein in the pathogenesis of cancer. Herein we describe experiments that utilize small molecule inhibitors, phenylcyclopropylamines, along with small interfering RNA to probe the role of LSD1 in breast cancer proliferation and in estrogen-dependent gene transcription. Surprisingly, whereas we have confirmed that inhibition of LSD1 strongly inhibits proliferation of breast cancer cells, we have determined that the cytostatic actions of LSD1 inhibition are not impacted by ER status. These data suggest that LSD1 may be a useful therapeutic target in several types of breast cancer; most notably, inhibitors of LSD1 may have utility in the treatment of ER-negative cancers for which there are minimal therapeutic options.
Subject(s)
Breast Neoplasms/enzymology , Breast Neoplasms/pathology , Estrogen Receptor alpha/physiology , Histone Demethylases/antagonists & inhibitors , Histone Demethylases/metabolism , Breast Neoplasms/drug therapy , Cell Line, Tumor , Cell Proliferation/drug effects , Enzyme Inhibitors/pharmacology , Enzyme Inhibitors/therapeutic use , Female , Humans , Pargyline/pharmacology , Pargyline/therapeutic useABSTRACT
p27 is a key regulator of cell proliferation. While it opposes cell cycle progression by binding to and inhibiting cyclin E-Cdk2, T157/T198 phosphorylation of p27 promotes its assembly of D-type cyclin-CDKs. In addition to its actions on the cell cycle, p27 regulates CDK-independent cytoplasmic functions. In human cancers, oncogenic activation of the PI3K signaling pathway often results in cytoplasmic mislocalization of p27. Cytoplasmic p27 plays an important role in cell motility and migration; it binds RhoA and modulates activation of the RhoA/ROCK cascade. p27:RhoA binding is facilitated by p27 phosphorylation at threonine 198. Accumulation of cytoplasmic p27 leads to increased cellular motility, a critical event in tumor metastasis. Further characterization of post-translational modifications governing p27 localization and its action on RhoA and the actin cytoskeleton may provide critical insights into human cancer metastasis.
Subject(s)
Calcium-Binding Proteins/metabolism , Cyclin-Dependent Kinase Inhibitor Proteins/metabolism , Cyclin-Dependent Kinase Inhibitor p27/metabolism , Cyclin-Dependent Kinases/metabolism , Neoplasms/metabolism , Protein Kinases/metabolism , Cell Cycle/physiology , Cell Movement/physiology , Humans , Signal Transduction/physiologyABSTRACT
p90 ribosomal S6 kinase (RSK1) is an effector of both Ras/MEK/MAPK and PI3K/PDK1 pathways. We present evidence that RSK1 drives p27 phosphorylation at T198 to increase RhoA-p27 binding and cell motility. RSK1 activation and p27pT198 both increase in early G(1). As for many kinase-substrate pairs, cellular RSK1 coprecipitates with p27. siRNA to RSK1 and RSK1 inhibition both rapidly reduce cellular p27pT198. RSK1 overexpression increases p27pT198, p27-cyclin D1-Cdk4 complexes, and p27 stability. Moreover, RSK1 transfectants show mislocalization of p27 to cytoplasm, increased motility, and reduced RhoA-GTP, phospho-cofilin, and actin stress fibers, all of which were reversed by shRNA to p27. Phosphorylation by RSK1 increased p27pT198 binding to RhoA in vitro, whereas p27T157A/T198A bound poorly to RhoA compared with WTp27 in cells. Coprecipitation of cellular p27-RhoA was increased in cells with constitutive PI3K activation and increased in early G(1). Thus T198 phosphorylation not only stabilizes p27 and mislocalizes p27 to the cytoplasm but also promotes RhoA-p27 interaction and RhoA pathway inhibition. These data link p27 phosphorylation at T198 and cell motility. As for other PI3K effectors, RSK1 phosphorylates p27 at T198. Because RSK1 is also activated by MAPK, the increased cell motility and metastatic potential of cancer cells with PI3K and/or MAPK pathway activation may result in part from RSK1 activation, leading to accumulation of p27T198 in the cytoplasm, p27:RhoA binding, inhibition of RhoA/Rock pathway activation, and loss of actomyosin stability.
Subject(s)
Cell Movement , Intracellular Signaling Peptides and Proteins/metabolism , Ribosomal Protein S6 Kinases, 90-kDa/metabolism , rhoA GTP-Binding Protein/metabolism , Actins/metabolism , Cell Line, Tumor , Cyclin-Dependent Kinase Inhibitor p27 , Cytoplasm/metabolism , G1 Phase , Humans , MAP Kinase Signaling System , Phosphorylation , Signal TransductionABSTRACT
p27 mediates Cdk2 inhibition and is also found in cyclin D1-Cdk4 complexes. The present data support a role for p27 in the assembly of D-type cyclin-Cdk complexes and indicate that both cyclin D1-Cdk4-p27 assembly and kinase activation are regulated by p27 phosphorylation. Prior work showed that p27 can be phosphorylated by protein kinase B/Akt (PKB/Akt) at T157 and T198. Here we show that PKB activation and the appearance of p27pT157 and p27pT198 precede p27-cyclin D1-Cdk4 assembly in early G(1). PI3K/PKB inhibition rapidly reduced p27pT157 and p27pT198 and dissociated cellular p27-cyclin D1-Cdk4. Mutant p27 allele products lacking phosphorylation at T157 and T198 bound poorly to cellular cyclin D1 and Cdk4. Cellular p27pT157 and p27pT198 coprecipitated with Cdk4 but were not detected in Cdk2 complexes. The addition of p27 to recombinant cyclin D1 and Cdk4 led to cyclin D1-Cdk4-p27 complex formation in vitro. p27 phosphorylation by PKB increased p27-cyclin D1-Cdk4 assembly in vitro but yielded inactive Cdk4. In contrast, Src pretreatment of p27 did not affect p27-cyclin D1-Cdk4 complex formation. However, Src treatment led to tyrosine phosphorylation of p27 and catalytic activation of assembled cyclin D1-Cdk4-p27 complexes. Thus, while PKB-dependent p27 phosphorylation appears to increase cyclin D1-Cdk4-p27 assembly or stabilize these complexes in vitro, cyclin D1-Cdk4-p27 activation requires the tyrosine phosphorylation of p27. Constitutive activation of PKB and Abl or Src family kinases in cancers would drive p27 phosphorylation, increase cyclin D1-Cdk4 assembly and activation, and reduce the cyclin E-Cdk2 inhibitory function of p27. Combined therapy with both Src and PI3K/PKB inhibitors may reverse this process.
Subject(s)
Cyclin D1/metabolism , Cyclin-Dependent Kinase 4/metabolism , Cyclin-Dependent Kinase Inhibitor p27/metabolism , Amino Acid Substitution/drug effects , Catalysis/drug effects , Cell Line, Tumor , Cyclin-Dependent Kinase 2/metabolism , Enzyme Activation/drug effects , G1 Phase/drug effects , Humans , Phosphoinositide-3 Kinase Inhibitors , Phosphorylation/drug effects , Phosphotyrosine/metabolism , Protein Binding/drug effects , Protein Kinase Inhibitors/pharmacology , Proto-Oncogene Proteins c-akt/antagonists & inhibitors , Proto-Oncogene Proteins c-akt/metabolism , Threonine/metabolismABSTRACT
The cell-cycle effects of mTORC1 are not fully understood. We provide evidence that mTOR-raptor phosphorylates SGK1 to modulate p27 function. Cellular mTOR activation, by refeeding of amino acid-deprived cells or by TSC2 shRNA, activated SGK1 and p27 phosphorylation at T157, and both were inhibited by short-term rapamycin treatment and by SGK1 shRNA. mTOR overexpression activated both Akt and SGK1, causing TGF-beta resistance through impaired nuclear import and cytoplasmic accumulation of p27. Rapamycin or raptor shRNA impaired mTOR-driven p70 and SGK1 activation, but not that of Akt, and decreased cytoplasmic p27. mTOR/raptor/SGK1 complexes were detected in cells. mTOR phosphorylated SGK1, but not SGK1-S422A, in vitro. SGK1 phosphorylated p27 in vitro. These data implicate SGK1 as an mTORC1 (mTOR-raptor) substrate. mTOR may promote G1 progression in part through SGK1 activation and deregulate the cell cycle in cancers through both Akt- and SGK-mediated p27 T157 phosphorylation and cytoplasmic p27 mislocalization.