The phosphodiesterase inhibitor SQ 20006 selectively blocks mitogen activation of p70S6k and transition to S phase of the cell division cycle without affecting the steady state phosphorylation of eIF-4E

Machilin D Promotes Apoptosis and Autophagy, and Inhibits Necroptosis in Human Oral Squamous Cell Carcinoma Cells

Oral squamous cell carcinoma (OSCC) accounts for about 90% of all head and neck cancers, the prognosis is very poor, and there are no effective targeted therapies. Herein, we isolated Machilin D (Mach), a lignin, from the roots of Saururus chinensis (S. chinensis) and assessed its inhibitory effects on OSCC. Herein, Mach had significant cytotoxicity against human OSCC cells and showed inhibitory effects against cell adhesion, migration, and invasion by inhibiting adhesion molecules, including the FAK/Src pathway. Mach suppressed the PI3K/AKT/mTOR/p70S6K pathway and MAPKs, leading to apoptotic cell death. We investigated other modes of programmed cell death in these cells and found that Mach increased LC3I/II and Beclin1 and decreased p62, leading to autophagosomes, and suppressed the necroptosis-regulatory proteins RIP1 and MLKL. Our findings provide evidence that the inhibitory effects of Mach against human YD-10B OSCC cells are related to the promotion of apoptosis and autophagy and inhibition of necroptosis and are mediated via focal adhesion molecules.

Latifolin, a Natural Flavonoid, Isolated from the Heartwood of Dalbergia odorifera Induces Bioactivities through Apoptosis, Autophagy, and Necroptosis in Human Oral Squamous Cell Carcinoma

Oral squamous cell carcinoma (OSCC) is the most common malignant neoplasm with frequent metastasis and high mortality in the oral cavity. Plant-derived natural compounds are actively progressing as a trend for cancer treatment. Latifolin (Latif), is a natural flavonoid isolated from the heartwood of Dalbergia odorifera T. Chen (D. odorifera) has been known to have beneficial effects on anti-aging, anti-carcinogenic, anti-inflammatory, and cardio-protective activities. However, the anti-cancer effects of Latif are unknown in OSCC. Herein, as a result of analysis in terms of the aggressive features of OSCCs, we found that Latif significantly inhibited the cell proliferation of human YD-8 and YD-10B OSCCs, and caused the anti-metastatic activities by effectively blocking cell migration, invasion, and adhesion via the inactivation of focal adhesion kinase (FAK)/non-receptor tyrosine kinase (Src). Moreover, we found that Latif induced apoptotic cell death to suppress the cell survival and proliferation of YD-10B OSCCs by targeting PI3K/AKT/mTOR/p70S6K signaling. Finally, we analyzed in terms of autophagy and necroptosis, which are other mechanisms of programmed cell death and survival compared to apoptosis in YD-10B OSCCs. We found that Latif suppressed autophagic-related proteins and autophagosome formation, and also Latif inhibited necroptosis by dephosphorylating necroptosis-regulatory proteins (RIP1, RIP3, and MLKL). Given these findings, our results provided new evidence for Latif's biological effect and mechanism in YD-10B OSCCs, suggesting that Latif may be a new candidate for patients with OSCCs.

Oxidant-induced hypertrophy of A549 cells is accompanied by alterations in eukaryotic translation initiation factor 4E and 4E-binding protein-1

Control of protein synthesis resides at the level of eukaryotic translation initiation (eIF) complex formation. Complex formation is regulated by the mRNA cap-binding protein, eIF4E, whose activity is influenced by phosphorylation and binding to 4E-binding protein 1 (4E-BP1). To provide a link between alterations in protein synthesis and the pathogenesis of oxidant-mediated lung disease, we investigated the effect of hydrogen peroxide (H2O2) on actively growing A549 cells. Cells were exposed to 200 or 400 microM H2O2 for 4 h and then assessed for changes in proliferation, protein synthesis, and eIF4E and 4E-BP1 status over 72 h. We found that both concentrations of H2O2 inhibited [3H]thymidine incorporation and cell division while inducing a G2/M-predominant growth arrest within 24 h. In addition, H2O2 increased cell size, [3H]leucine incorporation/cell, and total cell protein. Although time had little effect on eIF4E and 4E-BP1 expression and phosphorylation state of control cells, H2O2 induced a 2- to 3-fold increase in eIF4E and 4E-BP1 expression, a 5-fold increase in eIF4E phosphorylation, and a shift in the distribution of 4E-BP1 phosphorylation favoring lesser phosphorylated forms. These findings suggest that oxidant-mediated alterations in protein synthesis and cell morphology occur in concert with changes in factors known to regulate translation kinetics.

DNA binding of TEA/ATTS domain factors is regulated by protein kinase C phosphorylation in human choriocarcinoma cells

Transcription enhancer factor 1 (TEF-1) controls the expression of a diverse set of genes. Previous studies implicated protein kinase C (PKC)-mediated signal transduction in modulating TEF function. We demonstrate that in human choriocarcinoma BeWo cells, the PKC activator 12-O-tetradecanoyl phorbol 13-acetate and PKC inhibitor bisindolylmaleimide reciprocally down- and up-regulate, respectively, TEF-mediated GGAATG core enhancer activity. In vitro TEF-1 phosphorylation with several PKC isozymes and phosphoamino acid analysis confirmed that TEF-1 is a potential PKC substrate. TEF-1.DNA complexes formed by BeWo nuclear extracts are supershifted by phosphoserine- and phosphothreonine- but not phosphotyrosine-specific antibodies, indicating that TEF-1 is phosphorylated in vivo at serine and threonine residues. The TEF-1 phosphorylation domain was localized to the third alpha-helix of the DNA binding domain and adjacent hinge region by phosphopeptide analysis. TEF-1 phosphorylation significantly reduced its DNA binding activity both in vitro and in vivo, providing a possible mechanism for the inhibitory action of PKC. Finally, BeWo cells contained abundant levels of gamma and delta PKC isoforms, and their overexpression resulted in even greater inhibition of GGAATG core enhancer activity after 12-O-tetradecanoyl phorbol 13-acetate treatment. These data strongly suggest that PKC-mediated phosphorylation is a key factor controlling TEF function.

Attenuation of mammalian target of rapamycin activity by increased cAMP in 3T3-L1 adipocytes

Incubating 3T3-L1 adipocytes with forskolin, which increases intracellular cAMP by activating adenylate cyclase, mimicked rapamycin by attenuating the effect of insulin on stimulating the phosphorylation of four (S/T)P sites in PHAS-I, a downstream target of the mammalian target of rapamycin (mTOR) signaling pathway. To investigate the hypothesis that increasing cAMP inhibits mTOR, the protein kinase activity of mTOR was measured in an immune complex assay with recombinant PHAS-I as substrate. Both forskolin and 8-(4-chlorophenylthio)adenosine 3'-5'-monophosphate (CPT-cAMP) prevented the activation of mTOR by insulin in adipocytes, but neither agent affected mTOR activity when added directly to the immunopurified protein. In contrast, the cAMP phosphodiesterase inhibitor, theophylline, inhibited mTOR activity not only when added to intact adipocytes but also when added to immunopurified mTOR in vitro, demonstrating that certain methylxanthines are able to inhibit mTOR independently of increasing cAMP. Forskolin and CPT-cAMP blocked the effect of insulin on increasing mTOR phosphorylation, which was assessed using mTAb1, an antibody whose binding is inhibited by phosphorylation of mTOR. Although the mTAb1 epitope contains a consensus site for protein kinase B, neither agent inhibited the activation of protein kinase B produced by insulin. These findings support the interpretation that increasing cAMP attenuates the effects of insulin on PHAS-I, p70(S6K), and other downstream targets of the mTOR signaling pathway by inhibiting the phosphorylation and activation of mTOR.

Regulation of protein-synthesis elongation-factor-2 kinase by cAMP in adipocytes

Treatment of primary rat epididymal adipocytes or 3T3-L1 adipocytes with various agents which increase cAMP led to the phosphorylation of eukaryotic translation elongation factor-2 (eEF-2). The increase in eEF-2 phosphorylation was a consequence of the activation of eEF-2 kinase (eEF-2K), which is a Ca2+/calmodulin-dependent kinase. eEF-2K was shown to be essentially inactive at less than 0.1 microM free Ca2+ when measured in cell-free extracts. Treatment of adipocytes with isoproterenol induced Ca2+-independent eEF-2K activity, and an 8-10-fold activation of eEF-2K was observed at Ca2+ concentrations of less than 0.1 microM. Increased cAMP in 3T3-L1 adipocytes led to the inhibition of total protein synthesis and decreased the rate of polypeptide-chain elongation. We also show that the phosphorylation of eEF-2 and the activity of eEF-2K are insulin-regulated in adipocytes. These results demonstrate a novel mechanism for the control of protein synthesis by hormones which act by increasing cytoplasmic cAMP.

Cinnamaldehyde inhibits lymphocyte proliferation and modulates T-cell differentiation

Two kinds of cinnamaldehyde derivative, 2'-hydroxycinnamaldehyde (HCA) and 2'-benzoxy-cinnamaldehyde (BCA), were studied for their immunomodulatory effects. These compounds were screened as anticancer drug candidates from stem bark of Cinnamomum cassia for their inhibitory effect on farnesyl protein transferase activity. Ras activation, which is accompanied with its farnesylation, has been known to be important in immune cell activation as well as in carcinogenesis. Treatment of these cinnamaldehydes to mouse splenocyte cultures induced suppression of lymphoproliferation following both Con A and LPS stimulation in a dose-dependent manner. A dose of I microM of HCA and BCA inhibited the Con A-stimulated proliferation by 69% and 60%, and the LPS-induced proliferation by 29% and 21%, respectively. However, the proliferation induced by PMA plus ionomycin was affected by neither HCA nor BCA treatment. Decreased levels of antibody production by HCA or BCA treatment were observed in both SRBC-immunized mice and LPS-stimulated splenocyte cultures. The exposure of thymocytes to HCA or BCA for 48 h accelerated T-cell differentiation from CD4 and CD8 double positive cells to CD4 or CD8 single positive cells. The inhibitory effect of cinnamaldehyde on lymphoproliferation was specific to the early phase of cell activation, showing the strongest inhibition of Con A- or LPS-stimulated proliferation when added concomitantly with the mitogens. In addition, the treatment of HCA and BCA to splenocyte cultures attenuated the Con A-triggered progression of cell cycle at G1 phase with no inhibition of S to G2/M phase transition. Although cinnamaldehyde treatment had no effect on the IL-2 production by splenocyte cultures stimulated with Con A, it inhibited markedly and dose-dependently the expression of IL-2Ralpha and interferon-gamma. Taken together, the results in this study suggest both HCA and BCA inhibit the lymphoproliferation and induce a T-cell differentiation through the blockade of early steps in signaling pathway leading to cell growth.

Differential activation of p70 and p85 S6 kinase isoforms during cardiac hypertrophy in the adult mammal

An adult feline right ventricular pressure overload (RVPO) model was used to examine the two S6 kinase (S6K) isoforms, p70(S6K) and p85(S6K), that are involved in translational and transcriptional activation. Biochemical and confocal microscopy analyses at the level of the cardiocyte revealed that p70(S6K) is present predominantly in the cytosol, substantially activated in 1-h RVPO (>12 fold), and phosphorylated in the pseudosubstrate domain at the Ser-411, Thr-421, and Ser-424 sites. p85(S6K), which was localized exclusively in the nucleus, showed activation subsequent to p70(S6K), with a sustained increase in phosphorylation for up to 48 h of RVPO at equivalent sites of p70(S6K), Thr-421 and Ser-424, but not at Ser-411. Neither isoform translocated between the cytosol and the nucleus. Further studies to determine potential upstream elements of S6K activation revealed: (i) similar time course of activation for protein kinase C isoforms (alpha, gamma, and epsilon) and c-Raf, (ii) absence of accompanying phosphatidylinositol 3-kinase activation, (iii) activation of c-Src subsequent to p70(S6K), and (iv) similar changes in adult cardiocytes after treatment with 12-O-tetradecanoylphorbol-13-acetate. Thus, these studies suggest that a protein kinase C-mediated pathway couples pressure overload to growth induction via differential activation of S6K isoforms in cardiac hypertrophy.

Cloning and structural analysis of the gene encoding the ribosomal protein S6 from the parasite Leishmania infantum

We have cloned the S6 ribosomal protein encoding gene from a Leishmania infantum cDNA library. This parasite protozoon, responsible for leishmaniasis in Europe, is able to undergo developmental changes in vitro and results a good model to study cell differentiation processes. The LiS6 protein sequence indicates its pertinence to the S6 protein family, related to the early mechanisms of cell division, differentiation and activation, and shows an intermediate position between the yeasts and higher eukaryotes. Thus, LiS6 protein has the same amino acid length as that of the higher eukaryotes and certain common features such nucleus entrance sequences and several kinase phosphorylation sites. However, the key functional protein kinase C phosphorylation sites are at different locations and present several threonine instead of the usual serine residues. The gene structural analysis suggest the presence of three different encoding genes that do not present remarkable changes along the different phases of the parasite.

A rapid purification protocol for the mitogen-activated p70 S6 kinase

Reentry into the cell cycle from quiescence is stimulated by mitogens and supported by increased levels of protein synthesis. Initiation of polypeptide synthesis is the step most often subject to regulation. This is controlled by phosphorylation of a number of initiation factors and the single 40S ribosomal protein S6. The kinase responsible for S6 phosphorylation is p70(S6k). In the past, the apparent low cellular abundance of p70(S6k) resulted in the development of laborious large-scale purification procedures. Here a rapid and low-cost protocol which yields p70(S6k) purified to near homogeneity and with high specific activity is described. With respect to previous strategies, this novel scheme takes advantage of recently improved ion-exchange media in the first and second purification steps, thus allowing the effective removal of bulk proteins and the major Ser/Thr phosphatases. The third step of purification consists of a single affinity chromatography column in which the ligand is a 20-residue peptide containing the structural motif required for recognition and binding by p70(S6k). This novel protocol allows the rapid purification of large amounts of p70(S6k) and will facilitate the screening of libraries of natural or synthetic compounds aimed at the identification of p70(S6k) inhibitory molecules.

Rapamycin inhibition of the G1 to S transition is mediated by effects on cyclin D1 mRNA and protein stability

The immunosuppressant rapamycin has been shown previously to inhibit the G1/S transition in several cell types by prolonging the G1 phase of the cell cycle. This process appears to be controlled, in part, by the rapamycin-sensitive FK506-binding protein-rapamycin-associated protein-p70 S6 kinase (p70(S6k)) pathway and the cyclin-dependent kinases (Cdk). We now show that in serum-stimulated NIH 3T3 cells, rapamycin treatment delays the accumulation of cyclin D1 mRNA during progression through G1. Rapamycin also appears to affect stability of the transcript. The combined transcriptional and post-transcriptional effects of the drug ultimately result in decreased levels of cyclin D1 protein. Moreover, degradation of newly synthesized cyclin D1 protein is accelerated by rapamycin, a process prevented by inclusion of the proteasome inhibitor, N-acetyl-Leu-Leu-norleucinal. The overall effect of rapamycin on cyclin D1 leads, in turn, to impaired formation of active complexes with Cdk4, a process which triggers retargeting of the p27(Kip1) inhibitor to cyclin E/Cdk2. In view of this novel experimental evidence, we discuss a possible mechanism for the rapamycin-induced cell cycle arrest at the G1/S transition.

Phosphorylation and activation of B-Myb by cyclin A-Cdk2

BACKGROUND

Cyclins and their catalytic partners, the cyclin-dependent kinases (Cdks), function as key regulators of the eukaryotic cell cycle. Specific cyclin-Cdk complexes are active at successive stages during the cell cycle and control cell-cycle progression by phosphorylating specific target proteins, most of which have not yet been identified. B-Myb, a conserved member of the Myb oncoprotein family, is a sequence-specific DNA-binding protein expressed in virtually all proliferating mammalian cells. Increasing evidence suggests that B-Myb plays an important role during the late G1 and early S phases of the cell cycle. In this study, we have examined the regulation of B-Myb activity by cyclin-Cdks.

RESULTS

We found that the transcriptional transactivation potential of B-Myb was repressed by a regulatory domain located at the carboxyl terminus of the protein. Coexpression of B-Myb and cyclin A relieved this repression by phosphorylation of B-Myb in its carboxy-terminal region. Tryptic phosphopeptide mapping revealed that endogenous B-Myb was phosphorylated in cells undergoing S phase.

CONCLUSIONS

This work provides evidence for a link between the Myb oncoprotein family and the cell-cycle machinery. We have shown that the carboxyl terminus of B-Myb acts as a cell-cycle sensor that regulates the transactivation function of B-Myb. Moreover, our studies have identified B-Myb as a target of cyclin A-Cdk2 and have indicated that B-Myb activity is regulated by phosphorylation mediated by cyclin A-Cdk2.

Ribosomal S6 kinase p90rsk and mRNA cap-binding protein eIF4E phosphorylations correlate with MAP kinase activation during meiotic reinitiation of mouse oocytes

During meiotic reinitiation of the mouse oocyte, entry into M-phase is regulated by changes of protein phosphorylation and by the stimulation of selective mRNA translation following the nuclear membrane dissolution. Our results reveal that M-phase kinases (MAP kinase and histone H1 kinase) are being activated together with S6 kinase and with the phosphorylation of eIF4E, the cap-binding subunit of the initiation factor eIF-4F. In order to test which signaling pathway(s) is(are) involved, okadaic acid and cycloheximide have been used as tools for differentially modulating MAP and histone H1 kinase activities. A role for MAP kinases in the phosphorylation of eIF4E and the activation of S6 kinase is suggested. The possible implication of p90rsk and/or of p70s6k in the overall increase in S6 kinase activity has been examined. p70s6k does not appear to be involved since phosphorylated forms are found in prophase and maturing oocytes. In contrast, p90rsk is phosphorylated and activated in maturing oocytes. p90rSk phosphorylation correlates with the activation of S6 kinase. These results suggest that the overall increase of S6 kinase activity is mostly due to p90rsk activation. The roles of eIF4E phosphorylation and S6 kinase activation in the physiological induction of M-phase and in the okadaic acid-induced premature mitotic events are discussed.

Rapamycin dissociates p70(S6K) activation from DNA synthesis stimulated by bombesin and insulin in Swiss 3T3 cells

Phosphorylation and subsequent activation of p70 S6 kinase (S6K) are events that are highly conserved in the cellular response to mitogens. The neuropeptide bombesin, which is a potent mitogen for Swiss 3T3 cells, stimulated a time- and dose-dependent activation of p70(S6K) as determined by gel mobility shift and immune complex kinase assays. This effect was inhibited by the immunosuppressant rapamycin at 10 nM, which also completely abolished bombesin-stimulated DNA synthesis at identical concentrations. In striking contrast, the combination of bombesin and insulin in synergy stimulated maximum DNA synthesis in these cells despite persistent inhibition of p70(S6K) by rapamycin throughout G1. These results indicate that activation of p70(S6K) is not required for the transition of quiescent cells to the S phase of the cell cycle. The inhibitory effects of rapamycin on bombesin-stimulated cell cycle progression did not involve accumulation of the cyclin-dependent kinase inhibitor p27(kip1) but a striking inhibition of the expression of cyclin D1. This effect was circumvented by bombesin with insulin, suggesting that a rapamycin-insensitive pathway stimulated by this combination leads to cyclin D1 expression. Thus, these findings, dissociating the mitogenic effects of bombesin in synergy with insulin from activation of p70(S6K), support the hypothesis that this kinase is a component of one of the parallel pathways that can lead to DNA synthesis rather than an obligatory point of convergence in mitogenic signaling.

A Drosophila gene structurally and functionally homologous to the mammalian 70-kDa s6 kinase gene

cDNAs encoding the Drosophila 70-kDa S6 kinase (S6K) were isolated by low-stringency hybridization with mammalian p70S6k probes. Conceptual translation of S6k cDNA sequences yields a product containing all of the canonical features typical of serine/threonine kinases and has 78% amino acid identity in the catalytic domain with the human p70S6k homologue. The S6k gene, located at polytene chromosome site 65D, gives rise to two predominant transcripts of 3.0 and 5.0 kb and at least two smaller transcripts (< 3.0 kb) that are found in whole-animal RNAs at all stages of development. Blood cells derived from the hematopoietic organs of ribosomal protein S6 (RpS6air8) mutant animals express higher levels of the smaller S6k transcripts, suggesting tissue- or genotype-specific differences in the regulation of the S6k gene. Drosophila S6K expressed in COS or NIH 3T3 cells phosphorylates mammalian RPS6 in a mitogen-dependent wortmannin- and rapamycin-sensitive manner, suggesting that its regulation is similiar to mammalian p70S6k.

Control of the translational regulators PHAS-I and PHAS-II by insulin and cAMP in 3T3-L1 adipocytes

The eukaryotic initiation factor 4E (eIF-4E)-binding proteins PHAS-I and PHAS-II were found to have overlapping but different patterns of expression in tissues. Both PHAS proteins were expressed in 3T3-L1 adipocytes, in which insulin stimulated their phosphorylation, promoted dissociation of PHAS.eIF-4E complexes, and decreased the ability of both to bind exogenous eIF-4E. The effects of insulin were attenuated by rapamycin and wortmannin, two agents that block activation of p70(S6K). Unlike PHAS-I, PHAS-II was readily phosphorylated by cAMP-dependent protein kinase in vitro; however, the effects of insulin on both PHAS proteins were attenuated by agents that increase intracellular cAMP, by cAMP derivatives, and by phosphodiesterase inhibitors. These agents also markedly inhibited the activation of p70(S6K). In summary, our results indicate that PHAS-I and -II are controlled by the mammalian target of rapamycin and p70(S6K) signaling pathway and that in 3T3-L1 adipocytes this pathway is inhibited by increased cAMP.