Cell cycle regulation of p70 S6 kinase and p42/p44 mitogen-activated protein kinases in Swiss mouse 3T3 fibroblasts

Thermal Bioprinting Causes Ample Alterations of Expression of LUCAT1, IL6, CCL26, and NRN1L Genes and Massive Phosphorylation of Critical Oncogenic Drug Resistance Pathways in Breast Cancer Cells

Bioprinting technology merges engineering and biological fields and together, they possess a great translational potential, which can tremendously impact the future of regenerative medicine and drug discovery. However, the molecular effects elicited by thermal inkjet bioprinting in breast cancer cells remains elusive. Previous studies have suggested that bioprinting can be used to model tissues for drug discovery and pharmacology. We report viability, apoptosis, phosphorylation, and RNA sequence analysis of bioprinted MCF7 breast cancer cells at separate timepoints post-bioprinting. An Annexin A5-FITC apoptosis stain was used in combination with flow cytometry at 2 and 24 h post-bioprinting. Antibody arrays using a Human phospho-MAPK array kit was performed 24 h post-bioprinting. RNA sequence analysis was conducted in samples collected at 2, 7, and 24 h post-bioprinting. The post-bioprinting cell viability averages were 77 and 76% at 24 h and 48 h, with 31 and 64% apoptotic cells at 2 and 24 h after bioprinting. A total of 21 kinases were phosphorylated in the bioprinted cells and 9 were phosphorylated in the manually seeded controls. The RNA seq analysis in the bioprinted cells identified a total of 12,235 genes, of which 9.7% were significantly differentially expressed. Using a ±2-fold change as the cutoff, 266 upregulated and 206 downregulated genes were observed in the bioprinted cells, with the following 5 genes uniquely expressed NRN1L, LUCAT1, IL6, CCL26, and LOC401585. This suggests that thermal inkjet bioprinting is stimulating large scale gene alterations that could potentially be utilized for drug discovery. Moreover, bioprinting activates key pathways implicated in drug resistance, cell motility, proliferation, survival, and differentiation.

Multisite Phosphorylation of S6K1 Directs a Kinase Phospho-code that Determines Substrate Selection

Multisite phosphorylation of kinases can induce on-off or graded regulation of catalytic activity; however, its influence on substrate specificity remains unclear. Here, we show that multisite phosphorylation of ribosomal protein S6 kinase 1 (S6K1) alters target selection. Agonist-inducible phosphorylation of glutamyl-prolyl tRNA synthetase (EPRS) by S6K1 in monocytes and adipocytes requires not only canonical phosphorylation at Thr389 by mTORC1 but also phosphorylation at Ser424 and Ser429 in the C terminus by cyclin-dependent kinase 5 (Cdk5). S6K1 phosphorylation at these additional sites induces a conformational switch and is essential for high-affinity binding and phosphorylation of EPRS, but not canonical S6K1 targets, e.g., ribosomal protein S6. Unbiased proteomic analysis identified additional targets phosphorylated by multisite phosphorylated S6K1 in insulin-stimulated adipocytes-namely, coenzyme A synthase, lipocalin 2, and cortactin. Thus, embedded within S6K1 is a target-selective kinase phospho-code that integrates signals from mTORC1 and Cdk5 to direct an insulin-stimulated, post-translational metabolon determining adipocyte lipid metabolism.

Gomesin inhibits melanoma growth by manipulating key signaling cascades that control cell death and proliferation

Consistent with their diverse pharmacology, peptides derived from venomous animals have been developed as drugs to treat disorders as diverse as hypertension, diabetes and chronic pain. Melanoma has a poor prognosis due in part to its metastatic capacity, warranting further development of novel targeted therapies. This prompted us to examine the anti-melanoma activity of the spider peptides gomesin (AgGom) and a gomesin-like homolog (HiGom). AgGom and HiGom dose-dependently reduced the viability and proliferation of melanoma cells whereas it had no deleterious effects on non-transformed neonatal foreskin fibroblasts. Concordantly, gomesin-treated melanoma cells showed a reduced G0/G1 cell population. AgGom and HiGom compromised proliferation of melanoma cells via activation of the p53/p21 cell cycle check-point axis and the Hippo signaling cascade, together with attenuation of the MAP kinase pathway. We show that both gomesin peptides exhibit antitumoral activity in melanoma AVATAR-zebrafish xenograft tumors and that HiGom also reduces tumour progression in a melanoma xenograft mouse model. Taken together, our data highlight the potential of gomesin for development as a novel melanoma-targeted therapy.

CycD/Cdk4 and Discontinuities in Dpp Signaling Activate TORC1 in the Drosophila Wing Disc

The molecular mechanisms regulating animal tissue size during development are unclear. This question has been extensively studied in the Drosophila wing disc. Although cell growth is regulated by the kinase TORC1, no readout exists to visualize TORC1 activity in situ in Drosophila. Both the cell cycle and the morphogen Dpp are linked to tissue growth, but whether they regulate TORC1 activity is not known. We develop here an anti-phospho-dRpS6 antibody that detects TORC1 activity in situ. We find, unexpectedly, that TORC1 activity in the wing disc is patchy. This is caused by elevated TORC1 activity at the cell cycle G₁/S transition due to CycD/Cdk4 phosphorylating TSC1/2. We find that TORC1 is also activated independently of CycD/Cdk4 when cells with different levels of Dpp signaling or Brinker protein are juxtaposed. We thereby characterize the spatial distribution of TORC1 activity in a developing organ.

RB/PLK1-dependent induced pathway by SLAMF3 expression inhibits mitosis and control hepatocarcinoma cell proliferation

Polo-like kinase PLK1 is a cell cycle protein that plays multiple roles in promoting cell cycle progression. Among the many roles, the most prominent role of PLK1 is to regulate the mitotic spindle formation checkpoint at the M-phase. Recently we reported the expression of SLAMF3 in Hepatocytes and show that it is down regulated in tumor cells of hepatocellular carcinoma (HCC). We also show that the forced high expression level of SLAMF3 in HCC cells controls proliferation by inhibiting the MAPK ERK/JNK and the mTOR pathways. In the present study, we provide evidence that the inhibitory effect of SLAMF3 on HCC proliferation occurs through Retinoblastoma (RB) factor and PLK1-dependent pathway. In addition to the inhibition of MAPK ERK/JNK and the mTOR pathways, expression of SLAMF3 in HCC retains RB factor in its hypophosphorylated active form, which in turn inactivates E2F transcription factor, thereby repressing the expression and activation of PLK1. A clear inverse correlation was also observed between SLAMF3 and PLK expression in patients with HCC. In conclusion, the results presented here suggest that the tumor suppressor potential of SLAMF3 occurs through activation of RB that represses PLK1. We propose that the induction of a high expression level of SLAMF3 in cancerous cells could control cellular mitosis and block tumor progression.

Genistein promotes insulin action through adenosine monophosphate-activated protein kinase activation and p70 ribosomal protein S6 kinase 1 inhibition in the skeletal muscle of mice fed a high energy diet

Genistein (GEN), a soy isoflavone, exerts insulin-sensitizing actions in animals; however, the underlying mechanisms have not been determined. Because GEN is a known activator of adenosine monophosphate-activated protein kinase (AMPK), we hypothesize that GEN activates insulin signaling through AMPK activation. To test this hypothesis, a high fat-high fructose diet (HFFD)-fed mice model of insulin resistance was administered GEN, and the insulin signaling pathway proteins in the skeletal muscle were examined. Hyperglycemia and hyperinsulinemia observed in HFFD-fed mice were significantly lowered by GEN. GEN increased insulin-stimulated tyrosine phosphorylation of insulin receptor-β and insulin receptor substrate (IRS) 1 but down-regulated IRS-1 serine phosphorylation in the skeletal muscle of HFFD-fed mice. Furthermore, GEN treatment improved muscle IRS-1-associated phospatidylinositol-3 kinase expression, phosphorylation of Akt at Ser(473), and translocation of glucose transporter subtype 4. Phosphorylation of AMPK at Thr(172) and acetyl coenzyme A carboxylase (ACC) at Ser(79) was augmented, whereas phosphorylation of p70 ribosomal protein S6 kinase 1 at Thr(389) was significantly decreased after GEN treatment in the skeletal muscle of HFFD-fed mice. These results suggest that GEN might improve insulin action in the skeletal muscle by targeting AMPK.

CDNA cloning and characterization of S6 kinase and its effect on yolk protein gene expression in the oriental fruit fly Bactrocera dorsalis (Hendel)

p70 S6 kinase (S6K), a serine/threonine protein kinase, is a downstream target of target of rapamycin (TOR) gene and an important regulator of protein synthesis responsible for cell growth and reproduction. In this study, a S6K gene, named BdS6K (GenBank Accession No. GQ203802), was isolated from the oriental fruit fly Bactrocera dorsalis (Hendel). Quantitative RT-PCR showed that BdS6K mRNA is expressed at a higher level in egg than in other developmental stages, as well as in ovary than in fat body. Downregulation of BdS6K activity by rapamycin treatment in larval stage resulted in the developmental defects of larvae, pupae, and adults, with a reduced yolk protein (YP) expression in the fat body throughout the first reproductive cycle with a substantial reduction in ovary size, and also repressed the egg development in female fruit fly. Knockdown of BdS6K gene by RNA interference in the adult significantly decreased the YP expression. These observations support the involvement of BdS6K signaling in the regulation of the YP synthesis and egg development in B. dorsalis.

Nucleocytoplasmic localization of p70 S6K1, but not of its isoforms p85 and p31, is regulated by TSC2/mTOR

The tuberous sclerosis complex gene 2 (TSC2)/mammalian target of rapamycin (mTOR) pathway controls many cellular functions via phosphorylation of ribosomal protein S6 kinases (S6Ks). Alternative splicing and translation generate three S6K1 proteins. Although nuclear and cytoplasmic S6K targets are known, the nucleocytoplasmic localization of the S6K1 proteins has not been comparatively elucidated so far. We show that in primary fibroblasts p85 S6K1 is cytoplasmic, p70 can be found in both compartments and p31 is exclusively nuclear. As already known for p70 and p85, our data suggest that p31 is also a target of mTOR-mediated phosphorylation. Blocking mTOR kinase activity via rapamycin and its activation in TSC2(-/-) cells and via TSC2 small interfering RNAs revealed that it regulates the localization of p70, but not of p85 and p31. The mTOR-dependent phosphorylation of p70 S6K1 at T389 is essential for its nuclear localization and exclusively hyperphosphorylated p70 S6K1 can be found in the nucleus. We further demonstrate this mTOR-controlled p70 S6K1 localization to be growth factor dependent. During the cell-cycle phosphorylation and nuclear localization of p70 S6K1 occur in mid G1 phase. We report that the different S6K1 proteins exhibit different nucleocytoplasmic localizations and that the TSC2/mTOR cascade not only regulates p70 S6K1 activity, but also its localization. These findings provide new important insights into the temporal and spatial dynamics of TSC2/mTOR/S6K regulation.

Functions and regulation of the 70kDa ribosomal S6 kinases

The 70kDa ribosomal protein S6 kinases, S6K1 and S6K2 are two highly homologous serine/threonine kinases that are activated in response to growth factors, cytokines and nutrients. The S6 kinases have been linked to diverse cellular processes, including protein synthesis, mRNA processing, glucose homeostasis, cell growth and survival. Studies in model organisms have highlighted the roles that S6K activity plays in a number of pathologies, including obesity, diabetes, ageing and cancer. The importance of S6K function in human diseases has led to the development of S6K-specific inhibitors by a number of companies, offering the promise of improved tools with which to study these enzymes and potentially the effective targeting of deregulated S6K signalling in patients. Here we review the current literature on the role of S6Ks in the regulation of cell growth, survival and proliferation downstream of various signalling pathways and how their dysregulation contributes to the pathogenesis of human diseases.

Characterization of p70 S6 kinase 1 in early development of mouse embryos

The mTOR kinase controls cell growth, proliferation, and survival through two distinct multiprotein complexes mTORC1 and mTORC2. p70 S6 Kinase 1 (S6K1) is characterized as downstream effector of mTOR. Until recently, the connection between S6K1 and mTORC1 /mTORC2 during the early development of mouse embryos has not been well elucidated. Here, the expression level of total S6K1 and its phosphorylation at Thr389 was determined in four phases of one-cell embryos. S6K1 was active throughout the cell cycle especially with higher activity in G2 and M phases. Rapamycin decreased the activity of M-phase promoting factor (MPF) and delayed the first mitotic cleavage. Down-regulating mTOR and raptor reduced S6K1 phosphorylation at Thr389 in one-cell embryos. Furthermore, rapamycin and microinjection of raptor shRNA decreased the immunofluorescent staining of Thr389 phospho-S6K1. It is proposed that mTORC1 may be involved in the control of MPF by regulating S6K1 during the early development of mouse embryos.

Developmental regulation of p70 S6 kinase by a G protein-coupled receptor dynamically modelized in primary cells

The mechanisms whereby G protein-coupled receptors (GPCR) activate signalling pathways involved in mRNA translation are ill-defined, in contrast to tyrosine kinase receptors (TKR). We compared a GPCR and a TKR, both endogenously expressed, for their ability to mediate phosphorylation of 70-kDa ribosomal S6 kinase p70S6K in primary rat Sertoli cells at two developmental stages. In proliferating cells stimulated with follicle-stimulating hormone (FSH), active p70S6K was phosphorylated on T389 and T421/S424, through cAMP-dependent kinase (PKA) and phosphatidyl-inositide-3 kinase (PI3K) antagonizing actions. In FSH-stimulated differentiating cells, active p70S6K was phosphorylated solely on T389, PKA and PI3K independently enhancing its activity. At both developmental stages, insulin-induced p70S6K regulation was consistent with reported data. Therefore, TKR and GPCR trigger distinct p70S6K active conformations. p70S6K developmental regulation was formalized in a dynamic mathematical model fitting the data, which led to experimentally inaccessible predictions on p70S6K phosphorylation rate.

MAPK pathway activation delays G2/M progression by destabilizing Cdc25B

Activation of the mitogen-activated protein kinase (MAPK) pathway by growth factors or phorbol esters during G(2) phase delays entry into mitosis; however, the role of the MAPK pathway during G(2)/M progression remains controversial. Here, we demonstrate that activation of the MAPK pathway with either epidermal growth factor or 12-O-tetradecanoylphorbol-13-acetate induces a G(2) phase delay independent of known G(2) phase checkpoint pathways but was specifically dependent on MAPK/extracellular signal-regulated kinase kinase (MEK1). Activation of MAPK signaling also blocked exit from a G(2) phase checkpoint arrest. Both the G(2) phase delay and blocked exit from the G(2) checkpoint arrest were mediated by the MEK1-dependent destabilization of the critical G(2)/M regulator cdc25B. Reintroduction of cdc25B overcame the MEK1-dependent G(2) phase delay. Thus, we have demonstrated a new function for MEK1 that controls G(2)/M progression by regulating the stability of cdc25B. This represents a novel mechanism by which factors that activate MAPK signaling can influence the timing of entry into mitosis, particularly exit from a G(2) phase checkpoint arrest.

The stimulus-dependent co-localization of serum- and glucocorticoid-regulated protein kinase (Sgk) and Erk/MAPK in mammary tumor cells involves the mutual interaction with the importin-alpha nuclear import protein

In Con8 rat mammary epithelial tumor cells, indirect immunofluorescence revealed that Sgk (serum- and glucocorticoid-regulated kinase) and Erk/MAPK (extracellular signal-regulated protein kinase/mitogen activated protein kinase) co-localized to the nucleus in serum-treated cells and to the cytoplasmic compartment in cells treated with the synthetic glucocorticoid dexamethasone. Moreover, the subcellular distribution of the importin-alpha nuclear transport protein was similarly regulated in a signal-dependent manner. In vitro GST-pull down assays revealed the direct interaction of importin-alpha with either Sgk or Erk/MAPK, while RNA interference knockdown of importin-alpha expression disrupted the localization of both Sgk and Erk into the nucleus of serum-treated cells. Wild type or kinase dead forms of Sgk co-immunoprecipitated with Erk/MAPK from either serum- or dexamethasone-treated mammary tumor cells, suggesting the existence of a protein complex containing both kinases. In serum-treated cells, nucleus residing Sgk and Erk/MAPK were both hyperphosphorylated, indicative of their active states, whereas, in dexamethasone-treated cells Erk/MAPK, but not Sgk, was in its inactive hypophosphorylated state. Treatment with a MEK inhibitor, which inactivates Erk/MAPK, caused the relocalization of both Sgk and ERK to the cytoplasm. We therefore propose that the signal-dependent co-localization of Sgk and Erk/MAPK mediated by importin-alpha represents a new pathway of signal integration between steroid and serum/growth factor-regulated pathways.

Regulation of catalytic activity of S6 kinase 2 during cell cycle

Ribosomal S6 kinase 2 (S6K2) is one of the kinases regulated by the mammalian target of rapamycin (mTOR) signaling pathway. Although it has been identified as a kinase homologous to S6K1, evidence suggests that the two kinases have non-overlapping functions, and the biological function of S6K2 still remains unknown. In order to identify the cell cycle stage(s) during which S6K2 plays a role, we assessed changes in the catalytic activity of S6K2 throughout the cell cycle. Our data show that S6K2 is active throughout the cell cycle with higher activity in G2 and M phases. We also show that S6K1 activity peaks sharply during M phase. Our data suggest that S6K1 and S6K2 likely play yet-unknown roles in G2 and M phases.

Coordinate regulation of ribosome biogenesis and function by the ribosomal protein S6 kinase, a key mediator of mTOR function

Current understanding of the mechanisms by which cell growth is regulated lags significantly behind our knowledge of the complex processes controlling cell cycle progression. Recent studies suggest that the mammalian target of rapamycin (mTOR) pathway is a key regulator of cell growth via the regulation of protein synthesis. The key mTOR effectors of cell growth are eukaryotic initiation factor 4E-binding protein 1 (4EBP-1) and the ribosomal protein S6 kinase (S6K). Here we will review the current models for mTOR dependent regulation of ribosome function and biogenesis as well as its role in coordinating growth factor and nutrient signaling to facilitate homeostasis of cell growth and proliferation. We will place particular emphasis on the role of S6K1 signaling and will highlight the points of cross talk with other key growth control pathways. Finally, we will discuss the impact of S6K signaling and the consequent feedback regulation of the PI3K/Akt pathway on disease processes including cancer.

The ERK1/2 mitogen-activated protein kinase pathway as a master regulator of the G1- to S-phase transition

The Ras-dependent extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein (MAP) kinase pathway plays a central role in cell proliferation control. In normal cells, sustained activation of ERK1/ERK2 is necessary for G1- to S-phase progression and is associated with induction of positive regulators of the cell cycle and inactivation of antiproliferative genes. In cells expressing activated Ras or Raf mutants, hyperactivation of the ERK1/2 pathway elicits cell cycle arrest by inducing the accumulation of cyclin-dependent kinase inhibitors. In this review, we discuss the mechanisms by which activated ERK1/ERK2 regulate growth and cell cycle progression of mammalian somatic cells. We also highlight the findings obtained from gene disruption studies.

Phosphatidylcholine catabolism in the MCF-7 cell cycle

The catabolism of phosphatidylcholine (PtdCho) appears to play a key role in regulating the net accumulation of the lipid in the cell cycle. Current protocols for measuring the degradation of PtdCho at specific cell-cycle phases require prolonged periods of incubation with radiolabelled choline. To measure the degradation of PtdCho at the S and G2 phases in the MCF-7 cell cycle, protocols were developed with radiolabelled lysophosphatidylcholine (lysoPtdCho), which reduces the labelling period and minimizes the recycling of labelled components. Although most of the incubated lysoPtdCho was hydrolyzed to glycerophosphocholine (GroPCho) in the medium, the kinetics of the incorporation of label into PtdCho suggests that the labelled GroPCho did not contribute significantly to cellular PtdCho formation. A protocol which involved exposing the cells twice to hydroxyurea, was also developed to produce highly synchronized MCF-7 cells with a profile of G1:S:G2/M of 90:5:5. An analysis of PtdCho catabolism in the synchronized cells following labelling with lysoPtdCho revealed that there was increased degradation of PtdCho in early to mid-S phase, which was attenuated in the G2/M phase. The results suggest that the net accumulation of PtdCho in MCF-7 cells may occur in the G2 phase of the cell cycle.

Phospholipids are synthesized in the G2/M phase of the cell cycle

Very little is known about the metabolism of phospholipids in the G2 and M phases of the cell cycle, but limited studies have led to the postulation that phospholipid synthesis ceases during this period. To investigate whether phospholipids are synthesized in the G2/M phase of the cell cycle, protocols were developed to produce synchronized MCF-7 cell populations with greater than 80% of the cells in G1/S or G2/M phases that moved in synchrony following removal of the blocking agent. Analysis of the activities of key phosphatidylcholine and phosphatidylethanolamine biosynthetic enzymes in subcellular fractions obtained from MCF-7 cells at different cell cycle phases revealed that there was robust activity of key enzymes in the fractions prepared from MCF-7 cells in G2/M phase. Radiolabeled choline and ethanolamine were rapidly incorporated into cells maintained at G2/M phase with nocodazole, and the rates of incorporation were similar to those obtained in cells allowed to progress into the G1 phase. Furthermore, radiolabeled glycerol was incorporated into phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine and phosphatidic acid in MCF-7 cells maintained at G2/M phase with nocodazole. Similar results were obtained in CHO cells. These results demonstrate that glycerophospholipid synthesis is very active in the G2/M phase of these cells. Therefore, the postulated cessation of phospholipid synthesis in G2/M phases is not applicable to all cell types.

Extracellular signal-regulated kinase 1/2 activity is not required in mammalian cells during late G2 for timely entry into or exit from mitosis

Extracellular signal-regulated kinase (ERK)1/2 activity is reported to be required in mammalian cells for timely entry into and exit from mitosis (i.e., the G2-mitosis [G2/M] and metaphase-anaphase [M/A] transitions). However, it is unclear whether this involvement reflects a direct requirement for ERK1/2 activity during these transitions or for activating gene transcription programs at earlier stages of the cell cycle. To examine these possibilities, we followed live cells in which ERK1/2 activity was inhibited through late G2 and mitosis. We find that acute inhibition of ERK1/2 during late G2 and through mitosis does not affect the timing of the G2/M or M/A transitions in normal or transformed human cells, nor does it impede spindle assembly, inactivate the p38 stress-activated checkpoint during late G2 or the spindle assembly checkpoint during mitosis. Using CENP-F as a marker for progress through G2, we also show that sustained inhibition of ERK1/2 transiently delays the cell cycle in early/mid-G2 via a p53-dependent mechanism. Together, our data reveal that ERK1/2 activity is required in early G2 for a timely entry into mitosis but that it does not directly regulate cell cycle progression from late G2 through mitosis in normal or transformed mammalian cells.

Identification of G2/M targets for the MAP kinase pathway by functional proteomics

Although the importance of the extracellular signal-regulated kinase (ERK) pathway in regulating the transition from G1 to S has been extensively studied, its role during the G2/M transition is less well understood. Previous reports have shown that inhibition of the ERK pathway in mammalian cells delays entry as well as progression through mitosis, suggesting the existence of molecular targets of this pathway in M phase. In this report we employed 2-DE and MS to survey proteins and PTMs in the presence versus absence of MKK1/2 inhibitor. Targets of the ERK pathway in G2/M were identified as elongation factor 2 (EF2) and nuclear matrix protein, 55 kDa (Nmt55). Phosphorylation of each protein increased under conditions of ERK pathway inhibition, suggesting indirect control of these targets; regulation of EF2 was ascribed to phosphorylation and inactivation of upstream EF2 kinase, whereas regulation of Nmt55 was ascribed to a delay in normal mitotic phosphorylation and dephosphorylation. 2-DE Western blots probed using anti-phospho-Thr-Pro antibody demonstrated that the effect of ERK inhibition is not to delay the onset of phosphorylation controlled by cdc2 and other mitotic kinases, but rather to regulate a small subset of targets in M phase in a nonoverlapping manner with cdc2.

Role of mTOR signaling in intestinal cell migration

An early signaling event activated by amino acids and growth factors in many cell types is the phosphorylation of the mammalian target of rapamycin (mTOR; FRAP), which is functionally linked to ribosomal protein s6 kinase (p70(s6k)), a kinase that plays a critical regulatory role in the translation of mRNAs and protein synthesis. We previously showed that intestinal cell migration, the initial event in epithelial restitution, is enhanced by l-arginine (ARG). In this study, we used amino acids as prototypic activators of mTOR and ARG, IGF-1, or serum as recognized stimulators of intestinal cell migration. We found that 1) protein synthesis is required for intestinal cell migration, 2) mTOR/p70(s6k) pathway inhibitors (rapamycin, wortmannin, and intracellular Ca(2+) chelation) inhibit cell migration, 3) ARG activates migration and mTOR/p70(s6k) (but not ERK-2) in migrating enterocytes, and 4) immunocytochemistry reveals abundant p70(s6k) staining in cytoplasm, whereas phospho-p70(s6k) is virtually all intranuclear in resting cells but redistributes to the periphery on activation by ARG. We conclude that mTOR/p70(s6k) signaling is essential to intestinal cell migration, is activated by ARG, involves both nuclear and cytoplasmic events, and may play a role in intestinal repair.

Nuclear Export of S6K1 II Is Regulated by Protein Kinase CK2 Phosphorylation at Ser-17

Ribosomal S6 kinases (S6Ks) are principal players in the regulation of cell growth and energy metabolism. Signaling via phosphatidylinositol 3-kinase and mammalian target of rapamycin pathways mediates the activation of S6K in response to various mitogenic stimuli. The family of S6Ks consists of two forms, S6K1 and -2, that have cytoplasmic and nuclear splicing variants, S6K1 II and S6K1 I, respectively. Nuclear-cytoplasmic shuttling of both isoforms induced by mitogenic stimuli has been reported recently. Here we present the identification of protein kinase CK2 (CK2) as a novel binding and regulatory partner for S6K1 II. The interaction between S6K1 II and CK2beta regulatory subunit was initially identified in a yeast two-hybrid screen and further confirmed by co-immunoprecipitation of transiently expressed and endogenous proteins. The interaction between S6K1 II and CK2 was found to occur in serum-starved and serum-stimulated cells. In addition, we found that S6K1 II is a substrate for CK2. The localization of the CK2 phosphorylation site was narrowed down to Ser-17 in S6K1 II. Mutational analysis and the use of phosphospecific antibody indicate that Ser-17 is a major in vitro and in vivo phosphorylation site for CK2. Functional studies reveal that, in contrast to the wild type kinase, the phosphorylation-mimicking mutant of S6K1 II (S17E) retains its cytoplasmic localization in serum-stimulated cells. Treatment of cells with the nuclear export inhibitor leptomycin B revealed that the S17E mutant accumulates in the nucleus to the same extent as S6K1 II wild type. These results indicate that nuclear import of the S17E mutant is not affected, although the export is significantly enhanced. We also provide evidence that nuclear export of S6K1 is mediated by a CRM1-dependent mechanism. Taken together, this study establishes a functional link between S6K1 II and CK2 signaling, which involves the regulation of S6K1 II nuclear export by CK2-mediated phosphorylation of Ser-17.

ERK1c regulates Golgi fragmentation during mitosis

Extracellular signal-regulated kinase 1c (ERK1c) is an alternatively spliced form of ERK1 that is regulated differently than other ERK isoforms. We studied the Golgi functions of ERK1c and found that it plays a role in MEK-induced mitotic Golgi fragmentation. Thus, in late G2 and mitosis of synchronized cells, the expression and activity of ERK1c was increased and it colocalized mainly with Golgi markers. Small interfering RNA of ERK1c significantly attenuated, whereas ERK1c overexpression facilitated, mitotic Golgi fragmentation. These effects were also reflected in mitotic progression, indicating that ERK1c is involved in cell cycle regulation via modulation of Golgi fragmentation. Although ERK1 was activated in mitosis as well, it could not replace ERK1c in regulating Golgi fragmentation. Therefore, MEKs regulate mitosis via all three ERK isoforms, where ERK1c acts specifically in the Golgi, whereas ERK1 and 2 regulate other mitosis-related processes. Thus, ERK1c extends the specificity of the Ras-MEK cascade by activating ERK1/2-independent processes.

Tanshinone: an inhibitor of proliferation of vascular smooth muscle cells

Tanshinone (Tan) is one of the active components of Radix Salvia miltiorrhiza (Lamiaceae), which is clinically used to treat cardiovascular diseases in China. The aim of this study was to estimate the effect of Tan on the proliferation of cultured vascular smooth muscle cells (VSMCs) induced by fatal bovine serum (FBS). It was shown that various concentrations of Tan inhibited the VSMCs proliferation in a dose-dependent manner. Tan significantly blocked VSMCs cell cycle in G(0)/G(1) phase. The anti-proliferative effect of Tan was associated with the inhibition of the extracellular signal-regulated kinase1/2 (ERK1/2). On the other hand, the decrement of Tan on the cyclin D1 protein may be related to the high expression of p21(waf/cip1). The data suggest that the anti-proliferative effect of Tan on VSMCs proliferation was associated with ERK1/2 signaling pathway.

Nuclear phospholipase C-β1b activation during G2/M and late G1 phase in nocodazole-synchronized HL-60 cells

In this study, the activity of nuclear phosphatidylinositol-specific phosholipase C (PI-PLC) was investigated in HL-60 cells blocked at G(2)/M phase by the addition of nocodazole, and released into medium as synchronously progressing cells. Two peaks of an increase in the nuclear PI-PLC activities were detected; an early peak reached a maximum at 1 h after release from the nocodazole block, and a second increase was detected at 8.5 h after the release. Immunoprecipitation studies indicated that the increase in the activity was due to the activation of the nuclear PI-PLC-beta(1). Western blot analysis demonstrated no changes in the level of both a and b splicing variants of PI-PLC-beta(1) in the nuclei of cells isolated at either 1 h or 8.5 h after the block. However, an increase in the serine-phosphorylation of PI-PLC-beta(1b) was detected in the nuclei of HL-60 cells isolated at 1 and 8.5 h after the block, and the presence of MEK-inhibitor PD98059 completely inhibited both the serine phosphorylation and the increase in the PI-PLC activities in vitro. The presence of PI-PLC inhibitor prevented the progression of HL-60 cells through the G(1) into S phase of the cell cycle. These results demonstrate that two peaks of nuclear PI-PLC activities, which are due to a PD98059-sensitive phosphorylation of nuclear PLC-beta(1b) on serine, occur at the G(2)/M and late G(1) phase and are necessary for the progression of the cells through the cell cycle.

Colorectal tumors frequently express phosphorylated mitogen-activated protein kinase

Mounting evidence suggests that activation of the mitogen-activated protein (MAP) kinase pathway plays an important role in tumorigenesis. MAP kinase/ERK kinase (MEK), a crucial constituent of this pathway, is activated by phosphorylation, and the phosphorylated MEK (pMEK) in turn activates ERK kinase. The expression of pMEK has been described in some human malignancies, but not in primary human colon tumors. In this study, we analyzed the expression of pMEK in 123 colorectal tumors by immunohistochemistry. pMEK was detected either in the cytoplasm (63 cases) or nucleus (40 cases) in 93 of the 123 tumors (76%). Tubular adenomas and villous adenomas also expressed pMEK in 30% and 40% of the tumors, respectively. By contrast, the epithelial cells in the normal colonic mucosa showed no or only weak expression of pMEK in the cytoplasm. Taken together, these results indicate that MEK is frequently phosphorylated in colorectal tumors, and suggest that phosphorylation of MEK may play a role in the development of colorectal tumors.

N/A

N/A

A mitotic cascade of NIMA family kinases. Nercc1/Nek9 activates the Nek6 and Nek7 kinases

The Nek family of protein kinases in humans is composed of 11 members that share an amino-terminal catalytic domain related to NIMA, an Aspergillus kinase involved in the control of several aspects of mitosis, and divergent carboxyl-terminal tails of varying length. Nek6 (314AA) and Nek7 (303AA), 76% identical, have little noncatalytic sequence but bind to the carboxyl-terminal noncatalytic tail of Nercc1/Nek9, a NIMA family protein kinase that is activated in mitosis. Microinjection of anti-Nercc1 antibodies leads to spindle abnormalities and prometaphase arrest or chromosome missegregation. Herein we show that Nek6 is increased in abundance and activity during mitosis; activation requires the phosphorylation of Ser206 on the Nek6 activation loop. This phosphorylation and the activity of recombinant Nek6 is stimulated by coexpression with an activated mutant of Nercc1. Moreover, Nercc1 catalyzes the direct phosphorylation of prokaryotic recombinant Nek6 at Ser206 in vitro concomitant with 20-25-fold activation of Nek6 activity; Nercc1 activates Nek7 in vitro in a similar manner. Nercc1/Nek9 is likely to be responsible for the activation of Nek6 during mitosis and probably participates in the regulation of Nek7 as well. These findings support the conclusion that Nercc1/Nek9 and Nek6 represent a novel cascade of mitotic NIMA family protein kinases whose combined function is important for mitotic progression.

Mechanism of mitosis-specific activation of MEK1

Activation of cyclin B-Cdc2 is an absolute requirement for entry into mitosis, but other protein kinase pathways that also have mitotic functions are activated during G(2)/M progression. The MAPK cascade has well established roles in entry and exit from mitosis in Xenopus, but relatively little is known about the regulation and function of this pathway in mammalian mitosis. Here we report a detailed analysis of the activity of all components of the Ras/Raf/MEK/ERK pathway in HeLa cells during normal G(2)/M. The focus of this pathway is the dramatic activation of an endomembrane-associated MEK1 without the corresponding activation of the MEK substrate ERK. This is because of the uncoupling of MEK1 activation from ERK activation. The mechanism of this uncoupling involves the cyclin B-Cdc2-dependent proteolytic cleavage of the N-terminal ERK-binding domain of MEK1 and the phosphorylation of Thr(286). These results demonstrate that cyclin B-Cdc2 activity regulates signaling through the MAPK pathway in mitosis.

Mitotic regulation of ribosomal S6 kinase 1 involves Ser/Thr, Pro phosphorylation of consensus and non-consensus sites by Cdc2

During mitosis, the cyclin-dependent kinase, Cdc2, signals the inactivation of major anabolic processes such as transcription, mRNA processing, translation, and ribosome biogenesis, thereby providing energy needed for the radical and energetically costly structural reorganization of the cell. This is accomplished by phosphorylation and inactivation of several key anabolic elements, including TFIIIB, TFIID, RNA polymerase II, poly(A) polymerase, and translation elongation factor 1gamma. We report here that ribosomal S6 kinase 1 (S6K1), a protein kinase linked to the translation of ribosomal protein mRNAs, is also subject to regulation by Cdc2 in mitosis. In mitotic HeLa cells, when the activity of Cdc2 is high, S6K1 is phosphorylated at multiple Ser/Thr, Pro (S/TP) sites, including Ser(371), Ser(411), Thr(421), and Ser(424). Concomitant with this, the phosphorylation of the hydrophobic motif site, Thr(389), is reduced resulting in a decrease in the specific activity of S6K1. The mitotic S/TP phosphorylation sites are readily phosphorylated by Cdc2.cyclin B in vitro. These proline-directed phosphorylations are sensitive to chemical inhibitors of Cdc2 but not to inhibitors of mammalian target of rapamycin, phosphatidylinositol 3-kinase, MEK1/2, or p38. In murine FT210 cells arrested in mitosis, conditional inactivation of Cdc2 reduces phosphorylation of S6K1 at S/TP sites while simultaneously increasing phosphorylation of Thr(389) and of the S6K1 substrate, RPS6. A physical interaction exists between Cdc2 and S6K1, and this interaction is enhanced in mitotic cells. These results suggest that Cdc2 provides a signal that triggers inactivation of S6K1 in mitosis, presumably serving to spare energy for costly mitotic processes at the expense of ribosomal protein synthesis.

Paclitaxel induces inactivation of p70 S6 kinase and phosphorylation of Thr421 and Ser424 via multiple signaling pathways in mitosis

The 70 kDa ribosomal S6 kinase (p70S6K) is important for cell growth and survival. Activation of p70S6K requires sequential phosphorylation of multiple serine and threonine sites often triggered by growth factors and hormones. Here, we report that paclitaxel, a microtubule-damaging agent, induces phosphorylation of p70S6K at threonine 421 and serine 424 (T421/S424) in a concentration- and time-dependent manner in multiple breast and ovarian cancer cell lines demonstrated by a T421/S424 phospho-p70S6K antibody. Phosphoamino-acid analysis and Western blot analysis by serine-/threonine-specific antibodies further confirms that both serine and threonine residues are phosphorylated in p70S6K following treatment with paclitaxel. Paclitaxel-induced p70S6K(T421/S424) phosphorylation requires both de novo RNA and protein synthesis via multiple signaling pathways including ERK1/2 MAP kinase, JNK, PKC, Ca(++), PI3K, and mammalian target of rapamycin (mTOR). Despite phosphorylation of p70S6K(T421/S424), paclitaxel inactivates this kinase in a concentration- and time-dependent manner as illustrated by in vitro kinase assay. Inhibitors of mTOR, PI3K, and Ca(++) impair p70S6K activity, whereas inhibitors of JNK and PKC stimulate p70S6K activity. Inhibition of PKC and JNK prevents paclitaxel-induced p70S6K inactivation. Moreover, the paclitaxel-induced phosphorylation and low activity of p70S6K mainly occurs during mitosis. In summary, paclitaxel is able to induce p70S6K(T421/S424) phosphorylation and decrease its activity in mitotic cells via multiple signaling pathways. Our data suggest that paclitaxel-induced p70S6K(T421/S424) phosphorylation and kinase inactivation are differentially regulated. Our data also indicate that paclitaxel may exert its antitumor effect, at least in part, via inhibition of p70S6K.

Distinct cell cycle timing requirements for extracellular signal-regulated kinase and phosphoinositide 3-kinase signaling pathways in somatic cell mitosis

Mitogen-activated protein (MAP) kinase and phosphoinositide 3-kinase (PI3K) pathways are necessary for cell cycle progression into S phase; however the importance of these pathways after the restriction point is poorly understood. In this study, we examined the regulation and function of extracellular signal-regulated kinase (ERK) and PI3K during G(2)/M in synchronized HeLa and NIH 3T3 cells. Phosphorylation and activation of both the MAP kinase kinase/ERK and PI3K/Akt pathways occur in late S and persist until the end of mitosis. Signaling was rapidly reversed by cell-permeable inhibitors, indicating that both pathways are continuously activated and rapidly cycle between active and inactive states during G(2)/M. The serum-dependent behavior of PI3K/Akt versus ERK pathway activation indicates that their mechanisms of regulation differ during G(2)/M. Effects of cell-permeable inhibitors and dominant-negative mutants show that both pathways are needed for mitotic progression. However, inhibiting the PI3K pathway interferes with cdc2 activation, cyclin B1 expression, and mitotic entry, whereas inhibiting the ERK pathway interferes with mitotic entry but has little effect on cdc2 activation and cyclin B1 and retards progression from metaphase to anaphase. Thus, our study provides novel evidence that ERK and PI3K pathways both promote cell cycle progression during G(2)/M but have different regulatory mechanisms and function at distinct times.

p42/p44 MAPKs are intracellular targets of the CDK inhibitor purvalanol

Chemical inhibitors of cyclin-dependent kinases (CDKs) have a great therapeutic potential against various proliferative and neurodegenerative disorders. Intensive screening of a combinatorial chemistry library of 2,6,9-trisubstituted purines has led to the identification of purvalanol, one of the most potent and selective CDK inhibitors to date. In preliminary studies, this compound demonstrates definite anti-mitotic properties, consistent with its nanomolar range efficiency towards purified CDK1 and CDK2. However, the actual intracellular targets of purvalanol remain to be identified, and a method for the determination of its in vivo selectivity was developed. In this technique, cell extracts were screened for purvalanol-interacting proteins by affinity chromatography on immobilized inhibitor. In addition to CDK1, p42/p44 MAPK were found to be two major purvalanol-interacting proteins in five different mammalian cell lines (CCL39, PC12, HBL100, MCF-7 and Jurkat cells), suggesting the generality of the purvalanol/p42/p44 MAPK interaction. The Chinese hamster lung fibroblast cell line CCL39 was used as a model to investigate the anti-proliferative properties of purvalanol. The compound inhibited cell growth with a GI(50) value of 2.5 microM and induced a G2/M block when added to exponentially growing cells. It did not appear to trigger massive activation of caspase. We next tested whether CDKs and p42/p44 MAPK were actually targeted by the compound in vivo. p42/p44 MAPK activity was visualized using an Elk-Gal4 luciferase reporter system and CDK1 activity was detected by the phosphonucleolin level. When cells were treated with purvalanol, p42/p44 MAPK and CDK1 activities were inhibited in a dose-dependent manner. Furthermore, purvalanol inhibited the nuclear accumulation of p42/p44 MAPK, an event dependent on the catalytic activity of these kinases. We conclude that the anti-proliferative properties of purvalanol are mediated by inhibition of both p42/p44 MAPK and CDKs. These observations highlight the potency of moderate selectivity compounds and encourage the search for new therapeutics which simultaneously target distinct but relevant pathways of cell proliferation.

Ras-MAP kinase signaling pathways and control of cell proliferation: relevance to cancer therapy

The mitogen-activated protein (MAP) kinase pathways represent several families of signal transduction cascades that mediate information provided by extracellular stimuli. MAP kinase pathways regulate a wide range of physiological responses, including cell proliferation, apoptosis, cell differentiation, and tissue development. Constitutive activation of MAP kinase proteins in experimental models has been shown to cause cell transformation and is implicated in tumorigenesis. Of clinical importance, MAP kinase pathways are regulated by Ras G-proteins, which are found to be mutated and constitutively active in approximately 30% of all human cancers. Thus, a major goal in the treatment of cancer is the development of specific compounds that target Ras and critical downstream signaling proteins responsible for uncontrolled cell growth. A variety of biochemical, molecular, and structural approaches have been used to develop drug compounds that target signaling proteins important for MAP kinase pathway activation. These compounds have been useful tools for identifying the mechanisms of MAP kinase pathway signaling and hold promise for clinical use. This review will present an overview of the major proteins involved in Ras and MAP kinase signaling pathways and their function in regulating cell cycle events and proliferation. In addition, some of the relevant compounds that have been developed to inhibit the activities of these proteins and MAP kinase signaling are discussed.

The Src-family tyrosine kinase inhibitor PP1 interferes with the activation of ribosomal protein S6 kinases

Considerable biochemical and pharmacological evidence suggests that the activation of ribosomal protein S6 kinases (S6Ks) by activated receptor tyrosine kinases involves multiple co-ordinated input signals. However, the identities of many of these inputs remain poorly described, and their precise involvement in S6K activation has been the subject of great investigative effort. In the present study, we have shown that 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1), a selective inhibitor of the Src family of non-receptor tyrosine kinases, interferes with the activation of 70 and 85 kDa S6K gene products (p70S6K1 and p85S6K1) by insulin, insulin-like growth factor 1, sodium orthovanadate and activated alleles of phosphoinositide 3-kinase and H-Ras. PP1 also impedes the activation of AKT/protein kinase B and the extracellular signal-regulated protein kinases 1 and 2 by these various stimuli. Insulin-like growth factor 1 was observed to induce a sustained increase in c-Src autophosphorylation as revealed using anti-phospho-Y416 antisera, but this effect was absent from the cells treated with PP1. To conclude, an activated allele of p70S6K1 is compared with the wild-type allele, resistant to inhibition by PP1 when co-expressed with phosphoinositide-dependent kinase 1 (PDK1), suggesting that PP1 affects p70S6K1 via a PDK1-independent pathway. Thus activation of Src may supply a necessary signal for the activation of p70S6K1 and possibly other S6Ks.

Control of plant cytokinesis by an NPK1-mediated mitogen-activated protein kinase cascade

Cytokinesis is the last essential step in the distribution of genetic information to daughter cells and partition of the cytoplasm. In plant cells, various proteins have been found in the phragmoplast, which corresponds to the cytokinetic apparatus, and in the cell plate, which corresponds to a new cross wall, but our understanding of the functions of these proteins in cytokinesis remains incomplete. Reverse genetic analysis of NPK1 MAPKKK (nucleus- and phragmoplast-localized protein kinase 1 mitogen-activated protein kinase kinase kinase) and investigations of factors that might be functionally related to NPK1 have helped to clarify new aspects of the mechanisms of cytokinesis in plant cells. In this review, we summarize the evidence for the involvement of NPK1 in cytokinesis. We also describe the characteristics of a kinesin-like protein and the homologue of a mitogen-activated protein kinase that we identified recently, and we discuss possible relationships among these proteins in cytokinesis.

Inhibition of p70(S6) kinase during transforming growth factor-beta 1/vitamin D(3)-induced monocyte differentiation of HL-60 cells allows tumor necrosis factor-alpha to stimulate plasminogen activator inhibitor-1 synthesis

We investigated intracellular mechanisms involved in the up-regulation of plasminogen activator inhibitor I (PAI-1) synthesis by human recombinant tumor necrosis factor-alpha (TNF) during monocyte differentiation of HL-60 cells triggered by the transforming growth factor-beta1/vitamin D(3) (TGF/D3) mixture. TGF/D3-treated cells expressed surface monocytic markers and produced noticeable amounts of PAI-1 but stopped to proliferate. A reduced p70 S6 kinase (p70(S6K)) phosphorylation was also observed and, in this situation, TNF dramatically enhanced PAI-1 synthesis. Similarly, TNF significantly up-regulated PAI-1 synthesis when p70(S6K) phosphorylation was inhibited by rapamycin. This phenomenon was not due to a general decrease in protein synthesis but involved the activation of gene transcription rather than PAI-1 mRNA stabilization. The level of the transcriptional regulator factor E2F1, a repressor of PAI-1 gene expression, was shown to be down-modulated in TGF/D3- as well as in rapamycin-treated cells. Furthermore, the apoptotic effect of TNF in HL-60 cells appeared to be prevented by the addition of either TGF/D3 or rapamycin. In conclusion, these results indicate that inhibition of p70(S6K) phosphorylation during TGF/D3-induced monocyte differentiation of HL-60 cells is a determinant factor that allows TNF to exert its up-regulating effect on PAI-1 synthesis while protecting cells from apoptosis.

Coordinate involvement of cell cycle arrest and apoptosis strengthen the effect of FTY720

A novel reagent, FTY720 (2-amino-2-[2-(4-octylphenyl)ethyl]-1,3-propanediol hydrochloride), has been shown to induce a significant decrease of lymphocytes and lymphoma cells and is expected to be a potent immunosuppressant and anti-tumor drug. The decrease in lymphocytes and lymphoma cells is mainly the result of FTY720-induced apoptosis. FTY720 directly affects mitochondria and induces cell death. Moreover, FTY720 activates protein phosphatase (PP) 2A and affects anti-apoptotic intracellular signal transduction proteins to attenuate the anti-apoptotic effect. In this study, we examined the relationship between FTY720-induced apoptosis and cell cycle regulation. FTY720 induced apoptosis significantly at the G0 / G1 phase and caused G0 / G1 cell cycle arrest of the human lymphoma cell lines HL-60RG and Jurkat. Simultaneously, retinoblastoma protein (pRB) was dephosphorylated, suggesting that dephosphorylation of pRB was related to FTY720-induced G0 / G1 cell cycle arrest. Because this dephosphorylation was completely blocked by a specific PP1 / 2A inhibitor, okadaic acid, it appears that FTY720-activated PP2A is essential for FTY720-induced cell cycle arrest. FTY720-induced apoptosis was inhibited by Bcl-2 overexpression in Jurkat cells, but this did not prevent FTY720-induced cell cycle arrest, suggesting that the mechanism of FTY720-induced cell cycle arrest is independent of the mechanism of FTY720-induced apoptosis. These two independent pathways strengthen the effect of FTY720.

Rapamycin-insensitive regulation of 4e-BP1 in regenerating rat liver

In cultured cells, growth factor-induced phosphorylation of two translation modulators, p70 S6 kinase and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), is blocked by nanomolar concentrations of the immunosuppressant rapamycin. Rapamycin also attenuates liver regeneration after partial hepatectomy, but it is not known if this growth-suppressive effect is due to dephosphorylation of p70 S6 kinase and/or 4E-BP1. We found that partial hepatectomy induced a transient increase in liver p70 S6 kinase activity and 4E-BP1 phosphorylation as compared with sham-operated rats. The amount of p70 S6 kinase protein in regenerating liver did not increase, but active kinase from partially hepatectomized animals was highly phosphorylated. Phosphorylated 4E-BP1 from regenerating liver was unable to form an inhibitory complex with initiation factor 4E. Rapamycin blocked the activation of p70 S6 kinase in response to partial hepatectomy in a dose-dependent manner, but 4E-BP1 phosphorylation was not inhibited. By contrast, functional phosphorylation of 4E-BP1 induced by injection of cycloheximide or growth factors was partially reversed by the drug. The mammalian target of rapamycin (mTOR) has been proposed to directly phosphorylate 4E-BP1. Western blot analysis using phospho-specific antibodies showed that phosphorylation of Thr-36/45 and Ser-64 increased in response to partial hepatectomy in a rapamycin-resistant manner. Thus, rapamycin inhibits p70 S6 kinase activation and liver regeneration, but not functional phosphorylation of 4E-BP1, in response to partial hepatectomy. These results indicate that the effect of rapamycin on 4E-BP1 function in vivo can be significantly different from its effect in cultured cells.

The NPK1 mitogen-activated protein kinase kinase kinase is a regulator of cell-plate formation in plant cytokinesis

Mitogen-activated protein kinase (MAPK) cascades play important roles not only in the transduction of extracellular signals but in the progression of the cell cycle. However, evidence for their role in cytokinesis is limited. Here, we show that a tobacco MAPK kinase kinase (MAPKKK), designated NPK1, is required for cytokinesis. The activity of NPK1 increases in the late M phase of the tobacco cell cycle. During expansion of a new cross-wall (cell plate) toward the cell cortex, NPK1 is consistently localized to the equatorial zone of the phragmoplast, the cytokinetic apparatus where the cell plate is formed. Expression of a kinase-negative mutant of NPK1 results in the generation of multinucleate cells with incomplete cell plates. Phragmoplasts can be formed, but its expansion toward the cell cortex is also blocked. Thus, our results indicate that the NPK1 MAPKKK is essential for the formation of the cell plate, especially for its lateral growth.

TGF-beta inhibits p70 S6 kinase via protein phosphatase 2A to induce G(1) arrest

On TGF-beta binding, the TGF-beta receptor directly phosphorylates and activates the transcription factors Smad2/3, leading to G(1) arrest. Here, we present evidence for a second, parallel, TGF-beta-dependent pathway for cell cycle arrest, achieved via inhibition of p70(s6k). TGF-beta induces association of its receptor with protein phosphatase-2A (PP2A)-Balpha. Concomitantly, three PP2A-subunits, Balpha, Abeta, and Calpha, associate with p70(s6k), leading to its dephosphorylation and inactivation. Although either pathway is sufficient to induce G(1) arrest, abrogation of both, the inhibition of p70(s6k), and transcription through Smad proteins is required for release of epithelial cells from TGF-beta-induced G(1) arrest. TGF-beta thereby modulates the translational and posttranscriptional control of cell cycle progression.

Differential regulation of human papillomavirus E6 by protein kinase A: conditional degradation of human discs large protein by oncogenic E6

The protein Kinase A (PKA) pathway was found to selectively regulate the function of oncogenic but not non-oncogenic E6 proteins. High risk E6 proteins are phosphorylated at their Dlg/PDZ binding motif at the C-terminus by a PKA like activity. This PKA and PDZ binding module is found only for human PV, is strictly conserved in all the transforming HPVs and is absent in all the low risk HPV types. We present evidence of a conditional regulation of E6 induced degradation of Dlg. HPV18E6 positive but not HPV negative keratinocytes exhibit increased Dlg steady state levels under conditions of high PKA activity, with a concomitant increase in the presence of Dlg at tight junctions. In vitro binding experiments show that E6 phosphorylation by PKA reduces its binding to Dlg and molecular modelling can explain this observation in a structural context. E6 dependent degradation of Dlg in cells with high PKA levels is inhibited and this is dependent on phosphorylation of the PDZ binding site in E6. In contrast, the degradation of p53 induced by E6 is not affected by PKA. We propose a differential regulation of E6 for the ubiquitin mediated degradation of specific E6 target proteins.

Glucocorticoids abate p70(S6k) and eIF4E function in L6 skeletal myoblasts

The catabolic properties of glucocorticoid hormones are largely attributable to dual regulation of protein degradation and synthesis. With regard to the latter, glucocorticoids modulate the translational machinery, namely that component functional in translation initiation. This investigation revealed that in L6 myoblasts, dexamethasone, a synthetic glucocorticoid, deactivated the ribosomal protein S6 kinase (p70(S6k)) within 4 h, as evidenced by diminished phosphorylation of its physiological substrate, the 40S ribosomal protein S6. This deactivation correlated with dephosphorylation of p70(S6k) at Thr(389), whereas phosphorylation of Ser(411) was unaffected. Furthermore, glucocorticoid administration induced dephosphorylation of the cap-dependent translational repressor, eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1), thereby facilitating conjunction of the inhibitor and eIF4E. The mechanism of action is reminiscent of classical transcriptional regulation by steroid hormone receptors in that these effects were preceded by a temporal lag and were sensitive to inhibitors of glucocorticoid receptor function as well as transcriptional and translational inhibition. Okadaic acid and calyculin A corrected the dexamethasone-induced dephosphorylation of p70(S6k) and 4E-BP1, implicating a PP1- and/or PP2A-like protein phosphatase(s) in the observed phenomena. Hence, glucocorticoids attenuate distal constituents of the phosphatidylinositol-3 kinase signaling pathway and thereby encumber the protein synthetic apparatus.