Activation of a ribosomal protein S6 kinase in mouse fibroblasts during infection with herpesvirus

Ribosomal Protein S6: A Potential Therapeutic Target against Cancer?

Ribosomal protein S6 (RPS6) is a component of the 40S small ribosomal subunit and participates in the control of mRNA translation. Additionally, phospho (p)-RPS6 has been recognized as a surrogate marker for the activated PI3K/AKT/mTORC1 pathway, which occurs in many cancer types. However, downstream mechanisms regulated by RPS6 or p-RPS remains elusive, and the therapeutic implication of RPS6 is underappreciated despite an approximately half a century history of research on this protein. In addition, substantial evidence from RPS6 knockdown experiments suggests the potential role of RPS6 in maintaining cancer cell proliferation. This motivates us to investigate the current knowledge of RPS6 functions in cancer. In this review article, we reviewed the current information about the transcriptional regulation, upstream regulators, and extra-ribosomal roles of RPS6, with a focus on its involvement in cancer. We also discussed the therapeutic potential of RPS6 in cancer.

Alterations in Phosphorylation of Hepatocyte Ribosomal Protein S6 Control Plasmodium Liver Stage Infection

Plasmodium parasites are highly selective when infecting hepatocytes and induce many changes within the host cell upon infection. While several host cell factors have been identified that are important for liver infection, our understanding of what facilitates the maintenance of infection remains incomplete. Here, we describe a role for phosphorylated ribosomal protein S6 (Ser235/236) (p-RPS6) in Plasmodium yoelii-infected hepatocytes. Blocking RPS6 phosphorylation prior to infection decreases the number of liver stage parasites within 24 h. Infected hepatocytes exhibit elevated levels of p-RPS6 while simultaneously abrogating the induction of phosphorylation of RPS6 in response to insulin stimulation. This is in contrast with the regulation of p-RPS6 by Toxoplasma gondii, which elevates levels of p-RPS6 after infection but does not alter the response to insulin. Our data support a model in which RPS6 phosphorylation is uncoupled from canonical regulators in Plasmodium-infected hepatocytes and is relied on by the parasite to maintain infection.

Regulation of Ribosomal Proteins on Viral Infection

Ribosomal proteins (RPs), in conjunction with rRNA, are major components of ribosomes involved in the cellular process of protein biosynthesis, known as "translation". The viruses, as the small infectious pathogens with limited genomes, must recruit a variety of host factors to survive and propagate, including RPs. At present, more and more information is available on the functional relationship between RPs and virus infection. This review focuses on advancements in my own understanding of critical roles of RPs in the life cycle of viruses. Various RPs interact with viral mRNA and proteins to participate in viral protein biosynthesis and regulate the replication and infection of virus in host cells. Most interactions are essential for viral translation and replication, which promote viral infection and accumulation, whereas the minority represents the defense signaling of host cells by activating immune pathway against virus. RPs provide a new platform for antiviral therapy development, however, at present, antiviral therapeutics with RPs involving in virus infection as targets is limited, and exploring antiviral strategy based on RPs will be the guides for further study.

Systems Biology of Immunomodulation for Post-Stroke Neuroplasticity: Multimodal Implications of Pharmacotherapy and Neurorehabilitation

AIMS

Recent studies indicate that anti-inflammatory drugs, act as a double-edged sword, not only exacerbating secondary brain injury but also contributing to neurological recovery after stroke. Our aim is to explore whether there is a beneficial role for neuroprotection and functional recovery using anti-inflammatory drug along with neurorehabilitation therapy using transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS), so as to improve functional recovery after ischemic stroke.

METHODS

We develop a computational systems biology approach from preclinical data, using ordinary differential equations, to study the behavior of both phenotypes of microglia, such as M1 type (pro-inflammatory) vis-à-vis M2 type (anti-inflammatory) under anti-inflammatory drug action (minocycline). We explore whether pharmacological treatment along with cerebral stimulation using tDCS and rTMS is beneficial or not. We utilize the systems pathway analysis of minocycline in nuclear factor kappa beta (NF-κB) signaling and neurorehabilitation therapy using tDCS and rTMS that act through brain-derived neurotrophic factor (BDNF) and tropomyosin-related kinase B (TrkB) signaling pathways.

RESULTS

We demarcate the role of neuroinflammation and immunomodulation in post-stroke recovery, under minocycline activated-microglia and neuroprotection together with improved neurogenesis, synaptogenesis, and functional recovery under the action of rTMS or tDCS. We elucidate the feasibility of utilizing rTMS/tDCS to increase neuroprotection across the reperfusion stage during minocycline administration. We delineate that the signaling pathways of minocycline by modulation of inflammatory genes in NF-κB and proteins activated by tDCS and rTMS through BDNF, TrkB, and calmodulin kinase (CaMK) signaling. Utilizing systems biology approach, we show that the activation pathways for pharmacotherapy (minocycline) and neurorehabilitation (rTMS applied to ipsilesional cortex and tDCS) results into increased neuronal and synaptic activity that commonly occur through activation of N-methyl-d-aspartate receptors. We construe that considerable additive neuroprotection effect would be obtained and delayed reperfusion injury can be remedied, if one uses multimodal intervention of minocycline together with tDCS and rTMS.

CONCLUSION

Additive beneficial effect is, thus, noticed for pharmacotherapy along with neurorehabilitation therapy, by maneuvering the dynamics of immunomodulation using anti-inflammatory drug and cerebral stimulation for augmenting the functional recovery after stroke, which may engender clinical applicability for enhancing plasticity, rehabilitation, and neurorestoration.

Resistance of ribosomal protein mRNA translation to protein synthesis shutoff induced by poliovirus

Poliovirus infection induces an overall inhibition of host protein synthesis, although some mRNAs continue to be translated, suggesting different translation requirements for cellular mRNAs. It is known that ribosomal protein mRNAs are translationally regulated and that the phosphorylation of ribosomal protein S6 is involved in the regulation. Here, we report that the translation of ribosomal protein mRNAs resists poliovirus infection and correlates with an increase in p70(s6k) activity and phosphorylation of ribosomal protein S6.

The principal rapamycin-sensitive p70(s6k) phosphorylation sites, T-229 and T-389, are differentially regulated by rapamycin-insensitive kinase kinases

Mitogen-induced activation of p70(s6k) is associated with the phosphorylation of specific sites which are negatively affected by the immunosuppressant rapamycin, the fungal metabolite wortmannin, and the methylxanthine SQ20006. Recent reports have focused on the role of the amino terminus of the p85(s6k) isoform in mediating kinase activity, with the observation that amino-terminal truncation mutants are activated in the presence of rapamycin while retaining their sensitivity to wortmannin. Here we show that the effects of previously described amino- and carboxy-terminal truncations on kinase activity are ultimately reflected in the phosphorylation state of the enzyme. Mutation of the principal rapamycin-targeted phosphorylation site, T-389, to an acidic residue generates a form of the kinase which is as resistant to wortmannin or SQ20006 as it is to rapamycin, consistent with the previous observation that T-389 was a common target of all three inhibitors. Truncation of the first 54 residues of the amino terminus blocks the serum-induced phosphorylation of three rapamycin-sensitive sites, T-229 in the activation loop and T-389 and S-404 in the linker region. This correlates with a severe reduction in the ability of the kinase to be activated by serum. However, loss of mitogen activation conferred by the removal of the amino terminus is reversed by additional truncation of the carboxy-terminal domain, with the resulting mutant demonstrating phosphorylation of the remaining two rapamycin-sensitive sites, T-229 and T-389. In this double-truncation mutant, phosphorylation of T-229 occurs in the basal state, whereas mitogen stimulation is required to induce acute upregulation of T-389 phosphorylation. The phosphorylation of both sites proceeds unimpaired in the presence of rapamycin, indicating that the kinases responsible for the phosphorylation of these sites are not inhibited by the macrolide. In contrast, activation of the double-truncation mutant is blocked in the presence of wortmannin or SQ20006, and these agents completely block the phosphorylation of T-389 while having only a marginal effect on T-229 phosphorylation. When the T-389 site is mutated to an acidic residue in the double-truncation background, the activation of the resulting mutant is insensitive to the wortmannin and SQ20006 block, but interestingly, the mutant is activated to a significantly greater level than a control in the presence of rapamycin. These data are consistent with the hypothesis that T-389 is the principal regulatory phosphorylation site, which, in combination with hyperphosphorylation of the autoinhibitory domain S/TP sites, is acutely regulated by external effectors, whereas T-229 phosphorylation is regulated primarily by internal mechanisms.

Viral protein kinases and protein phosphatases

Certain large DNA viruses (e.g. herpesviruses and poxviruses) encode proteins related to cellular protein-serine/threonine kinases, and Hepatitis B virus and vesicular stomatitis virus may encode structurally different protein kinases. Other viruses activate cellular protein kinases, e.g. interferon-induced eukaryotic initiation factor-2 kinase, growth factor-induced kinases and protein kinases that regulate mitosis. Protein phosphatases are encoded by vaccinia virus and bacteriophage lambda and must also play a role in viral infection--as do cellular protein phosphatases. The functions of many of these viral enzymes remain to be determined, but they represent possible new targets for anti-viral therapy.

Activation of p70s6k is associated with phosphorylation of four clustered sites displaying Ser/Thr-Pro motifs

Partial amino acid sequences were obtained from 22 internal tryptic peptides of rat liver p70s6k (M(r) 70,000 ribosomal protein S6 kinase), 3 of which were found to contain phosphorylated residues. To determine whether these sites were associated with p70s6k activation, the kinase was labeled to high specific activity with 32P(i) in Swiss mouse 3T3 cells. By sequential cleavage with CNBr and endoproteinase Lys-C followed by two-dimensional tryptic peptide analysis, it could be shown that all of the sites were located in a small endoproteinase Lys-C peptide of M(r) 2400. Analysis of the p70s6k protein sequence revealed a single candidate that could represent this peptide. Three tryptic peptides derived from the endoproteinase Lys-C fragment were chosen by a newly described computer program as the most likely candidates to contain the in vivo sites of phosphorylation. Synthetic peptides based on these sequences were phosphorylated either chemically or enzymatically and found to comigrate by two-dimensional thin-layer electrophoresis/chromatography with the four major in vivo labeled tryptic phosphopeptides. Three of the phosphorylation sites in these peptides were equivalent to those sequenced in the rat liver p70s6k. In addition, all four sites display the motif Ser/Thr-Pro, typical of cell cycle-regulated sites, and are clustered in a putative autoinhibitory domain of the enzyme.

Effects of pseudorabies virus on the neuronal properties of PC12 cells

The effect of pseudorabies virus on neuronal functions was investigated in PC12 cells. During the period investigated, choline acetyltransferase was not affected, while the acetylcholinesterase activity declined steadily starting at 12 h post infection (p.i.), reaching its minimal level of 40% of the control value at 24 h p.i. In contrast, the activity of tyrosine hydroxylase, the key enzyme in catecholamine synthesis, increased to 150% of the control level by 15 h p.i., dropping off slowly with the appearance of viral cytopathology. In parallel, the infection induced, by a process independent of the extracellular Ca2+, an increased release of dopamine at 11 h p.i., followed by noradrenaline at 20 h p.i. In the infected cells, the intracellular content of catecholamine was maintained only in the presence of a high amount of catecholamine precursors in the culture medium. Three plaque-forming units per cell was the minimal multiplicity of infection required to obtain the maximal changes in enzyme activities; higher multiplicities induced more rapidly the maximal effects on tyrosine hydroxylase and acetylcholinesterase. Inhibition of DNA synthesis did not prevent the increase in tyrosine hydroxylase activity; however, protein synthesis was required. In conclusion, infection of the PC12 cells with pseudorabies virus induced significant changes in catecholaminergic and cholinergic metabolism, indicating the ability of this virus to interfere selectively with specialized neuronal functions.

Further definition of the substrate specificity of the alpha-herpesvirus protein kinase and comparison with protein kinases A and C

The pseudorabies virus protein kinase prefers model substrates containing arginyl residues on the amino-terminal side of a target seryl or threonyl residue. We have defined this substrate specificity more precisely in experiments using a new series of synthetic model peptides. When the number of arginyl residues was varied from two to four in substrates of the type RnASVA it was found that peptides with four arginyl residues constituted the best substrates, although the most marked decrease in Km was seen on increasing the number of arginyl residues from two to three. The effect of varying the number of 'spacer' alanyl residues from zero to three was investigated in peptides of the type R4AmSVA, and the peptide with one alanyl residue was found to be the best substrate, making R4X the optimal amino-terminal environment for this enzyme. A similar substrate specificity was observed with the herpes simplex type 1 protein kinase. Protein kinase C was found to have a quite similar substrate preference to the viral enzyme as far as the number and position of the amino-terminal basic residues was concerned; but, unlike the viral protein kinase, it also requires carboxy-terminal basic residues in optimal peptide substrates, and can tolerate the substitution of lysyl for arginyl residues. The cyclic AMP-dependent protein kinase, like the viral enzyme, had favourable kinetic constants for this series of peptides, but differed from the latter in being able to catalyze the phosphorylation of the peptides with two to four arginyl residues with similar efficiency. Studies with the protein, clupeine Y1, as substrate indicated that the pseudorabies virus protein kinase can tolerate arginyl residues on the carboxyl-terminal side of its target residue when there are suitable amino-terminal arginyl determinants. In this respect the virus protein kinase resembled protein kinase C but differed from the cyclic AMP-dependent protein kinase which cannot tolerate such carboxyl-terminal basic residues. The relationship of substrate specificity with model peptides to the ability of the pseudorabies virus protein kinase to phosphorylate proteins in vitro and in vivo is discussed.

Herpes simplex virus type-1-induced stimulation of ribosomal protein S6 phosphorylation is inhibited in neomycin-treated human epidermoid carcinoma 2 cells and in ras-transformed cells

Neomycin, an inhibitor of inositol phospholipid turnover, prevents Herpes-simplex-virus-type-1 (HSV-1)-induced stimulation of ribosomal protein S6 phosphorylation, but does not impair the S6 phosphorylation induced by serum. Long-term treatment with phorbol 12-myristate 13-acetate, which down-regulates protein kinase C activity, does not inhibit virus-induced S6 phosphorylation. In ras-transformed cells, S6 phosphorylation is not stimulated after HSV-1 infection. These results suggest that activation of the inositol phospholipid pathway is involved in the HSV-1-induced stimulation of S6 phosphorylation. However, protein kinase C activation does not appear to be necessary for HSV-1-induced S6 phosphorylation.

The activity of protein kinases from hamster fibroblasts towards a synthetic peptide based on a carboxy-terminal portion of ribosomal protein S6

A synthetic decapeptide, S6(231-240), based on a region near the C-terminus of eukaryotic ribosomal protein S6, was used as a substrate for protein kinases (EC 2.7.1.37) from hamster fibroblasts stimulated with fresh medium. Consistent with the results of others using shorter peptides from this region, it was found that the cyclic AMP-dependent protein kinase preferentially phosphorylated the residue corresponding to Ser-235, whereas protein kinase C preferentially phosphorylated the residue corresponding to Ser-236 in this peptide. The peptide did not serve as a substrate for the growth-associated protein kinase from hamster fibroblasts that phosphorylated ribosomal protein S6 in 40S ribosomal subunits, but did serve as a substrate for a previously undetected protein kinase activity that was resolved from the latter by DEAE-cellulose chromatography. This S6(231-240) protein kinase activity did not phosphorylate ribosomal protein S6 in 40S ribosomal subunits, but is possibly a proteolytic fragment of the 40S ribosomal subunit S6 kinase as the latter activity acquired the ability to phosphorylate the decapeptide after partial tryptic proteolysis. The S6(231-240) protein kinase activity preferentially phosphorylated the residue corresponding to Ser-236 with an apparent Km of 15 microM. These results suggest that specific interactions with the ribosome may be required to activate the growth-associated ribosomal protein S6 kinase.

Ribosome and protein synthesis modifications after infection of human epidermoid carcinoma cells with herpes simplex virus type 1

Modifications of ribosomes have been investigated in human epidermoid carcinoma-2 cells at different stages of herpes simplex virus type 1 infection. Very early in infection, there is an increase in ribosomal protein S6 phosphorylation even in the absence of serum. The same result is obtained in the presence of actinomycin D. At early infection time, ribosomal proteins S2, S3a and Sa are newly phosphorylated. At early and early-late times, three phosphorylated non-ribosomal proteins (v1, v2 and v3) are differently associated temporally to ribosomes. Analyses of proteins extracted from 40S subunits, 80S ribosomes and polysomes show that v1 and v2 are distributed differently among the different ribosomal populations. S6 phosphopeptides were found to be identical after serum stimulation and after viral infection. In every case phosphoserine and phosphothreonine were identified in S6. Only phosphoserine was found in other phosphorylated proteins. Our results indicate that herpes simplex virus type 1 is able to modify pre-existing ribosomes: (i) by stimulating a pre-existing kinase for S6 phosphorylation even in the absence of serum and of viral genome expression; (ii) by inducing new specific kinase activity(ies); and (iii) by association of new, phosphorylated proteins to ribosomes. These ribosomal modifications are correlated with changes in protein synthesis, as shown by two-dimensional electrophoretic analyses of newly synthesized 35S-labelled proteins.