Temporal control of NF-kappaB activation by ERK differentially regulates interleukin-1beta-induced gene expression

In cultured rat vascular smooth muscle cells, sustained activation of ERK is required for interleukin-1beta to persistently activate NF-kappaB. Without ERK activation, interleukin-1beta induces only acute and transient NF-kappaB activation. The present study examined whether the temporal control of NF-kappaB activation by ERK could differentially regulate the expression of NF-kappaB-dependent genes, including inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), vascular cell adhesion molecule-1 (VCAM-1), and manganese-containing superoxide dismutase (Mn-SOD). Treatment of vascular smooth muscle cells with interleukin-1beta induced the expression of iNOS, COX-2, VCAM-1, and Mn-SOD in a time-dependent manner, but with different patterns. Either PD98059 or U0126, selective inhibitors of MEK, or overexpression of a dominant negative MEK-1 inhibited interleukin-1beta- induced ERK activation and the expression of iNOS and COX-2 but had essentially no effect on the expression of VCAM-1 and Mn-SOD. The expression of these genes was inhibited when NF-kappaB activation was down-regulated by MG132, a proteasome inhibitor, or by overexpression of an I-kappaBalpha mutant that prevented both the transient and the persistent activation of NF-kappaB. Inhibition of ERK did not affect interleukin-1beta-induced I-kappaBalpha phosphorylation and degradation but attenuated I-kappaBbeta degradation. Thus, although NF-kappaB activation was essential for interleukin-1beta induction of each of the proteins studied, gene expression was differentially regulated by ERK and by the duration of NF-kappaB activation. These results reveal a novel functional role for ERK as an important temporal regulator of NF-kappaB activation and NF-kappaB-dependent gene expression.     

Methotrexate suppresses NF-kappaB activation through inhibition of IkappaBalpha phosphorylation and degradation

Methotrexate (MTX), a folate antagonist, is a commonly used anti-inflammatory, antiproliferative, and immunosuppressive drug whose mode of action is not fully established. Due to the central role of NF-kappaB in these responses, we postulated that MTX must mediate its effects through suppression of NF-kappaB activation. We investigated the effects of MTX on NF-kappaB activation induced by TNF in Jurkat cells. The treatment of these cells with MTX suppressed TNF-induced NF-kappaB activation with optimum effects occurring at 10 microM MTX for 60 min. These effects were not restricted to Jurkat cells because other cell types were also inhibited. Besides TNF, MTX also suppressed the NF-kappaB activation induced by various other inflammatory stimuli. The suppression of TNF-induced NF-kappaB activation by MTX correlated with inhibition of IkappaBalpha degradation, suppression of IkappaBalpha phosphorylation, abrogation of IkappaBalpha kinase activation, and inhibition of NF-kappaB-dependent reporter gene expression. Because ecto 5' nucleotidase inhibitor (alpha,beta-methylene adenosine-5'-diphosphate) blocked the effect of MTX, adenosine mimicked the effect of MTX, and adenosine A2b receptor antagonist (3,7-dimethyl-1-propargylxanthine) reversed the inhibitory effect of MTX, we suggest that MTX suppresses NF-kappaB activation by releasing adenosine. A partial reversal of MTX-induced NF-kappaB suppression by thymidine and folinic acid indicates the role of the thymidylate synthase pathway also. Overall, our results clearly demonstrate that MTX suppresses NF-kappaB activation through the release of adenosine, which may contribute to the role of MTX in anti-inflammatory, immunomodulatory, and antiproliferative effects.     

SHP-2 modulates interleukin-1-induced Ca2+ flux and ERK activation via phosphorylation of phospholipase Cgamma1

Interleukin-1 (IL-1) signaling is dependent on focal adhesions, structures that are enriched with tyrosine kinases and phosphatases. Because the non-receptor tyrosine phosphatase Src homology 2 domain-containing protein tyrosine phosphatase-2 (SHP-2) is enriched in focal adhesions and IL-1-induced ERK activation requires increased Ca(2+), we determined whether SHP-2 modulates IL-1-induced Ca(2+) signaling. In SHP-2-deficient fibroblasts, IL-1-induced Ca(2+) signaling and ERK activation were markedly diminished compared with cells expressing SHP-2. IL-1-induced Ca(2+) release from the endoplasmic reticulum occurred in the vicinity of focal adhesions and was strongly inhibited by the blockage of phospholipase C (PLC) catalytic activity. Immunoprecipitation and immunostaining showed that SHP-2, the endoplasmic reticulum-specific protein calnexin, and PLCgamma1 were associated with focal adhesions; however, these associations and IL-1-induced ERK activation dissipated after cells were plated on non-integrin substrates. IL-1 promoted phosphorylation of SHP-2 and PLCgamma1. IL-1-induced phosphorylation of PLCgamma1 was diminished in SHP-2-deficient cells but was restored by stable transfection with SHP-2. BAPTA/AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester)) blocked IL-1-induced phosphorylation of SHP-2 and PLCgamma1, indicating mutually dependent interactive roles for Ca(2+), SHP-2, and PLCgamma1 in IL-1 signaling. We conclude that SHP-2 is critical for IL-1-induced phosphorylation of PLCgamma1 and thereby enhances IL-1-induced Ca(2+) release and ERK activation. Focal adhesions co-localizing with the endoplasmic reticulum may provide molecular staging sites required for ERK activation.     

Control sites of ribosomal S6 kinase B and persistent activation through tumor necrosis factor

RSKB, a 90-kDa ribosomal S6 protein kinase family (RSK) member with two complete catalytic domains connected by a linker, is activated through p38- and ERK-mitogen-activated protein kinases. The N-terminal kinases of RSKs phosphorylate substrates; activation requires phosphorylation of linker and C-terminal kinase sites. Unlike other RSKs, the activation loop phosphorylation sites of both catalytic domains of RSKB, Ser(196) and Thr(568), were required for activity. RSKB activation depended on phosphorylation of linker Ser(343) and Ser(360) and associated with phosphorylation of nonconserved Ser(347), but Ser(347)-deficient RSKB retained partial activity. The known protein kinase A and protein kinase C inhibitors, H89 and Ro31-8220, blocked RSKB activity. Treatment of HeLa cells with tumor necrosis factor, epidermal growth factor, phorbol 12-myristate 13-acetate, and ionomycin but not with insulin resulted in strong activation of endogenous RSKB. High RSKB activity and Ser(347)/Ser(360) phosphorylation persisted for 3 h in tumor necrosis factor-treated cells, in contrast to the short bursts of p38, ERK, and RSK1-3 activities. In conclusion, a variety of stimuli induced phosphorylation and activation of RSKB through both p38 and ERK pathways; the persistence of activation indicated that RSKB selectively escaped cell mechanisms causing rapid deactivation of upstream p38 and ERK and other RSKs.     

Ribosomal protein S6 phosphorylation: from protein synthesis to cell size

Recent studies are beginning to disclose a signaling network involved in regulating cell size. Although many links and effectors are still unknown, central components of this network include the mammalian target of rapamycin (mTOR) and its downstream effectors - the ribosomal protein S6 kinase (S6K) and the translational repressor eukaryotic initiation factor 4E-binding protein. Until recently, the role of S6K and its many substrates in cell-size control remained obscure; however, a knockin mouse carrying mutations at all phosphorylation sites in the primary S6K substrate, ribosomal protein S6 (rpS6), has provided insight into the physiological role of this protein phosphorylation event. In addition to its role in glucose homeostasis in the whole mouse, phosphorylation of rpS6 is essential for regulating the size of at least some cell types, but is dispensable for translational control of mRNAs with a 5' terminal oligopyrimidine tract (TOP mRNAs) - its previously assigned targets. It therefore seems that establishing the function of the phosphorylation of other effectors of mTOR or S6K will inevitably require genetic manipulation of the respective sites within these targets.     

Hydrophobic motif phosphorylation is not required for activation loop phosphorylation of p70 ribosomal protein S6 kinase 1 (S6K1)

p70 ribosomal protein S6 kinase 1 (S6K1) is regulated by multiple phosphorylation events. Three of these sites are highly conserved among AGC kinases (cAMP dependent Protein Kinase, cGMP dependent Protein Kinase, and Protein Kinase C subfamily): the activation loop in the kinase domain, and two C-terminal sites, the turn motif and the hydrophobic motif. The common dogma has been that phosphorylation of the hydrophobic motif primes S6K1 for the phosphorylation at the activation loop by phosphoinositide-dependent protein kinase 1 (PDK1). Here, we show that the turn motif is, in fact, phosphorylated first, the activation loop second, and the hydrophobic motif is third. Specifically, biochemical analyses of a construct of S6K1 lacking the C-terminal autoinhibitory domain as well as full-length S6K1, reveals that S6K1 is constitutively phosphorylated at the turn motif when expressed in insect cells and becomes phosphorylated in vitro by purified PDK1 at the activation loop. Only the species phosphorylated at the activation loop by PDK1 gets phosphorylated at the hydrophobic motif by mammalian target of rapamycin (mTOR) in vitro. These data are consistent with a previous model in which constitutive phosphorylation of the turn motif provides the key priming step in the phosphorylation of S6K1. The data provide evidence for regulation of S6K1, where hydrophobic motif phosphorylation is not required for PDK1 to phosphorylate S6K1 at the activation loop, but instead activation loop phosphorylation of S6K1 is required for mTOR to phosphorylate the hydrophobic motif of S6K1.     

Mechanisms of persistent NF-kappaB activation by HTLV-I tax

Human T cell leukemia virus type I (HTLV-I) is the causative agent of a fatal malignancy known as adult T cell leukemia (ATL). The HTLV-I Tax protein is thought to play a significant role in the initiation and pathogenesis of HTLV-I-mediated disease. Tax is a potent oncogene that deregulates cellular gene expression by persistently activating signaling pathways such as NF-kappaB. Tax activation of NF-kappaB is critical for the immortalization and survival of HTLV-I-infected T cells. In this review, we describe recent insights into the mechanisms employed by Tax to activate the canonical and noncanonical NF-kappaB signaling pathways. The adaptor function of Tax appears to be a common and important mechanism for the pathological activation of both NF-kappaB pathways.     

Ceramide inhibits insulin-stimulated Akt phosphorylation through activation of Rheb/mTORC1/S6K signaling in skeletal muscle

Ceramide is a negative regulator of insulin activity. At the molecular level, it causes a decrease in insulin-stimulated Akt Ser473 phosphorylation in C2C12 myotubes. Interestingly, we found that the phosphorylation of S6K at Thr389 was increased under the same conditions. Utilizing both rapamycin to inhibit mTORC1 activity and shRNA to knock down Rheb, we demonstrated that the decrease in Akt Ser473 phosphorylation stimulated by insulin after C2-ceramide incubation can be prevented. The mechanism by which C2-ceramide impairs signaling would seem to involve a negative feedback of activated S6K via phosphorylation of insulin receptor substrate-1 at Ser636/639, since S6K inhibitor can block this phenomenon. Finally, rapamycin treatment was found not to affect C2-ceramide-induced PKCζ activation, suggesting that the pathway revealed in this study is parallel to the one involving PKCζ activation. We proposed a novel pathway/mechanism involving Rheb/mTORC1/S6K signaling to explain how C2-ceramide impairs insulin signaling via Akt phosphorylation. The existence of multiple pathways involved in insulin signaling impairment by C2-ceramide treatment implies that different strategies might be needed to ameliorate insulin resistance caused by C2-ceramide.     

The activation and inhibition of cyclin-dependent kinase-5 by phosphorylation

Despite the very similar 3-dimensional structures as reflected by the more than 60% identity in amino acid sequences, CDK2 and CDK5 have very different functions and characteristics. Phosphorylation on a conserved Thr14 can inhibit activities of both the kinases, but phosphorylating another conserved Tyr15, however, can lead to totally opposite inhibition and stimulation consequences in CDK2 and CDK5. Our molecular dynamics (MD) simulations suggest a similar inhibition mechanism of phosphorylation on the Thr14 as in the CDK2 system. In both the systems, the kinase activities are inhibited by the phosphorylation because it causes ATP phosphate moiety misalignment and changes in the Mg2+ ion coordination sphere, which have been proven to be critical for the phosphate group of the ATP transferring to the hydroxyl group on the serine in the substrate peptide. The calculations indicate that ATP adopts a more favorable conformation and location in the phosphorylated Tyr15 complex to facilitate the interactions with the substrate and the Mg2+ is wrapped more strongly by the phosphate group than in the unphosphorylated system, which might be favored by the transfer reaction.     

Signal pathways involved in activation of p70S6K and phosphorylation of 4E-BP1 following exposure of multiple myeloma tumor cells to interleukin-6

Interleukin-6 (IL-6) is a prominent tumor growth factor for malignant multiple myeloma cells. In addition to its known activation of the Janus tyrosine kinase-STAT and RAS-MEK-ERK pathways, recent work suggests that IL-6 can also activate the phosphatidylinositol 3-kinase (PI3-K)/AKT kinase pathway in myeloma cells. Because activation of the PI3-K/AKT as well as RAS-MEK-ERK pathways may result in downstream stimulation of the p70(S6K) (p70) and phosphorylation of the 4E-BP1 translational repressor, we assessed these potential molecular targets in IL-6-treated myeloma cells. IL-6 rapidly activated p70 kinase activity and p70 phosphorylation. Activation was inhibited by wortmannin, rapamycin, and the ERK inhibitors PD98059 and UO126, as well as by a dominant negative mutant of AKT. The concurrent requirements for both ERK and PI3-K/AKT appeared to be a result of their ability to phosphorylate p70 on different residues. In contrast, IL-6-induced phosphorylation of 4E-BP1 was inhibited by rapamycin, wortmannin, and dominant negative AKT but ERK inhibitors had no effect, indicating ERK function was dispensable. In keeping with these data, a dominant active AKT mutant was sufficient to induce 4E-BP1 phosphorylation but could not by itself activate p70 kinase activity. Prevention of IL-6-induced p70 activation and 4E-BP1 phosphorylation by the mammalian target of rapamycin inhibitors rapamycin and CCI-779 resulted in inhibition of IL-6-induced myeloma cell growth. These results indicate that both ERK and PI3-K/AKT pathways are required for optimal IL-6-induced p70 activity, but PI3-K/AKT is sufficient for 4E-BP1 phosphorylation. Both effects are mediated via mammalian target of rapamycin function, and, furthermore, these effects are critical for IL-6-induced tumor cell growth.