Feedback activation of neurofibromin terminates growth factor-induced Ras activation

Background

Growth factors induce a characteristically short-lived Ras activation in cells emerging from quiescence. Extensive work has shown that transient as opposed to sustained Ras activation is critical for the induction of mitogenic programs. Mitogen-induced accumulation of active Ras-GTP results from increased nucleotide exchange driven by the nucleotide exchange factor Sos. In contrast, the mechanism accounting for signal termination and prompt restoration of basal Ras-GTP levels is unclear, but has been inferred to involve feedback inhibition of Sos. Remarkably, how GTP-hydrolase activating proteins (GAPs) participate in controlling the rise and fall of Ras-GTP levels is unknown.

Results

Monitoring nucleotide exchange of Ras in permeabilized cells we find, unexpectedly, that the decline of growth factor-induced Ras-GTP levels proceeds in the presence of unabated high nucleotide exchange, pointing to GAP activation as a major mechanism of signal termination. Experiments with non-hydrolysable GTP analogues and mathematical modeling confirmed and rationalized the presence of high GAP activity as Ras-GTP levels decline in a background of high nucleotide exchange. Using pharmacological and genetic approaches we document a raised activity of the neurofibromatosis type I tumor suppressor Ras-GAP neurofibromin and an involvement of Rsk1 and Rsk2 in the down-regulation of Ras-GTP levels.

Conclusions

Our findings show that, in addition to feedback inhibition of Sos, feedback stimulation of the RasGAP neurofibromin enforces termination of the Ras signal in the context of growth-factor signaling. These findings ascribe a precise role to neurofibromin in growth factor-dependent control of Ras activity and illustrate how, by engaging Ras-GAP activity, mitogen-challenged cells play safe to ensure a timely termination of the Ras signal irrespectively of the reigning rate of nucleotide exchange.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-016-0128-z) contains supplementary material, which is available to authorized users.

Immunosuppression after sepsis: systemic inflammation and sepsis induce a loss of naïve T-cells but no enduring cell-autonomous defects in T-cell function

Sepsis describes the life-threatening systemic inflammatory response (SIRS) of an organism to an infection and is the leading cause of mortality on intensive care units (ICU) worldwide. An acute episode of sepsis is characterized by the extensive release of cytokines and other mediators resulting in a dysregulated immune response leading to organ damage and/or death. This initial pro-inflammatory burst often transits into a state of immune suppression characterised by loss of immune cells and T-cell dysfunction at later disease stages in sepsis survivors. However, despite these appreciations, the precise nature of the evoked defect in T-cell immunity in post-acute phases of SIRS remains unknown. Here we present an in-depth functional analysis of T-cell function in post-acute SIRS/sepsis. We document that T-cell function is not compromised on a per cell basis in experimental rodent models of infection-free SIRS (LPS or CpG) or septic peritonitis. Transgenic antigen-specific T-cells feature an unaltered cytokine response if challenged in vivo and ex vivo with cognate antigens. Isolated CD4(+)/CD8(+) T-cells from post-acute septic animals do not exhibit defects in T-cell receptor-mediated activation at the the level of receptor-proximal signalling, activation marker upregulation or expansion. However, SIRS/sepsis induced transient lymphopenia and gave rise to an environment of immune attenuation at post acute disease stages. Thus, systemic inflammation has an acute impact on T-cell numbers and adaptive immunity, but does not cause major cell-autonomous enduring functional defects in T-cells.

Maternal protective behavior of zebu type cattle (Bos indicus) and its association with temperament

The objective of the study was to evaluate the maternal protective behavior of zebu-type cattle (Bos indicus) and its association with temperament. A total of 40 cow-calf pairs raised under extensive conditions were randomly assigned to 1 of 4 groups (n = 10), which were evaluated at 30, 60, 90, and 120 d postpartum (dpp), respectively. Measures obtained were defense responses of cows protecting their calves assessed by categorizing the behavioral response of the dams during handling of their calves and chute exit score and facial hair whorl (HW) position as indirect measures associated with temperament. No association was found between protective behavior and exit score or HW measures (rs < 0.22 and 0.13, respectively; P > 0.05). At 30 dpp, 90% of the cows responded to the stimulus of calves being handled, 40% reacted exclusively to alien calves, and 50% responded to their own or alien calves. Sixty days later, the proportion of cows responding to alien calves decreased (P < 0.05) to 10%, and at 120 dpp, cows responding to any calf decreased (P < 0.05) to 20%, while the nonresponding cows increased (P < 0.05) to 60%. Similarly, as dpp increased, the intensity of the reaction of the cows to the manipulation of their calves declined. The intensity of the response was exacerbated (P < 0.05) when a human being was less than 1 m distance from the calf, also when the calf was its own or when the calf vocalized. Furthermore, independent of the sex of their own calf, cows reacted more to male than female calves (P < 0.05). It was concluded that zebu cows may display maternal protective behavior to their own or alien calves, which weakens about 120 dpp and is not influenced by individual temperament.

Graded inhibition of oncogenic Ras-signaling by multivalent Ras-binding domains

BACKGROUND

Ras is a membrane-associated small G-protein that funnels growth and differentiation signals into downstream signal transduction pathways by cycling between an inactive, GDP-bound and an active, GTP-bound state. Aberrant Ras activity as a result of oncogenic mutations causes de novo cell transformation and promotes tumor growth and progression.

RESULTS

Here, we describe a novel strategy to block deregulated Ras activity by means of oligomerized cognate protein modules derived from the Ras-binding domain of c-Raf (RBD), which we named MSOR for multivalent scavengers of oncogenic Ras. The introduction of well-characterized mutations into RBD was used to adjust the affinity and hence the blocking potency of MSOR towards activated Ras. MSOR inhibited several oncogenic Ras-stimulated processes including downstream activation of Erk1/2, induction of matrix-degrading enzymes, cell motility and invasiveness in a graded fashion depending on the oligomerization grade and the nature of the individual RBD-modules. The amenability to accurate experimental regulation was further improved by engineering an inducible MSOR-expression system to render the reversal of oncogenic Ras effects controllable.

CONCLUSION

MSOR represent a new tool for the experimental and possibly therapeutic selective blockade of oncogenic Ras signals.

Regulation of ras exchange factors and cellular localization of ras activation by lipid messengers in T cells

The Ras-MAPK signaling pathway is highly conserved throughout evolution and is activated downstream of a wide range of receptor stimuli. Ras guanine nucleotide exchange factors (RasGEFs) catalyze GTP loading of Ras and play a pivotal role in regulating receptor-ligand induced Ras activity. In T cells, three families of functionally important RasGEFs are expressed: RasGRF, RasGRP, and Son of Sevenless (SOS)-family GEFs. Early on it was recognized that Ras activation is critical for T cell development and that the RasGEFs play an important role herein. More recent work has revealed that nuances in Ras activation appear to significantly impact T cell development and selection. These nuances include distinct biochemical patterns of analog versus digital Ras activation, differences in cellular localization of Ras activation, and intricate interplays between the RasGEFs during distinct T cell developmental stages as revealed by various new mouse models. In many instances, the exact nature of these nuances in Ras activation or how these may result from fine-tuning of the RasGEFs is not understood. One large group of biomolecules critically involved in the control of RasGEFs functions are lipid second messengers. Multiple, yet distinct lipid products are generated following T cell receptor (TCR) stimulation and bind to different domains in the RasGRP and SOS RasGEFs to facilitate the activation of the membrane-anchored Ras GTPases. In this review we highlight how different lipid-based elements are generated by various enzymes downstream of the TCR and other receptors and how these dynamic and interrelated lipid products may fine-tune Ras activation by RasGEFs in developing T cells.

Chemokine unresponsiveness of chronic lymphocytic leukemia cells results from impaired endosomal recycling of Rap1 and is associated with a distinctive type of immunological anergy

Trafficking of malignant lymphocytes is fundamental to the biology of chronic lymphocytic leukemia (CLL). Transendothelial migration (TEM) of normal lymphocytes into lymph nodes requires the chemokine-induced activation of Rap1 and αLβ2 integrin. However, in most cases of CLL, Rap1 is refractory to chemokine stimulation, resulting in failed αLβ2 activation and TEM unless α4β1 is coexpressed. In this study, we show that the inability of CXCL12 to induce Rap1 GTP loading in CLL cells results from failure of Rap1-containing endosomes to translocate to the plasma membrane. Furthermore, failure of chemokine-induced Rap1 translocation/GTP loading was associated with a specific pattern of cellular IgD distribution resembling that observed in normal B cells anergized by DNA-based Ags. Anergic features and chemokine unresponsiveness could be simultaneously reversed by culturing CLL cells ex vivo, suggesting that these two features are coupled and driven by stimuli present in the in vivo microenvironment. Finally, we show that failure of Rap1 translocation/GTP loading is linked to defective activation of phospholipase D1 and its upstream activator Arf1. Taken together, our findings indicate that chemokine unresponsiveness in CLL lymphocytes results from failure of Arf1/phospholipase D1-mediated translocation of Rap1 to the plasma membrane for GTP loading and may be a specific feature of anergy induced by DNA Ags.

Mechanisms and functional consequences of liver failure substantially differ between endotoxaemia and faecal peritonitis in rats

BACKGROUND

Many of the concepts describing molecular mechanisms of sepsis-induced liver failure are derived from endotoxin models. However, the biological significance of such models is questionable as the complexity of clinical sepsis and associated organ failure is only partially replicated.

AIMS

Comparison of cytokine response, leucocyte recruitment, oxidative stress and markers of hepatic organ dysfunction in rat models of endotoxaemia or peritoneal contamination and infection (PCI).

METHODS

Endotoxemia and polymicrobial sepsis were induced in rats by intraperitoneal injection of lipopolysaccharide (LPS) or stool suspension, respectively.

RESULTS

Both insults produced clinical and laboratory signs of multiple organ dysfunction, including hepatic excretory dysfunction. However, TNF alpha, oxidative stress responses and the degree of cell death were significantly higher in endotoxaemia compared to PCI (e.g. serum TNF levels (pg/ml) at 1.5 h post-insult: sham 5 ± 1.4, LPS 1 mg/kg bw 2176.92 ± 373.78, sepsis below detection limit; P P < 0.05). Cholestasis was significantly more pronounced in polymicrobial sepsis whereas serum bilirubin in endotoxaemic animals did not differ from sham-operated controls (plasma levels of bilirubin (μmol/L) at 15 h after the insult: sham 7.1 ± 0.6, LPS 30 mg/kg 9.1 ± 0.6, sepsis 15.2 ± 1.3).

CONCLUSIONS

Polymicrobial sepsis produces profound hepatocellular dysfunction in the absence of traditional cytokine-mediated mechanisms of cellular injury. This questions the central role of cytokines and the ensuing oxidative stress as key molecular events in mediating liver dysfunction.

Features of Ras activation by a mislocalized oncogenic tyrosine kinase: FLT3 ITD signals via K-Ras at the plasma membrane of Acute Myeloid Leukemia cells

FLT3 ITD (FMS-like tyrosine kinase 3 with internal tandem duplication) is an important oncoprotein in Acute Myeloid Leukemia (AML). Owing to its constitutive kinase activity FLT3 ITD accumulates partially at endomembranes, a feature shared with other disease-associated, mutated receptor tyrosine kinases. Since Ras proteins also transit through endomembranes we have investigated the possible existence of an intracellular FLT3 ITD/Ras signaling pathway by comparing Ras signaling of FLT3 ITD with that of wild-type FLT3. Ligand stimulation activated both K- and N-Ras in cells expressing wild-type FLT3. Life-cell Ras-GTP imaging revealed ligand-induced Ras activation at the plasma membrane (PM). FLT3 ITD dependent constitutive activation of K-Ras and N-Ras was also observed primarily at the PM, supporting the view that the PM-resident pool of FLT3 ITD engaged the Ras/Erk pathway in AML cells. Accordingly, specific interference with FLT3 ITD/Ras signaling at the PM using PM-restricted dominant negative K-RasS17N potently inhibited cell proliferation and promoted apoptosis, corroborating that Ras signalling is crucial for FLT3 ITD dependent cell transformation and confirming that FLT3 ITD addresses PM-bound Ras despite its pronounced mislocalization to endomembranes.

Abstract P1-01-29: Intraoperative molecular analysis of sentinel lymph node as a new predictor of axillary status in early breast cancer patients

The one-step nucleic acid amplification (OSNA) assay (Sysmex Corporation, Kobe, Japan) is a new diagnostic device that uses molecular biological techniques to analyze sentinel lymph node (SLN). Intraoperative SLN assessed by OSNA has been validated as an accurate method for detection of SLN metastasis compared to conventional histological examination. Although recent reports have shown that breast cancer patients with &lt;2 positive SLNs can be spared of a complete axillary lymph node dissection (cALND), there are still a number of patients for whom prediction of non SLN metastasis may be helpful for cALND decision making.

The aim of the present study is to assess the intraoperative positive SLN total tumor load (TTL, defined as the amount of CK19 mRNA copies [copies/μL] in all positive SLNs) obtained by OSNA and to determine whether it is predictive of non-SLNs metastasis independently of the number of affected SLN and the type of surgery.

Data were collected during the month of June 2012 from medical records and include age, tumor size and grade, estrogen and progesterone receptor status, HER2 status, Ki67, presence of lymphovascular invasion (LVI), total number of SLN and non-SLN, number of positive and negative non-SLN, size of SLN and non-SLN metastasis, and TTL in each SLN. A total number of 701 patients were recruited, of which 697 (99,4%) met the study selection criteria. Univariate logistic regression showed that, in addition to TTL (p &lt; 0,001), the number of affected SLNs (p &lt; 0,001), tumor size (p &lt; 0,001), HER2 status (p = 0,007), and LVI (p &lt; 0,001) were predictive of ALND status. The multivariate logistic regression analysis showed that TTL is an independent predictor of metastatic non-SLNs, after adjusting for the tumor size, HER2 status, LVI and, in particular, the number of affected SLNs.

Moreover, the ROC curve analysis showed that, as compared to the number of affected SLN, TTL has a better ROC curve, as measured by the AUC: LogTTL 0.709 (95% CI, 0.667–0.760); number of affected SLN 0.610 (95% CI, 0.570–0.652), p &lt; 0.001. Furthermore, in patients possessing a TTL&lt;15000, the frequency of non-SLN metastasis was 14,7% (NPV = 85,3%, PPV = 41,1%, Sensitivity = 76,7%, Specificity = 55,2%). Taking this value as a cutpoint, 85 patients with mastectomy may have spared a cALND considering the predictive results of the TTL. In seven patients with &gt; 3 positive SLN the TTL was &lt; 15000 so this group, even with 3 positive SLNs, have 14.7% of having additional non SLN metastasis.

In conclusion, TTL by OSNA is a newly standardized, automated, and reproducible tool that predicts axillary node status better and independently of the number of affected SLNs and the type of surgery. This value can then help clinicians to personalize surgical treatment. Prospective studies will be carried out to determine the clinical impact of this variable in the management of patients.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P1-01-29.

First Report of the Crucifer Pathogen Pseudomonas cannabina pv. alisalensis Causing Bacterial Blight on Radish (Raphanus sativus) in Germany

In 2008, a bacterial blight was observed on Raphanus sativus in the Pfalz region in Germany. Disease was sporadic but severe when present within R. sativus fields, which resulted in unmarketable crops. Symptoms consisted of small, angular, water-soaked flecks that often were surrounded by chlorotic haloes. Lesions were visible from adaxial and abaxial leaf surfaces and generally retained chlorotic borders. A gram-negative, bluefluorescing bacterium was isolated from surface-disinfested leaf tissue on King's medium B agar. The radish isolate was levan positive, oxidase negative, and arginine dihydrolase negative. The isolate did not rot potato slices but induced a hypersensitive reaction in tobacco. These reactions corresponded to Lelliot's LOPAT group 1 (2). Repetitive extragenic palindromic sequence (rep)-PCR assays using the BOXA1R primer resulted in different DNA fragment banding patterns between the radish isolate and the pathotype strain of Pseudomonas syringae pv. maculicola (CFBP 1657), but identical DNA fragment banding patterns between the radish isolate and the pathotype strain of P. cannabina pv. alisalensis (CFBP 6866). Unlike P. syringae pv. maculicola, P. cannabina pv. alisalensis and the radish isolate were lysed by bacteriophage PBS1 (1). Pathogenicity was evaluated on two hosts, radish (R. sativus cv. Comet) and broccoli raab (Brassica rapa cv. Sorrento). In each of two independent experiments, 3-week-old radish and broccoli raab plants were inoculated with either the radish isolate, P. cannabina pv. alisalensis, or P. syringae pv. maculicola. Inoculum was prepared by growing the bacteria on nutrient agar for 48 h at 27°C, suspending the bacteria in 0.01 M phosphate buffer (pH 7.0), and adjusting each suspension to 0.6 OD at 600 nm (approximately 1 × 10⁸ CFU/ml). All plants were inoculated by spraying until runoff, incubated in a humidity chamber for 48 h, then placed in a greenhouse at 20 to 25°C for symptom development. Plants inoculated with P. cannabina pv. alisalensis or sprayed with buffer served as positive and negative control treatments, respectively. Seven to ten days postinoculation, the development of symptoms similar to those originally observed in the field were observed on plants inoculated with the radish isolate. In addition, symptoms on radish and broccoli raab plants caused by the radish isolate were similar to symptoms caused by P. cannabina pv. alisalensis in contrast to the lack of symptoms on plants inoculated with P. syringae pv. maculicola. Bacteria isolated from symptomatic tissue and surface-disinfested with sodium hypochlorite (0.525%) had identical characteristics to the radish isolate used to inoculate plants and to the P. cannabina pv. alisalensis pathotype for LOPAT reactions, rep-PCR DNA fragment banding pattern analysis, and sensitivity to phage PBS1, thus fulfilling Koch's postulates. To our knowledge, this is the first report of P. cannabina pv. alisalensis isolated from diseased crucifers in Germany. Verification of P. cannabina pv. alisalensis in Germany indicates that German crucifer growers should differentiate between outbreaks caused by P. cannabina pv. alisalensis and P. syringae pv. maculicola and apply appropriate, specific management strategies. References: (1) C. T. Bull et al. Syst. Appl. Microbiol. 33:105, 2010. (2) R. A. Lelliott. J. Appl. Bacteriol. 29:470, 1966.

Activation of Ras requires the ERM-dependent link of actin to the plasma membrane

BACKGROUND

Receptor tyrosine kinases (RTKs) participate in a multitude of signaling pathways, some of them via the small G-protein Ras. An important component in the activation of Ras is Son of sevenless (SOS), which catalyzes the nucleotide exchange on Ras.

PRINCIPAL FINDINGS

We can now demonstrate that the activation of Ras requires, in addition, the essential participation of ezrin, radixin and/or moesin (ERM), a family of actin-binding proteins, and of actin. Disrupting either the interaction of the ERM proteins with co-receptors, down-regulation of ERM proteins by siRNA, expression of dominant-negative mutants of the ERM proteins or disruption of F-actin, abolishes growth factor-induced Ras activation. Ezrin/actin catalyzes the formation of a multiprotein complex consisting of RTK, co-receptor, Grb2, SOS and Ras. We also identify binding sites for both Ras and SOS on ezrin; mutations of these binding sites destroy the interactions and inhibit Ras activation. Finally, we show that the formation of the ezrin-dependent complex is necessary to enhance the catalytic activity of SOS and thereby Ras activation.

CONCLUSIONS

Taking these findings together, we propose that the ERM proteins are novel scaffolds at the level of SOS activity control, which is relevant for both normal Ras function and dysfunction known to occur in several human cancers.

SAP-mediated inhibition of diacylglycerol kinase α regulates TCR-induced diacylglycerol signaling

Diacylglycerol kinases (DGKs) metabolize diacylglycerol to phosphatidic acid. In T lymphocytes, DGKα acts as a negative regulator of TCR signaling by decreasing diacylglycerol levels and inducing anergy. In this study, we show that upon costimulation of the TCR with CD28 or signaling lymphocyte activation molecule (SLAM), DGKα, but not DGKζ, exits from the nucleus and undergoes rapid negative regulation of its enzymatic activity. Inhibition of DGKα is dependent on the expression of SAP, an adaptor protein mutated in X-linked lymphoproliferative disease, which is essential for SLAM-mediated signaling and contributes to TCR/CD28-induced signaling and T cell activation. Accordingly, overexpression of SAP is sufficient to inhibit DGKα, whereas SAP mutants unable to bind either phospho-tyrosine residues or SH3 domain are ineffective. Moreover, phospholipase C activity and calcium, but not Src-family tyrosine kinases, are also required for negative regulation of DGKα. Finally, inhibition of DGKα in SAP-deficient cells partially rescues defective TCR/CD28 signaling, including Ras and ERK1/2 activation, protein kinase C membrane recruitment, induction of NF-AT transcriptional activity, and IL-2 production. Thus SAP-mediated inhibition of DGKα sustains diacylglycerol signaling, thereby regulating T cell activation, and it may represent a novel pharmacological strategy for X-linked lymphoproliferative disease treatment.

Hormetic signaling patterns

Organisms are perpetually facing noxious insults but exhibit surprising diverse reaction patterns. Depending on the strength, frequency and quality of the stress stimuli biological systems may react with increased vitality, future stress resistance or with injury and degeneration. Whereas a multitude of such specific stress responses has been observed in diverse biological systems the underlying molecular mechanisms are mainly unknown. These knowledge restrictions urge the exploration of specific molecular signaling reactions controlling the ambivalent responses of cells and organisms to noxious effects. The adaptive responses of signaling networks to defined stress stimuli need to be investigated in a time-and dose-resolved manner in cellular and organismic models. Anticipated results are expected to significantly advance the understanding of the molecular signatures of stress responses and may also promote ongoing efforts for the effective use of the organism's preventive and regenerative potentials in modern medicine.