A Sos proteomimetic as a pan-Ras inhibitor

Aberrant Ras signaling is linked to a wide spectrum of hyperproliferative diseases, and components of the signaling pathway, including Ras, have been the subject of intense and ongoing drug discovery efforts. The cellular activity of Ras is modulated by its association with the guanine nucleotide exchange factor Son of sevenless (Sos), and the high-resolution crystal structure of the Ras–Sos complex provides a basis for the rational design of orthosteric Ras ligands. We constructed a synthetic Sos protein mimic that engages the wild-type and oncogenic forms of nucleotide-bound Ras and modulates downstream kinase signaling. The Sos mimic was designed to capture the conformation of the Sos helix–loop–helix motif that makes critical contacts with Ras in its switch region. Chemoproteomic studies illustrate that the proteomimetic engages Ras and other cellular GTPases. The synthetic proteomimetic resists proteolytic degradation and enters cells through macropinocytosis. As such, it is selectively toxic to cancer cells with up-regulated macropinocytosis, including those that feature oncogenic Ras mutations.

The Ras-specific guanine nucleotide exchange factor Son of sevenless (Sos) mediates the conversion of Ras from its inactive GDP-bound form to the active GTP-bound state (1). Sos catalyzes nucleotide exchange via insertion of a critical helical segment (αH) between the conformationally dynamic Switch I and II regions that flank the Ras nucleotide binding pocket leading to disruption of water-mediated and direct interactions between the protein and the cofactor (Fig. 1A) (2). Given the biomedical importance of the conformationally dynamic Sos-binding interface of Ras, several rational design and screening strategies have been attempted to develop ligands for this interface (3, 4). Recent efforts to engage the Ras G12C (5, 6) and the G12D isoforms (7, 8) suggest that targeted screens may afford small molecules and peptide macrocycles as potential leads. The structure of the Ras–Sos complex provides a basis for the rational design of Sos helix mimics that engage the Ras switch regions. Our group has previously developed a conformationally stabilized α-helix mimic to target the Ras–Sos protein–protein interaction (PPI) (9, 10). The stabilized α-helix was shown to bind Ras at the orthosteric binding site and inhibit Sos-mediated nucleotide exchange, Ras activation, and phosphorylation of the downstream effector protein ERK (11), a well-characterized kinase implicated in cell proliferation and differentiation. However, this compound preferred to bind Ras in its nucleotide-free form, suggesting that a single Sos helix is likely insufficient to properly engage the dynamic Ras interface in its nucleotide-bound form. While wild-type Ras toggles between its two nucleotide-bound forms (Fig. 1A), the oncogenic forms of Ras remain activated in their GTP-bound states (3). Therefore, a compound that preferentially engages the nucleotide-free form of Ras may have limited biological utility.Open in a separate windowFig. 1.Overview of the Ras activation cycle and design of a Sos-based proteomimetic. (A) The cellular activity of Ras is tightly controlled as part of a balanced feedback loop. Oncogenic mutations shift this balance and increase the cellular concentration of Ras-GTP leading to aberrant downstream signaling. The molecular model shows the complex between Ras (gray ribbon) and its guanine exchange factor Sos (green). Sos inserts a helical hairpin (pink and blue helices) into the nucleotide binding pocket of Ras to mediate nucleotide exchange. The Ras nucleotide binding pocket is highlighted in yellow. Segments of Sos are not shown to highlight interactions of the helical hairpin with Ras (PDB code: 1NVW). (B) The molecular models depict critical Sos helices and the design of a constrained Sos proteomimetic as a Ras inhibitor. GDP, guanosine 5′-diphosphate; GTP, guanosine 5′-triphosphate; GAP, GTPase activating protein; GEF, guanine nucleotide exchange factor.Our prior results with the Sos helix mimic encapsulate a critical challenge in developing minimal protein secondary-structure mimics. Although mimics of protein secondary structures have proven to be a potent class of PPI inhibitors (12–15), many protein interfaces feature binding epitope complexity beyond what can be captured by reproduction of minimal elements of protein structure (16–19). We hypothesized that the introduction of additional contact residues from Sos may allow engagement of nucleotide-bound Ras (Fig. 1B). Sos inserts αH into the switch region of Ras, but analysis of the complex shows that several other residues from Sos also interact with Ras (SI Appendix, Fig. S1). The conformation of the αH helix, itself, is controlled by the αI domain as part of a hairpin helix organization. The αI helix makes important electrostatic contacts with the Ras effector loop in the Switch I region.We sought to develop a tertiary-structure mimic of Sos that encompasses critical binding residues from the helix–loop–helix motif to determine if the additional contacts allow engagement of nucleotide-bound Ras (Fig. 1B). We utilized a recently described synthetic approach from our group to capture the conformation of the Sos αH and αI helices. In prior efforts, we learned that helix dimers may be stabilized by judicious substitution of a surface salt bridge with a covalent bond and appropriate sculpting of the dimeric interface to coerce knob-into-hole helix packing (20, 21). These stabilized proteomimetics are termed crosslinked helix dimers or CHDs. Here, we show that Sos CHDs are proteolytically stable, selectively cell permeable, and engage the Sos-binding surface of nucleotide-bound Ras with high specificity in biochemical and cellular contexts. We utilized a combination of rational design principles and computational modeling to exploit previously unexplored and underutilized pockets at the target interface (22, 23). The optimized proteomimetic binds wild-type and mutant Ras forms with nanomolar to low micromolar affinities, modulates nucleotide exchange, engages Ras and other Ras subfamily GTPases as demonstrated by chemoproteomic assays, inhibits downstream activation of the Ras-mediated signaling cascade, and is selectively toxic to cancer cells with oncogenic Ras mutations.     

Is gastric cancer a new indication for surveillance colonoscopy? Colon cancer is increased in gastric cancer patients]

BACKGROUND/AIMS: It has been reported that the risk of gastric polyp is increased in various colonic polyposis syndromes or in series of patients with sporadic colonic polyps. However, there are only a few large case controlled studies of colon cancer incidence in gastric cancer patients who underwent colonoscopy. The aims of this study were to determine the incidence of colorectal neoplasm and to evaluate the necessity of colonoscopic surveillance in patients with gastric cancer. METHODS: We performed colonoscopy in 105 patients with gastric cancer who agreed to undergo colonoscopy before or after 6 months from gastric resection between January 2002 and December 2004 in Kangbuk Samsung hospital. As a control group, 269 consecutive, age and sex matched patients without gastric neoplasm on gastroscopy who underwent colonoscopy within 6 months for the evaluation of various gastrointestinal symptoms during the year 2004 were included. Endoscopic reports and pathological results were reviewed retrospectively. RESULTS: In the patient group, adenomatous polyps were diagnosed in 24/105 patients (22.9%) and colorectal adenocarcinoma in 10/105 patients (9.5%). In the control group, adenomatous polyps were diagnosed in 78/269 patients (29.0%) and colorectal adenocarcinoma in 2/269 patients (0.7%). The incidence of colorectal adenocarcinoma between the patient group and control group showed significant differences (odds ratio 11.04, p=0.003). CONCLUSIONS: The risk of colorectal adenocarcinoma increases significantly in patients with gastric cancer. We suggest that the patients with gastric cancer might carry a high risk for colorectal cancer whom require surveillance colonoscopy.     

The effect of age and calorie restriction on HIF-1-responsive genes in aged liver

Hypoxia inducible factor-1 (HIF-1) regulates transactivation of several genes in response to hypoxia condition. We explore hepatic HIF-1 responsive gene regulation during aging and the age-related changes of the HIF-1 related gene activation in young and old rats. Results indicate that the aging process induces the activation of HIF-1, which is accompanied by increased HIF-1 DNA binding. This increased binding activity is accompanied by the increase of HIF-1-dependent genes, heme oxygenase-1 (HO-1), vascular endothelial growth factor (VEGF), erythropoietin (EPO), and inducible nitric oxide synthase (iNOS), which all showed remarkable up-regulation during aging process. In contrast, the increased HIF-1 related gene expression was effectively blunted by the anti-oxidative action of calorie restriction in aged rat liver. We propose that age-related HIF-1 binding activity may well be influenced by the increased pro-oxidative conditions of aged animals, which up-regulate HIF-1-dependent gene expression.Min Ju Kang and Hyon Jeen Kim Contributed equally to this work     

Response to empirical anti-tuberculosis treatment in patients with sputum smear-negative presumptive pulmonary tuberculosis

BACKGROUND: In many cases, physicians initiate anti-tuberculosis (TB) treatment based only on symptoms or radiographic findings without confirmation of pulmonary TB by acid-fast bacilli (AFB) smear. It has not been well known which clinical characteristics could be used as predictors for positive culture or real TB in patients with sputum smear-negative presumptive pulmonary TB. OBJECTIVE: We tried to elucidate treatment outcomes in patients with sputum smear-negative presumptive pulmonary TB and to find predictors of positive culture results. METHODS: We reviewed data of the patients who had been treated as presumptive TB with negative AFB smear on the basis of clinical and radiographic features from December 1998 to December 2000 at a university hospital in Korea. We reviewed medical records and radiographs of patients and analyzed possible predictors for positive culture. RESULTS: One hundred and one patients were enrolled. Among them, pulmonary TB was confirmed by culture in 32 patients (31%). Thirty-one (96.9%) out of 32 culture-positive patients showed clinical or radiographic improvement as did 50 (72.5%) out of 69 culture-negative patients. The predictor for a positive culture result is the presence of patchy consolidation in an initial radiograph (p = 0.025; OR 2.89; 95% CI 1.14-7.28). CONCLUSIONS: The empirical anti-TB treatment in patients with sputum smear-negative presumptive pulmonary TB was effective and adequate, especially presented with patchy consolidation in initial chest radiographs in Korea.     

Efficacy of Magnesium Trihydrate of Ursodeoxycholic Acid and Chenodeoxycholic Acid for Gallstone Dissolution: A Prospective Multicenter Trial

Background/Aims

Cholecystectomy is necessary for the treatment of symptomatic or complicated gallbladder (GB) stones, but oral litholysis with bile acids is an attractive alternative therapeutic option for asymptomatic or mildly symptomatic patients. This study was conducted to evaluate the efficacy of magnesium trihydrate of ursodeoxycholic acid (UDCA) and chenodeoxycholic acid (CDCA) on gallstone dissolution and to investigate improvements in gallstone-related symptoms.

Methods

A prospective, multicenter, phase 4 clinical study to determine the efficacy of orally administered magnesium trihydrate of UDCA and CDCA was performed from January 2011 to June 2013. The inclusion criteria were GB stone diameter ≤15 mm, GB ejection fraction ≥50%, radiolucency on plain X-ray, and asymptomatic/mildly symptomatic patients. The patients were prescribed one capsule of magnesium trihydrate of UDCA and CDCA at breakfast and two capsules at bedtime for 6 months. The dissolution rate, response rate, and change in symptom score were evaluated.

Results

A total of 237 subjects were enrolled, and 195 subjects completed the treatment. The dissolution rate was 45.1% and the response rate was 47.2% (92/195) after 6 months of administration of magnesium trihydrate of UDCA and CDCA. Only the stone diameter was significantly associated with the response rate. Both the symptom score and the number of patients with symptoms significantly decreased regardless of stone dissolution. Adverse events necessitating discontinuation of the drug, surgery, or endoscopic management occurred in 2.5% (6/237) of patients.

Conclusions

Magnesium trihydrate of UDCA and CDCA is a well-tolerated bile acid that showed similar efficacy for gallstone dissolution and improvement of gallstone-related symptoms as that shown in previous studies.     

The efficacy and safety of prone positioning in adults patients with acute respiratory distress syndrome: a meta-analysis of randomized controlled trials

Background

Prone positioning for acute respiratory distress syndrome (ARDS) has no impact on mortality despite significant improvements in oxygenation. However, a recent trial demonstrated reduced mortality rates in the prone position for severe ARDS. We evaluated effects of prone position duration and protective lung strategies on mortality rates in ARDS.

Methods

We extensively searched MEDLINE, EMBASE, and the Cochrane Central Register of Controlled Trials to identify randomized controlled trials (RCTs) reporting on prone positioning during acute respiratory failure in adults for inclusion in our meta-analysis.

Results

Eight trials met our inclusion criteria, Totals of 1,099 and 1,042 patients were randomized to the prone and supine ventilation positions. The mortality rates associated with the prone and supine positions were 41% and 47% [risk ratio (RR), 0.90; 95% confidence interval (CI), 0.82-0.98, P=0.02], but the heterogeneity was moderate (P=0.01, I²=61%). In a subgroup analysis, the mortality rates for lung protective ventilation (RR 0.73, 95% CI, 0.62-0.86, P=0.0002) and duration of prone positioning >12 h (RR 0.75, 95% CI, 0.65-0.87, P<0.0001) were reduced in the prone position. Prone positioning was not associated with an increased incidence of cardiac events (RR 1.01, 95% CI, 0.87-1.17) or ventilator associated pneumonia (RR 0.88, 95% CI, 0.71-1.09), but it was associated with an increased incidence of pressure sores (RR 1.23, 95% CI, 1.07-1.41) and endotracheal dislocation (RR 1.33, 95% CI, 1.02-1.74).

Conclusions

Prone positioning tends to reduce the mortality rates in ARDS patients, especially when used in conjunction with a lung protective strategy and longer prone position durations. Prone positioning for ARDS patients should be prioritized over other invasive procedures because related life-threatening complications are rare. However, further additional randomized controlled design to study are required for confirm benefit of prone position in ARDS.