Postnatal liver growth and regeneration are independent of c-myc in a mouse model of conditional hepatic c-myc deletion

<p>Abstract</p> <p>Background</p> <p>The transcription factor <it>c-myc </it>regulates genes involved in hepatocyte growth, proliferation, metabolism, and differentiation. It has also been assigned roles in liver development and regeneration. In previous studies, we made the unexpected observation that c-Myc protein levels were similar in proliferating fetal liver and quiescent adult liver with c-Myc displaying nucleolar localization in the latter. In order to investigate the functional role of c-Myc in adult liver, we have developed a hepatocyte-specific <it>c-myc </it>knockout mouse, <it>c-myc^fl/fl</it>;<it>Alb</it>-<it>Cre</it>.</p> <p>Results</p> <p>Liver weight to body weight ratios were similar in control and <it>c-myc </it>deficient mice. Liver architecture was unaffected. Conditional <it>c-myc </it>deletion did not result in compensatory induction of other <it>myc </it>family members or in c-Myc's binding partner Max. Floxed <it>c-myc </it>did have a negative effect on <it>Alb</it>-Cre expression at 4 weeks of age. To explore this relationship further, we used the Rosa26 reporter line to assay Cre activity in the <it>c-myc </it>floxed mice. No significant difference in Alb-Cre activity was found between control and <it>c-myc^fl/fl</it>mice. c<it>-myc </it>deficient mice were studied in a nonproliferative model of liver growth, fasting for 48 hr followed by a 24 hr refeeding period. Fasting resulted in a decrease in liver mass and liver protein, both of which recovered upon 24 h of refeeding in the c<it>-myc^fl/fl;Alb</it>-Cre animals. There was also no effect of reducing <it>c-myc </it>on recovery of liver mass following 2/3 partial hepatectomy.</p> <p>Conclusions</p> <p>c-Myc appears to be dispensable for normal liver growth during the postnatal period, restoration of liver mass following partial hepatectomy and recovery from fasting.</p

Regulation of Gene Expression in Hepatic Cells by the Mammalian Target of Rapamycin (mTOR)

We investigated mTOR regulation of gene expression by studying rapamycin effect in two hepatic cell lines, the non-tumorigenic WB-F344 cells and the tumorigenic WB311 cells. The latter are resistant to the growth inhibitory effects of rapamycin, thus providing us with an opportunity to study the gene expression effects of rapamycin without confounding effects on cell proliferation.The hepatic cells were exposed to rapamycin for 24 hr. Microarray analysis on total RNA preparations identified genes that were affected by rapamycin in both cell lines and, therefore, modulated independent of growth arrest. Further studies showed that the promoter regions of these genes included E-box-containing transcription factor binding sites at higher than expected rates. Based on this, we tested the hypothesis that c-Myc is involved in regulation of gene expression by mTOR by comparing genes altered by rapamycin in the hepatic cells and by c-Myc induction in fibroblasts engineered to express c-myc in an inducible manner. Results showed enrichment for c-Myc targets among rapamycin sensitive genes in both hepatic cell lines. However, microarray analyses on wild type and c-myc null fibroblasts showed similar rapamycin effect, with the set of rapamycin-sensitive genes being enriched for c-Myc targets in both cases.There is considerable overlap in the regulation of gene expression by mTOR and c-Myc. However, regulation of gene expression through mTOR is c-Myc-independent and cannot be attributed to the involvement of specific transcription factors regulated by the rapamycin-sensitive mTOR Complex 1

Bone marrow-specific loss of ABI1 induces myeloproliferative neoplasm with features resembling, human myelofibrosis

Although the pathogenesis of primary myelofibrosis (PMF) and other myeloproliferative neoplasms (MPNs) is linked to constitutive activation of the JAK-STAT pathway, JAK inhibitors have neither curative nor MPN-stem cell-eradicating potential, indicating that other targetable mechanisms are contributing to the pathophysiology of MPNs. We previously demonstrated that Abelson interactor 1 (Abi-1), a negative regulator of Abelson kinase 1, functions as a tumor suppressor. Here we present data showing that bone marrow-specific deletion of Abi1 in a novel mouse model leads to development of an MPNlike phenotype resembling human PMF. Abi1 loss resulted in a significant increase in the activity of the Src family kinases (SFKs), STAT3, and NF-κB signaling. We also observed impairment of hematopoietic stem cell self-renewal and fitness, as evidenced in noncompetitive and competitive bone marrow transplant experiments. CD34 + hematopoietic progenitors and granulocytes from patients with PMF showed decreased levels of ABI1 transcript as well as increased activity of SFKs, STAT3, and NF-κB. In aggregate, our data link the loss of Abi-1 function to hyperactive SFKs/STAT3/NF-κB signaling and suggest that this signaling axis may represent a regulatory module involved in the molecular pathophysiology of PMF

Rapamycin Response in Tumorigenic and Non-Tumorigenic Hepatic Cell Lines

The mTOR inhibitor rapamycin has anti-tumor activity across a variety of human cancers, including hepatocellular carcinoma. However, resistance to its growth inhibitory effects is common. We hypothesized that hepatic cell lines with varying rapamycin responsiveness would show common characteristics accounting for resistance to the drug.We profiled a total of 13 cell lines for rapamycin-induced growth inhibition. The non-tumorigenic rat liver epithelial cell line WB-F344 was highly sensitive while the tumorigenic WB311 cell line, originally derived from the WB-F344 line, was highly resistant. The other 11 cell lines showed a wide range of sensitivities. Rapamycin induced inhibition of cyclin E-dependent kinase activity in some cell lines, but the ability to do so did not correlate with sensitivity. Inhibition of cyclin E-dependent kinase activity was related to incorporation of p27(Kip1) into cyclin E-containing complexes in some but not all cell lines. Similarly, sensitivity of global protein synthesis to rapamycin did not correlate with its anti-proliferative effect. However, rapamycin potently inhibited phosphorylation of two key substrates, ribosomal protein S6 and 4E-BP1, in all cases, indicating that the locus of rapamycin resistance was downstream from inhibition of mTOR Complex 1. Microarray analysis did not disclose a unifying mechanism for rapamycin resistance, although the glycolytic pathway was downregulated in all four cell lines studied.We conclude that the mechanisms of rapamycin resistance in hepatic cells involve alterations of signaling downstream from mTOR and that the mechanisms are highly heterogeneous, thus predicting that maintaining or promoting sensitivity will be highly challenging

Phosphoproteomic Profiling of In Vivo Signaling in Liver by the Mammalian Target of Rapamycin Complex 1 (mTORC1)

Our understanding of signal transduction networks in the physiological context of an organism remains limited, partly due to the technical challenge of identifying serine/threonine phosphorylated peptides from complex tissue samples. In the present study, we focused on signaling through the mammalian target of rapamycin (mTOR) complex 1 (mTORC1), which is at the center of a nutrient- and growth factor-responsive cell signaling network. Though studied extensively, the mechanisms involved in many mTORC1 biological functions remain poorly understood.We developed a phosphoproteomic strategy to purify, enrich and identify phosphopeptides from rat liver homogenates. Using the anticancer drug rapamycin, the only known target of which is mTORC1, we characterized signaling in liver from rats in which the complex was maximally activated by refeeding following 48 hr of starvation. Using protein and peptide fractionation methods, TiO(2) affinity purification of phosphopeptides and mass spectrometry, we reproducibly identified and quantified over four thousand phosphopeptides. Along with 5 known rapamycin-sensitive phosphorylation events, we identified 62 new rapamycin-responsive candidate phosphorylation sites. Among these were PRAS40, gephyrin, and AMP kinase 2. We observed similar proportions of increased and reduced phosphorylation in response to rapamycin. Gene ontology analysis revealed over-representation of mTOR pathway components among rapamycin-sensitive phosphopeptide candidates.In addition to identifying potential new mTORC1-mediated phosphorylation events, and providing information relevant to the biology of this signaling network, our experimental and analytical approaches indicate the feasibility of large-scale phosphoproteomic profiling of tissue samples to study physiological signaling events in vivo

Resveratrol Inhibits Protein Translation in Hepatic Cells

Resveratrol is a plant-derived polyphenol that extends lifespan and healthspan in model organism. Despite extensive investigation, the biological processes mediating resveratrol's effects have yet to be elucidated. Because repression of translation shares many of resveratrol's beneficial effects, we hypothesized that resveratrol was a modulator of protein synthesis. We studied the effect of the drug on the H4-II-E rat hepatoma cell line. Initial studies showed that resveratrol inhibited global protein synthesis. Given the role of the mammalian Target of Rapamycin (mTOR) in regulating protein synthesis, we examined the effect of resveratrol on mTOR signaling. Resveratrol inhibited mTOR self-phosphorylation and the phosphorylation of mTOR targets S6K1 and eIF4E-BP1. It attenuated the formation of the translation initiation complex eIF4F and increased the phosphorylation of eIF2α. The latter event, also a mechanism for translation inhibition, was not recapitulated by mTOR inhibitors. The effects on mTOR signaling were independent of effects on AMP-activated kinase or AKT. We conclude that resveratrol is an inhibitor of global protein synthesis, and that this effect is mediated through modulation of mTOR-dependent and independent signaling

Required vs. Elective Research and In-Depth Scholarship Programs in the Medical Student Curriculum

The ability to understand and integrate new knowledge into clinical practice is a necessary quality of good physicians. Student participation in in-depth scholarship could enhance this skill in physicians while also creating a larger cadre of physician-scientists prepared to advance the field of medicine. However, because no definitive data exist demonstrating that in-depth scholarship in medical school leads to improved patient care or to productive academic careers, whether such scholarship should be required as part of the medical school curriculum is unclear. In this article, the authors present both sides of this debate. Theoretical benefits to students of a required scholarly program include closer mentorship by individual faculty, enhanced capabilities in critical interpretation of research findings, and increased confidence to investigate conundrums encountered in clinical care. Society may also benefit by having physicians available to create and apply new knowledge related to biomedicine. These theoretical benefits must be balanced, however, by pragmatic considerations of required scholarly projects including their impact on medical school applications, their effect on the medical curriculum, their costs, the availability of mentors, and their effects on the school's educational culture.\ud \u

A 5-year experience with an elective scholarly concentrations program

Problem: Programs that encourage scholarly activities beyond the core curriculum and traditional biomedical research are now commonplace among US medical schools. Few studies have generated outcome data for these programs. The goal of the present study was to address this gap. Intervention: The Scholarly Concentration (SC) Program, established in 2006 at the Warren Alpert Medical School of Brown University, is a 4-year elective program that not only encourages students to pursue scholarly work that may include traditional biomedical research but also seeks to broaden students’ focus to include less traditional areas. We compared characteristics and academic performance of SC students and non-SC students for the graduating classes of 2010–2014. Context: Approximately one-third of our students opt to complete an SC during their 4-year undergraduate medical education. Because this program is additional to the regular MD curriculum, we sought to investigate whether SC students sustained the academic achievement of non-SC students while at the same time producing scholarly work as part of the program. Outcome: Over 5 years, 35% of students elected to enter the program and approximately 81% of these students completed the program. The parameters that were similar for both SC and non-SC students were age at matriculation, admission route, proportion of undergraduate science majors, and number of undergraduate science courses. Most academic indicators, including United States Medical Licensing Examinations scores, were similar for the two groups; however, SC students achieved more honors in the six core clerkships and were more likely to be inducted into the medical school's two honor societies. Residency specialties selected by graduates in the two groups were similar. SC students published an average of 1.3 peer-reviewed manuscripts per student, higher than the 0.8 manuscripts per non-SC student (p=0.013). Conclusions: An elective, interdisciplinary scholarly program with a focus beyond traditional biomedical research offers students the opportunity to expand the scope of their medical education without an untoward effect on academic performance or residency placement