Mitochondrial ACSS1-K635 acetylation knock-in mice exhibit altered metabolism, cell senescence, and nonalcoholic fatty liver disease

Acetyl-CoA synthetase short-chain family member 1 (ACSS1) uses acetate to generate mitochondrial acetyl-CoA and is regulated by deacetylation by sirtuin 3. We generated an ACSS1-acetylation (Ac) mimic mouse, where lysine-635 was mutated to glutamine (K635Q). Male Acss1K635Q/K635Q mice were smaller with higher metabolic rate and blood acetate and decreased liver/serum ATP and lactate levels. After a 48-hour fast, Acss1K635Q/K635Q mice presented hypothermia and liver aberrations, including enlargement, discoloration, lipid droplet accumulation, and microsteatosis, consistent with nonalcoholic fatty liver disease (NAFLD). RNA sequencing analysis suggested dysregulation of fatty acid metabolism, cellular senescence, and hepatic steatosis networks, consistent with NAFLD. Fasted Acss1K635Q/K635Q mouse livers showed increased fatty acid synthase (FASN) and stearoyl-CoA desaturase 1 (SCD1), both associated with NAFLD, and increased carbohydrate response element-binding protein binding to Fasn and Scd1 enhancer regions. Last, liver lipidomics showed elevated ceramide, lysophosphatidylethanolamine, and lysophosphatidylcholine, all associated with NAFLD. Thus, we propose that ACSS1-K635-Ac dysregulation leads to aberrant lipid metabolism, cellular senescence, and NAFLD.

Ketogenic diet induces p53-dependent cellular senescence in multiple organs

A ketogenic diet (KD) is a high-fat, low-carbohydrate diet that leads to the generation of ketones. While KDs improve certain health conditions and are popular for weight loss, detrimental effects have also been reported. Here, we show mice on two different KDs and, at different ages, induce cellular senescence in multiple organs, including the heart and kidney. This effect is mediated through adenosine monophosphate-activated protein kinase (AMPK) and inactivation of mouse double minute 2 (MDM2) by caspase-2, leading to p53 accumulation and p21 induction. This was established using p53 and caspase-2 knockout mice and inhibitors to AMPK, p21, and caspase-2. In addition, senescence-associated secretory phenotype biomarkers were elevated in serum from mice on a KD and in plasma samples from patients on a KD clinical trial. Cellular senescence was eliminated by a senolytic and prevented by an intermittent KD. These results have important clinical implications, suggesting that the effects of a KD are contextual and likely require individual optimization.

MYOD-SKP2 axis boosts tumorigenesis in fusion negative rhabdomyosarcoma by preventing differentiation through p57Kip2 targeting

Rhabdomyosarcomas (RMS) are pediatric mesenchymal-derived malignancies encompassing PAX3/7-FOXO1 Fusion Positive (FP)-RMS, and Fusion Negative (FN)-RMS with frequent RAS pathway mutations. RMS express the master myogenic transcription factor MYOD that, whilst essential for survival, cannot support differentiation. Here we discover SKP2, an oncogenic E3-ubiquitin ligase, as a critical pro-tumorigenic driver in FN-RMS. We show that SKP2 is overexpressed in RMS through the binding of MYOD to an intronic enhancer. SKP2 in FN-RMS promotes cell cycle progression and prevents differentiation by directly targeting p27Kip1 and p57Kip2, respectively. SKP2 depletion unlocks a partly MYOD-dependent myogenic transcriptional program and strongly affects stemness and tumorigenic features and prevents in vivo tumor growth. These effects are mirrored by the investigational NEDDylation inhibitor MLN4924. Results demonstrate a crucial crosstalk between transcriptional and post-translational mechanisms through the MYOD-SKP2 axis that contributes to tumorigenesis in FN-RMS. Finally, NEDDylation inhibition is identified as a potential therapeutic vulnerability in FN-RMS.

A SNAI2/CTCF Interaction is Required for NOTCH1 Expression in Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a pediatric malignancy of the muscle with characteristics of cells blocked in differentiation. NOTCH1 is an oncogene that promotes self-renewal and blocks differentiation in the fusion negative-RMS sub-type. However, how NOTCH1 expression is transcriptionally maintained in tumors is unknown. Analyses of SNAI2 and CTCF chromatin binding and HiC analyses revealed a conserved SNAI2/CTCF overlapping peak downstream of the NOTCH1 locus marking a sub-topologically associating domain (TAD) boundary. Deletion of the SNAI2-CTCF peak showed that it is essential for NOTCH1 expression and viability of FN-RMS cells. Reintroducing constitutively activated NOTCH1-ΔE in cells with the SNAI2-CTCF peak deleted restored cell-viability. Ablation of SNAI2 using CRISPR/Cas9 reagents resulted in the loss of majority of RD and SMS-CTR FN-RMS cells. However, the few surviving clones that repopulate cultures have recovered NOTCH1. Cells that re-establish NOTCH1 expression after SNAI2 ablation are unable to differentiate robustly as SNAI2 shRNA knockdown cells; yet, SNAI2-ablated cells continued to be exquisitely sensitive to ionizing radiation. Thus, we have uncovered a novel mechanism by which SNAI2 and CTCF maintenance of a sub-TAD boundary promotes rather than represses NOTCH1 expression. Further, we demonstrate that SNAI2 suppression of apoptosis post-radiation is independent of SNAI2/NOTCH1 effects on self-renewal and differentiation.

The 3D chromatin landscape of rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a pediatric soft tissue cancer with a lack of precision therapy options for patients. We hypothesized that with a general paucity of known mutations in RMS, chromatin structural driving mechanisms are essential for tumor proliferation. Thus, we carried out high-depth in situ Hi-C in representative cell lines and patient-derived xenografts (PDXs) to define chromatin architecture in each major RMS subtype. We report a comprehensive 3D chromatin structural analysis and characterization of fusion-positive (FP-RMS) and fusion-negative RMS (FN-RMS). We have generated spike-in in situ Hi-C chromatin interaction maps for the most common FP-RMS and FN-RMS cell lines and compared our data with PDX models. In our studies, we uncover common and distinct structural elements in large Mb-scale chromatin compartments, tumor-essential genes within variable topologically associating domains and unique patterns of structural variation. Our high-depth chromatin interactivity maps and comprehensive analyses provide context for gene regulatory events and reveal functional chromatin domains in RMS.

Defining function of wild-type and three patient-specific TP53 mutations in a zebrafish model of embryonal rhabdomyosarcoma

In embryonal rhabdomyosarcoma (ERMS) and generally in sarcomas, the role of wild-type and loss- or gain-of-function TP53 mutations remains largely undefined. Eliminating mutant or restoring wild-type p53 is challenging; nevertheless, understanding p53 variant effects on tumorigenesis remains central to realizing better treatment outcomes. In ERMS, >70% of patients retain wild-type TP53, yet mutations when present are associated with worse prognosis. Employing a kRASG12D-driven ERMS tumor model and tp53 null (tp53-/-) zebrafish, we define wild-type and patient-specific TP53 mutant effects on tumorigenesis. We demonstrate that tp53 is a major suppressor of tumorigenesis, where tp53 loss expands tumor initiation from <35% to >97% of animals. Characterizing three patient-specific alleles reveals that TP53C176F partially retains wild-type p53 apoptotic activity that can be exploited, whereas TP53P153Δ and TP53Y220C encode two structurally related proteins with gain-of-function effects that predispose to head musculature ERMS. TP53P153Δ unexpectedly also predisposes to hedgehog-expressing medulloblastomas in the kRASG12D-driven ERMS-model.

Abstract 1465: A SNAI2/CDKN1A pathway protects rhabdomyosarcoma tumors from radiation

Rhabdomyosarcoma (RMS) is tumor of the muscle and is the most common soft tissue cancer in children and teens, with approximately 400 to 500 new cases every year in the United States. There are two main subtypes of rhabdomyosarcoma: 1) Alveolar rhabdomyosarcoma (fusion-positive RMS FP-RMS), is classified primarily by the presence of the fusion between PAX3/PAX7 and FOXO1 proteins. 2) Embryonal rhabdomyosarcoma (fusion-negative FN-RMS) is the more common subtype. The survival rate for patients with RMS is 70%, but at relapse it is less than 30%. The standard of care for this disease includes radiation, chemotherapy and surgery. To better understand how RMS tumors survive therapy, we sought to define the role of SNAI2 in the response to radiation treatment. We identified SNAI2 as a critical oncogene that protects RMS tumors from radiation induced apoptosis by repressing pro-apoptotic BIM expression (Cancer Research 2021). SNAI2 additionally prevents FN-RMS tumors and cells from undergoing myogenic differentiation (Nature Comm. 2021). We also showed that MEK maintained SNAI2 post translationally in RAS mutant FN-RMS tumors and combining MEK inhibitor with radiation led to complete tumor regression and delayed event free survival in xenograft and PDX models. However, rare tumor cells grow back after a 2-4 weeks delay (Molecular Cancer Therapeutics, 2022). Hence, we hypothesized that there are additional genes or pathways in cells lacking SNAI2 that are co-opted to drive relapse disease. We have identified that CDKN1A or p21 expression is robustly increased in SNAI2 null cells. CDKN1A is thought to be a tumor suppressor in RMS and other cancers. However, our preliminary data indicate that CDKN1A is an oncogene and in RMS patients, high expression in tumors is associated with poor outcome. We found that p21 expression in combination with SNAI2 can predict resistance to radiation across RMS cell lines. We next developed control RD, Rh30 and Rh18 cells either CDKN1A or both CDKN1A and SNAI2 ablated using CRISPR/Cas9 reagents with control cells expressing control gRNAs. Ablation of CDKN1A while not effecting proliferation led to a significant increase in apoptosis post-radiation and this effect was increased in SNAI2/CDKN1A double knockout cells. Further, in colony forming assays we find that double ablated cells form significantly fewer colonies than control and single knockout cells. Rather unexpectedly, loss of CDKN1A, SNAI2, or CDKN1A/SNAI2 led to a G2/M cell cycle block, indicating that the cell cycle effects of p21 might not be responsible for the phenotypes observed. Ongoing xenograft experiments will determine if the effect we observe in vitro will also translate in vivo. Additionally, we are performing experiments to test how SNAI2 modulates CDKN1A expression. In summary, our study identifies CDKN1A as an oncogene that protects SNAI2 deficient cells from ionizing radiation, thus enabling the survival or rare relapse driving tumor cells.

Citation Format: Paulomi S. Modi, Long Wang, Prethish Sreenivas, Myron Ignatius. A SNAI2/CDKN1A pathway protects rhabdomyosarcoma tumors from radiation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1465.

Sensitization to Ionizing Radiation by MEK Inhibition Is Dependent on SNAI2 in Fusion-Negative Rhabdomyosarcoma

In fusion-negative rhabdomyosarcoma (FN-RMS), a pediatric malignancy with skeletal muscle characteristics, >90% of high-risk patients have mutations that activate the RAS/MEK signaling pathway. We recently discovered that SNAI2, in addition to blocking myogenic differentiation downstream of MEK signaling in FN-RMS, represses proapoptotic BIM expression to protect RMS tumors from ionizing radiation (IR). As clinically relevant concentrations of the MEK inhibitor trametinib elicit poor responses in preclinical xenograft models, we investigated the utility of low-dose trametinib in combination with IR for the treatment of RAS-mutant FN-RMS. We hypothesized that trametinib would sensitize FN-RMS to IR through its downregulation of SNAI2 expression. While we observed little to no difference in myogenic differentiation or cell survival with trametinib treatment alone, robust differentiation and reduced survival were observed after IR. In addition, IR-induced apoptosis was significantly increased in FN-RMS cells treated concurrently with trametinib, as was increased BIM expression. SNAI2's role in these processes was established using overexpression rescue experiments, where overexpression of SNAI2 prevented IR-induced myogenic differentiation and apoptosis. Moreover, combining MEK inhibitor with IR resulted in complete tumor regression and a 2- to 4-week delay in event-free survival (EFS) in preclinical xenograft and patient-derived xenograft models. Our findings demonstrate that the combination of MEK inhibition and IR results in robust differentiation and apoptosis, due to the reduction of SNAI2, which leads to extended EFS in FN-RMS. SNAI2 thus is a potential biomarker of IR insensitivity and target for future therapies to sensitize aggressive sarcomas to IR.

SNAI2-Mediated Repression of BIM Protects Rhabdomyosarcoma from Ionizing Radiation

Ionizing radiation (IR) and chemotherapy are mainstays of treatment for patients with rhabdomyosarcoma, yet the molecular mechanisms that underlie the success or failure of radiotherapy remain unclear. The transcriptional repressor SNAI2 was previously identified as a key regulator of IR sensitivity in normal and malignant stem cells through its repression of the proapoptotic BH3-only gene PUMA/BBC3. Here, we demonstrate a clear correlation between SNAI2 expression levels and radiosensitivity across multiple rhabdomyosarcoma cell lines. Modulating SNAI2 levels in rhabdomyosarcoma cells through its overexpression or knockdown altered radiosensitivity in vitro and in vivo. SNAI2 expression reliably promoted overall cell growth and inhibited mitochondrial apoptosis following exposure to IR, with either variable or minimal effects on differentiation and senescence, respectively. Importantly, SNAI2 knockdown increased expression of the proapoptotic BH3-only gene BIM, and chromatin immunoprecipitation sequencing experiments established that SNAI2 is a direct repressor of BIM/BCL2L11. Because the p53 pathway is nonfunctional in the rhabdomyosarcoma cells used in this study, we have identified a new, p53-independent SNAI2/BIM signaling axis that could potentially predict clinical responses to IR treatment and be exploited to improve rhabdomyosarcoma therapy. SIGNIFICANCE: SNAI2 is identified as a major regulator of radiation-induced apoptosis in rhabdomyosarcoma through previously unknown mechanisms independent of p53.

Interaction between SNAI2 and MYOD enhances oncogenesis and suppresses differentiation in Fusion Negative Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is an aggressive pediatric malignancy of the muscle, that includes Fusion Positive (FP)-RMS harboring PAX3/7-FOXO1 and Fusion Negative (FN)-RMS commonly with RAS pathway mutations. RMS express myogenic master transcription factors MYOD and MYOG yet are unable to terminally differentiate. Here, we report that SNAI2 is highly expressed in FN-RMS, is oncogenic, blocks myogenic differentiation, and promotes growth. MYOD activates SNAI2 transcription via super enhancers with striped 3D contact architecture. Genome wide chromatin binding analysis demonstrates that SNAI2 preferentially binds enhancer elements and competes with MYOD at a subset of myogenic enhancers required for terminal differentiation. SNAI2 also suppresses expression of a muscle differentiation program modulated by MYOG, MEF2, and CDKN1A. Further, RAS/MEK-signaling modulates SNAI2 levels and binding to chromatin, suggesting that the differentiation blockade by oncogenic RAS is mediated in part by SNAI2. Thus, an interplay between SNAI2, MYOD, and RAS prevents myogenic differentiation and promotes tumorigenesis.

Abstract B35: Liaison between SNAI2 and MYOD enhances oncogenesis and suppresses differentiation in fusion-negative rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a pediatric malignancy of the muscle, and patients with high-risk fusion-negative RMS (FN-RMS), the major subtype of this disease, are associated with RAS pathway activating mutations and have a poor survival rate of &lt;30%. RMS cells express the myogenic master transcription factors MYOD and MYOG and yet are unable to differentiate. Here, we report an oncogenic role for SNAI2, which is highly expressed in FN-RMS, that blocks differentiation and promotes tumor growth through inhibition of a MYOD, MYOG, and MEF2 program. HiC analyses of the tridimensional-structure around the SNAI2 locus in IMR-90 cells find that it is regulated by a 1.2 mb enhancer regulatory region. In RMS tumors, MYOD can engage multiple SNAI2 enhancers and directly induce SNAI2 expression, while SNAI2 knockdown in RMS RD and JR1 results in increased expression of MYOD, indicating that SNAI2 in turn can repress MYOD expression. Employing two validated shRNAs to knock down SNAI2 in FN-RMS RD, JR1, and RD18 cells, we find that SNAI2 plays an oncogenic role by blocking myogenic differentiation and promoting growth both in vitro and in vivo in murine xenograft experiments. SNAI2 knockdown potentiates vincristine treatments and expands differentiation and greatly reduces tumor growth in vivo. In order to understand molecularly how SNAI2 blocks differentiation and promotes growth, we optimized ChIP-seq experiments in FN-RMS RD, JR1, and SMS-CTR cells to define genome-wide chromatin binding of SNAI2 and MYOD, with or without SNAI2 knockdown. Our ChIP-sequencing experiments confirmed that SNAI2 binds EBoxes, including the SNAI2 motif and motifs that MYOD and MYOG engage. Importantly, combining ChIPseq and RNAseq analyses, we discovered that SNAI2 preferentially binds EBox elements associated with enhancer elements and regulates gene expression by dampening enhancer function. SNAI2 competes with MYOD at a subset of myogenic enhancers associated with terminal differentiation, thus blocking differentiation in FN-RMS cells while potentially enabling progrowth effects of MYOD. SNAI2 also downregulates the expression of a MYOG, MEF2A/C/D, and CDKN1A differentiation program that is suppressed in FN-RMS cells, which upon reactivation along with MYOD drives robust differentiation and inhibits tumor xenograft growth in mice. Finally, we establish that SNAI2 function is downstream of the RAS program involved in blocking differentiation. In summary, SNAI2, through inhibition of a MYOD, MYOG, MEF2, and CDNK1A program, blocks tumor differentiation and promotes growth in FN-RMS.

Citation Format: Silvia Pomella, Prethish Sreenivas, Berkley E. Gryder, Long Wang, Matteo Cassandri, Kunal Baxi, Nicole R. Hensch, Elena Carcarino, Young Song, Marielle Yohe, Bruno Amadio, Ignazio Caruana, Cristiano De Stefanis, Rita De Vito, Franco Locatelli, Yidong Chen, Eleanor Y. Chen, Peter Houghton, Javed Khan, Rossella Rota, Myron S. Ignatius. Liaison between SNAI2 and MYOD enhances oncogenesis and suppresses differentiation in fusion-negative rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr B35.

Abstract 3124: SNAI2 function in embryonal RMS

Rhabdomyosarcoma (RMS) is a pediatric malignancy of the muscle and a key feature of the histology is muscle cells blocked in differentiation despite robust expression of diagnostic muscle differentiation factors MYOD1 and Myogenin. Thus, there are mechanisms operating in tumors that block myogenic differentiation. We previously defined roles in differentiation, self-renewal and growth for a NOTCH1/SNAI1/MEF2C pathway in Embryonal RMS, the major RMS subtype driven predominantly by Ras signaling. However, we observed that SNAI1 knockdown did not result in as robust differentiation as in NOTCH1 shRNA knockdown cells. We hypothesized that SNAI1 and SNAI2 function might be redundant in ERMS. Analysis of SNAI2 expression in primary tumors and cell lines finds that indeed SNAI2 is highly expressed in RMS and ERMS tumors typically express higher SNAI2 compared to SNAI1. To address SNAI2 function, we knocked down SNAI2 using 2 independent shRNAs and assessed effects on differentiation, self-renewal and growth in ERMS RD, SMS-CTR and JR1 cells. Knockdown of SNAI2 both in stable and transient experiments resulted in robust differentiation (10 fold increase) as assessed by differentiated myosin MF20 expression in RD, JR1 and SMS-CTR cells p&lt;0.001). This increase in differentiation was associated with increased expression differentiation genes including MYOD1, MYOGENIN, MEF2C, MEF2D and differentiated myosins and a loss of precursor gene PAX7 as assessed by quantitative RT-PCR and protein expression. SNAI2 knockdown RD and JR1 cells also formed significantly fewer rhabdospheres (p&lt;0.01). Finally, SNAI2 knockdown with 2 independent shRNAs resulted in significantly smaller and more differentiated tumors when xenografted subcutaneously in vivo in SCID mice. Since SNAI2 is a known DNA binding transcriptional repressor, we performed ChIPseq for SNAI2 and H3K27acetyl in SMS-CTR and RD cells coupled with RNAseq to define direct and indirect SNAI2 regulated genes. Our ChIPseq results identified the known SNAI2 DNA binding motif, however additionally we find that SNAI2 chromatin binding significantly enriched for myogenic E box elements bound by MYOD1, E2A and Myogenin. Additionally, SNAI2 binding was more significantly associated with the muscle differentiation program. Given the differential roles of MYOD1 and SNAI2 on gene activation vs. gene repression, we hypothesized that SNAI2 by competing with MYOD1 at terminally differentiated genes maintains early cell cycle effects of MYOD1 but blocks terminal differentiation. Analysis of MYOD1 expression in SNAI2 knockdown cells finds a redistribution of MYOD1 binding from cell cycle to more differentiated muscle genes and is associated with a concomitant exit from the cell cycle and robust differentiation. In summary, SNAI2 is a robust driver of ERMS differentiation and in vivo growth. High SNAI2 expression competes with MYOD1 at terminally differentiated genes blocking differentiation and exit from the cell cycle in ERMS.

Citation Format: Silvia Pomelo, Prethish Sreenivas, Berkley Gryder, Long Wang, Baxi Kunal, Nicole Hensch, Eleanor Chen, Peter Houghton, Rossella Rota, Javed Khan, Myron S. Ignatius. SNAI2 function in embryonal RMS [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3124.

The NOTCH1/SNAIL1/MEF2C Pathway Regulates Growth and Self-Renewal in Embryonal Rhabdomyosarcoma

Tumor-propagating cells (TPCs) share self-renewal properties with normal stem cells and drive continued tumor growth. However, mechanisms regulating TPC self-renewal are largely unknown, especially in embryonal rhabdomyosarcoma (ERMS)-a common pediatric cancer of muscle. Here, we used a zebrafish transgenic model of ERMS to identify a role for intracellular NOTCH1 (ICN1) in increasing TPCs by 23-fold. ICN1 expanded TPCs by enabling the de-differentiation of zebrafish ERMS cells into self-renewing myf5+ TPCs, breaking the rigid differentiation hierarchies reported in normal muscle. ICN1 also had conserved roles in regulating human ERMS self-renewal and growth. Mechanistically, ICN1 upregulated expression of SNAIL1, a transcriptional repressor, to increase TPC number in human ERMS and to block muscle differentiation through suppressing MEF2C, a myogenic differentiation transcription factor. Our data implicate the NOTCH1/SNAI1/MEF2C signaling axis as a major determinant of TPC self-renewal and differentiation in ERMS, raising hope of therapeutically targeting this pathway in the future.

A fine balance: epigenetic control of cellular quiescence by the tumor suppressor PRDM2/RIZ at a bivalent domain in the cyclin a gene

Adult stem cell quiescence is critical to ensure regeneration while minimizing tumorigenesis. Epigenetic regulation contributes to cell cycle control and differentiation, but few regulators of the chromatin state in quiescent cells are known. Here we report that the tumor suppressor PRDM2/RIZ, an H3K9 methyltransferase, is enriched in quiescent muscle stem cells invivo and controls reversible quiescence in cultured myoblasts. We find that PRDM2 associates with >4400 promoters in G₀ myoblasts, 55% of which are also marked with H3K9me2 and enriched for myogenic, cell cycle and developmental regulators. Knockdown of PRDM2 alters histone methylation at key promoters such as Myogenin and CyclinA2 (CCNA2), and subverts the quiescence program via global de-repression of myogenesis, and hyper-repression of the cell cycle. Further, PRDM2 acts upstream of the repressive PRC2 complex in G₀. We identify a novel G₀-specific bivalent chromatin domain in the CCNA2 locus. PRDM2 protein interacts with the PRC2 protein EZH2 and regulates its association with the bivalent domain in the CCNA2 gene. Our results suggest that induction of PRDM2 in G₀ ensures that two antagonistic programs—myogenesis and the cell cycle—while stalled, are poised for reactivation. Together, these results indicate that epigenetic regulation by PRDM2 preserves key functions of the quiescent state, with implications for stem cell self-renewal.

Distinct transcriptional networks in quiescent myoblasts: a role for Wnt signaling in reversible vs. irreversible arrest

Most cells in adult mammals are non-dividing: differentiated cells exit the cell cycle permanently, but stem cells exist in a state of reversible arrest called quiescence. In damaged skeletal muscle, quiescent satellite stem cells re-enter the cell cycle, proliferate and subsequently execute divergent programs to regenerate both post-mitotic myofibers and quiescent stem cells. The molecular basis for these alternative programs of arrest is poorly understood. In this study, we used an established myogenic culture model (C2C12 myoblasts) to generate cells in alternative states of arrest and investigate their global transcriptional profiles. Using cDNA microarrays, we compared G₀ myoblasts with post-mitotic myotubes. Our findings define the transcriptional program of quiescent myoblasts in culture and establish that distinct gene expression profiles, especially of tumour suppressor genes and inhibitors of differentiation characterize reversible arrest, distinguishing this state from irreversibly arrested myotubes. We also reveal the existence of a tissue-specific quiescence program by comparing G₀ C2C12 myoblasts to isogenic G₀ fibroblasts (10T1/2). Intriguingly, in myoblasts but not fibroblasts, quiescence is associated with a signature of Wnt pathway genes. We provide evidence that different levels of signaling via the canonical Wnt pathway characterize distinct cellular states (proliferation vs. quiescence vs. differentiation). Moderate induction of Wnt signaling in quiescence is associated with critical properties such as clonogenic self-renewal. Exogenous Wnt treatment subverts the quiescence program and negatively affects clonogenicity. Finally, we identify two new quiescence-induced regulators of canonical Wnt signaling, Rgs2 and Dkk3, whose induction in G₀ is required for clonogenic self-renewal. These results support the concept that active signal-mediated regulation of quiescence contributes to stem cell properties, and have implications for pathological states such as cancer and degenerative disease.

Quercetin attenuates Monocyte Chemoattractant Protein-1 gene expression in glucose primed aortic endothelial cells through NF-kappaB and AP-1

Monocyte Chemoattractant Protein-1 (MCP-1) is involved in the diapedesis of blood monocytes into the arterial intima, an early critical event in atherogenesis. Modulating MCP-1 expression can be a key strategy to decrease the risk for atherosclerosis in diabetes. We hypothesized that quercetin, an anti-inflammatory molecule could modulate high glucose concentration (HG) induced MCP-1 expression in aortic endothelial cells in vitro because of its regulatory effects on Activator Protein-1 (AP-1) and Nuclear Factor-kappaB (NF-kappaB). Rat aortic endothelial cells (RAECs) were exposed to HG in the presence or absence of quercetin. Quercetin attenuated HG induced MCP-1 mRNA (42%) and protein synthesis (45%) when estimated using real-time reverse transcriptase-polymerase chain reaction and enzyme-linked immunosorbent assay respectively. Western blot analysis found quercetin to maintain cytosolic p65 protein levels to that seen in control. Quercetin was found to attenuate HG induced increased NF-kappaB and AP-1 DNA binding activity in electrophoretic mobility shift assay. Immunofluorescence studies revealed quercetin to prevent HG induced nuclear localization of p65 and c-jun. Quercetin was also found to decrease HG induced activation of NF-kappaB (71%+/-14%), AP-1 (69%+/-24%) and MCP-1 promoter (79%+/-25%) in EA.hy926 cells when analyzed using luciferase reporter assay. We conclude that quercetin attenuates MCP-1 expression in HG treated RAECs, probably by regulating both NF-kappaB and AP-1 pathways. The findings provide new insights into HG induced MCP-1 gene regulation in aortic endothelial cells and the potential of quercetin in abating the risk for atherosclerosis in diabetes.