SUMMARY Studies support the importance of microRNAs in physiological and pathological processes. Here we describe the regulation and function of miR-29 in myogenesis and Rhabdomyosarcoma (RMS). Results demonstrate that in myoblasts miR-29 is repressed by NF-κB acting through YY1 and the Polycomb. During myogenesis, NF-κB and YY1 downregulation causes derepression of miR-29, which in turn accelerates differentiation by targeting its repressor YY1. However, in RMS cells and primary tumors that possess impaired differentiation, miR-29 is epigenetically silenced by an activated NF-κB-YY1 pathway. Reconstitution of miR-29 in RMS in mice inhibits tumor growth and stimulates differentiation, suggesting that miR-29 acts as a tumor suppressor through its pro-myogenic function. Together, results identify a NF-κB–YY1–miR-29 regulatory circuit whose disruption may contribute to RMS. SIGNIFICANCE MicroRNAs regulate skeletal myogenesis, but their impact in muscle diseases is not well understood. Here we describe miR-29 as an enhancer of myogenic differentiation and a suppressor of RMS. We find that miR-29 exists in a regulatory circuit involving NF-κB and YY1. In myoblasts NF-B acts through YY1 to epigenetically suppress miR-29, while during differentiation miR-29 is induced to facilitate myogenesis by a negative feedback on YY1. Significantly, RMS tumors lose miR-29 due to an elevation in NF-B and YY1, and readjustment of miR-29 levels in RMS stimulates differentiation. Thus, myogenesis is dependent on NF-κB–YY1–miR-29 circuitry whose dysfunction may contribute to RMS pathogenesis. Such findings offer potential avenues for the diagnosis and treatment of muscle relevant cancers.

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Nuclear factor κB (NF-κB) is involved in multiple skeletal muscle disorders, but how it functions in differentiation remains elusive given that both anti- and promyogenic activities have been described. In this study, we resolve this by showing that myogenesis is controlled by opposing NF-κB signaling pathways. We find that myogenesis is enhanced in MyoD-expressing fibroblasts deficient in classical pathway components RelA/p65, inhibitor of κB kinase β (IKKβ), or IKKγ. Similar increases occur in myoblasts lacking RelA/p65 or IKKβ, and muscles from RelA/p65 or IKKβ mutant mice also contain higher fiber numbers. Moreover, we show that during differentiation, classical NF-κB signaling decreases, whereas the induction of alternative members IKKα, RelB, and p52 occurs late in myogenesis. Myotube formation does not require alternative signaling, but it is important for myotube maintenance in response to metabolic stress. Furthermore, overexpression or knockdown of IKKα regulates mitochondrial content and function, suggesting that alternative signaling stimulates mitochondrial biogenesis. Together, these data reveal a unique IKK/NF-κB signaling switch that functions to both inhibit differentiation and promote myotube homeostasis.

There is growing recognition that psychological stress influences pain. Hormones that comprise the physiological response to stress (e.g. corticosterone; CORT) may interact with effectors of neuropathic pain. To test this hypothesis, mice received a spared nerve injury (SNI) after exposure to 60 min restraint stress. In stressed mice, allodynia was consistently increased. The mechanism(s) underlying the exacerbated pain response involves CORT acting via glucocorticoid receptors (GRs); RU486, a GR antagonist, prevented the stress-induced increase in allodynia whereas exogenous administration of CORT to non-stressed mice reproduced the allodynic response caused by stress. Since nerve injury-induced microglial activation has been implicated in the onset and propagation of neuropathic pain, we evaluated cellular and molecular indices of microglial activation in the context of stress. Activation of dorsal horn microglia was accelerated by stress; however, this effect was transient and was not associated with the onset or maintenance of a pro-inflammatory phenotype. Stress-enhanced allodynia was associated with increased dorsal horn extracellular signal-regulated kinase phosphorylation (pERK). ERK activation could indicate a stress-mediated increase in glutamatergic signaling, therefore mice were treated prior to SNI and stress with memantine, an Nmethyl-D-aspartate receptor (NMDAR) antagonist. Memantine prevented stress-induced enhancement of allodynia after SNI. These data suggest that the hormonal responses elicited by stress exacerbate neuropathic pain through enhanced central sensitization. Moreover, drugs that inhibit glucocorticoids (GCs) and/or NMDAR signaling could ameliorate pain syndromes caused by stress.

Figure 3. (F) Primary or C2C12 myoblasts were transfected with siControl (siCont) or siRNA against p65 (sip65) along with Tn-luc reporter. Cells were switched to DM, and luciferase assays were performed. Error bars represent SEM.

To determine whether mice unable to mount an intact inflammatory response because of a Toll-like receptor (TLR) pathway defect will develop less severe cancer cachexia.

Classical NF-B activity functions as an inhibitor of the skeletal muscle myogenic program. Recent findings reveal that even in newborn RelA/p65 ؊/؊ mice, myofiber numbers are increased over that of wild type mice, suggesting that NF-B may be a contributing factor in early postnatal skeletal muscle development. Here we show that in addition to p65 deficiency, repression of NF-B with the IB␣-SR transdominant inhibitor or with muscle-specific deletion of IKK␤ resulted in similar increases in total fiber numbers as well as an up-regulation of myogenic gene products. Upon further characterization of early postnatal muscle, we observed that NF-B activity progressively declines within the first few weeks of development. At birth, the majority of this activity is compartmentalized to muscle fibers, but by neonatal day 8 NF-B activity from the myofibers diminishes, and instead, stromal fibroblasts become the main cellular compartment within the muscle that contains active NF-B. We find that NF-B functions in these fibroblasts to regulate inducible nitric-oxide synthase expression, which we show is important for myoblast fusion during the growth and maturation process of skeletal muscle. Together, these data broaden our understanding of NF-B during development by showing that in addition to its role as a negative regulator of myogenesis, NF-B also regulates nitric-oxide synthase expression within stromal fibroblasts to stimulate myoblast fusion and muscle hypertrophy.

Studies support that NF-kappaB functions in cellular growth through the transcriptional regulation of cyclin D1, but whether such regulation is attributed to a single NF-kappaB subunit remains unclear. To address this issue we examined endogenous cyclin D1 levels during cell cycle re-entry in mouse embryonic fibroblasts (MEFs) lacking specific NF-kappaB signaling subunits. Results showed that each of these subunits were dispensable for regulating cyclin D1 transcription. However, we found that resulting cyclin D1 protein was severely reduced in MEFs lacking only RelA/p65. Cyclohexamide treatment revealed that this regulation was due to an increase in protein turnover. Similar downregulation of cyclin D1 protein, but not RNA, was observed in vivo in multiple tissues lacking RelA/p65. Co-immunoprecipitation analysis also showed that RelA/p65 and cyclin D1 were capable of interacting, thus providing a possible explanation for cyclin D1 protein stability. In addition, although the decrease in cyclin D1 in RelA/p65(-/-) MEFs was concomitant with lower CDK4 activity during cell cycle re-entry, this was not sufficient to affect S phase progression. Nevertheless, similar decreases in cyclin D1 protein in primary RelA/p65(-/-) myoblasts was adequate to accelerate cell cycle exit and differentiation of these cells. Based on these findings we conclude that RelA/p65 functions as a specific regulator of cyclin D1 protein stability, necessary for proper cell cycle withdrawal during skeletal myogenesis.