Linking Hedgehog, Translation, and mTORC1 in Medulloblastoma

The role and mechanism of RNA-binding proteins in bone metabolism and osteoporosis

Osteoporosis is a prevalent chronic metabolic bone disease that poses a significant risk of fractures or mortality in elderly individuals. Its pathophysiological basis is often attributed to postmenopausal estrogen deficiency and natural aging, making the progression of primary osteoporosis among elderly people, especially older women, seemingly inevitable. The treatment and prevention of osteoporosis progression have been extensively discussed. Recently, as researchers delve deeper into the molecular biological mechanisms of bone remodeling, they have come to realize the crucial role of posttranscriptional gene control in bone metabolism homeostasis. RNA-binding proteins, as essential actors in posttranscriptional activities, may exert influence on osteoporosis progression by regulating the RNA life cycle. This review compiles recent findings on the involvement of RNA-binding proteins in abnormal bone metabolism in osteoporosis and describes the impact of some key RNA-binding proteins on bone metabolism regulation. Additionally, we explore the potential and rationale for modulating RNA-binding proteins as a means of treating osteoporosis, with an overview of drugs that target these proteins.

Preclinical efficacy of ribavirin in SHH and group 3 medulloblastoma

OBJECTIVE

Medulloblastoma, the most common pediatric brain malignancy, has Sonic Hedgehog (SHH) and group 3 (Myc driven) subtypes that are associated with the activity of eukaryotic initiation factor 4E (eIF4E), a critical mediator of translation, and enhancer of zeste homolog 2 (EZH2), a histone methyltransferase and master regulator of transcription. Recent drug repurposing efforts in multiple solid and hematologic malignancies have demonstrated that eIF4E and EZH2 are both pharmacologically inhibited by the FDA-approved antiviral drug ribavirin. Given the molecular overlap between medulloblastoma biology and known ribavirin activity, the authors investigated the preclinical efficacy of repurposing ribavirin as a targeted therapeutic in cell and animal models of medulloblastoma.

METHODS

Multiple in vitro assays were performed using human ONS-76 (a primitive SHH model) and D425 (an aggressive group 3 model) cells. The impacts of ribavirin on cellular growth, death, migration, and invasion were quantified using proliferation and Cell Counting Kit-8 (CCK-8) assays, flow cytometry with annexin V (AnnV) staining, scratch wound assays, and Matrigel invasion chambers, respectively. Survival following daily ribavirin treatment (100 mg/kg) was assessed in vivo in immunodeficient mice intracranially implanted with D425 cells.

RESULTS

Compared to controls, ribavirin treatment led to a significant reduction in medulloblastoma cell growth (ONS-76 proliferation assay, p = 0.0001; D425 CCK-8 assay, p < 0.0001) and a significant increase in cell death (flow cytometry for AnnV, ONS-76, p = 0.0010; D425, p = 0.0284). In ONS-76 cells, compared to controls, ribavirin significantly decreased cell migration and invasion (Matrigel invasion chamber assay, p = 0.0012). In vivo, ribavirin significantly extended survival in an aggressive group 3 medulloblastoma mouse model compared to vehicle-treated controls (p = 0.0004).

CONCLUSIONS

The authors demonstrate that ribavirin, a clinically used drug known to inhibit eIF4E and EZH2, has significant antitumor effects in multiple preclinical models of medulloblastoma, including an aggressive group 3 animal model. Ribavirin may represent a promising targeted therapeutic in medulloblastoma.

Regulation of the RNA-binding protein Smaug by the GPCR Smoothened via the kinase Fused

From fly to mammals, the Smaug/Samd4 family of prion-like RNA-binding proteins control gene expression by destabilizing and/or repressing the translation of numerous target transcripts. However, the regulation of its activity remains poorly understood. We show that Smaug's protein levels and mRNA repressive activity are downregulated by Hedgehog signaling in tissue culture cells. These effects rely on the interaction of Smaug with the G-protein coupled receptor Smoothened, which promotes the phosphorylation of Smaug by recruiting the kinase Fused. The activation of Fused and its binding to Smaug are sufficient to suppress its ability to form cytosolic bodies and to antagonize its negative effects on endogenous targets. Importantly, we demonstrate in vivo that HH reduces the levels of smaug mRNA and increases the level of several mRNAs downregulated by Smaug. Finally, we show that Smaug acts as a positive regulator of Hedgehog signaling during wing morphogenesis. These data constitute the first evidence for a post-translational regulation of Smaug and reveal that the fate of several mRNAs bound to Smaug is modulated by a major signaling pathway.