Cachexia, or muscle-wasting syndrome, is one of the major causes of death in patients affected by diseases such as cancer, AIDS and sepsis. However, no effective anti-cachectic treatment is currently available. Here we show that a low dose of pateamine A, an inhibitor of translation initiation, prevents muscle wasting caused by the cytokines interferon γ and tumour necrosis factor α or by C26-adenocarcinoma tumours. Surprisingly, although high doses of pateamine A abrogate general translation, low doses selectively inhibit the expression of pro-cachectic factors such as inducible nitric oxide synthase. This selectivity depends on the 5′UTR of inducible nitric oxide synthase messenger RNA (mRNA) that, unlike the 5′UTR of MyoD mRNA, promotes the recruitment of inducible nitric oxide synthase mRNA to stress granules, where its translation is repressed. Collectively, our data provide a proof of principle that nontoxic doses of compounds such as pateamine A could be used as novel drugs to combat cachexia-induced muscle wasting.

HuR promotes myogenesis by stabilizing the MyoD, Myogenin and p21 mRNAs during the fusion of muscle cells to form myotubes. Here we show that HuR, via a novel mRNA destabilizing activity, promotes the early steps of myogenesis by reducing the expression of the cell cycle promoter nucleophosmin (NPM). Depletion of HuR stabilizes the NPM mRNA, increases NPM protein levels and inhibits myogenesis, while its overexpression elicits the opposite effects. NPM mRNA destabilization involves the association of HuR with the decay factor KSRP as well as the ribonuclease PARN and the exosome. The C-terminus of HuR mediates the formation of the HuR-KSRP complex and is sufficient for maintaining a low level of the NPM mRNA as well as promoting the commitment of muscle cells to myogenesis. We therefore propose a model whereby the downregulation of the NPM mRNA, mediated by HuR, KSRP and its associated ribonucleases, is required for proper myogenesis.

Very little is known as to how the accumulation of senescent cells during aging may affect our ability to cope with various stresses. Here we show that the assembly of stress granules (SGs) is part of the early events used by senescent cells to respond to certain stresses. Although SGs can form in response to stress during senescence activation, their number significantly increases once the cells are fully senescent. This increase correlates with a rapid decrease in the expression levels of the cyclin kinase inhibitor p21, an important activator of senescence. Throughout stress, p21 mRNA is stabilized and localizes to SGs, but only during late senescence does this localization interferes with its translation. Additionally, we observed that when the stress is relieved, senescent cells produce lower levels of p21 protein, which correlates with a small delay in SG disassembly. Therefore, our data suggest that SG formation and the reduction in p21 protein levels represent two main events by which senescent cells respond to stress.

Upon muscle injury the high mobility group box 1 (HMGB1) protein is up-regulated and secreted to initiate reparative responses. Here we show that HMGB1 controls myogenesis both in vitro and in vivo, during development and after adult muscle injury. HMGB1 expression in muscle cells is regulated at the translational level: the miRNA miR-1192 inhibits HMGB1 translation and the RNA-binding protein HuR promotes it. HuR binds to a cis-element, HuRBS, located in the 3′UTR of the HMGB1 transcript, and at the same time miR-1192 is recruited to an adjacent seed element. The binding of HuR to the HuRBS prevents the recruitment of Argonaute 2 (Ago2), overriding miR-1192-mediated translation inhibition. Depleting HuR reduces myoblast fusion and silencing miR-1192 re-establishes the fusion potential of HuR-depleted cells. We propose that HuR promotes the commitment of myoblasts to myogenesis by enhancing the translation of HMGB1 and suppressing the translation inhibition mediated by miR-1192.

Little is known about the cellular mechanisms modulating the shift in balance from a state of survival to cell death by caspasemediated apoptosis in response to a lethal stress. Here we show that the RNA-binding protein HuR has an important function in mediating this switch. During caspase-mediated apoptosis, HuR is cleaved to generate two cleavage products (CPs). Our data demonstrate that the cleavage of HuR switches its function from being a prosurvival factor under normal conditions to becoming a promoter of apoptosis in response to a lethal stress. In the absence of an apoptotic stimuli, HuR associates with and promotes the expression of caspase-9 and prothymosin a (ProT) mRNAs, and pro-and antiapoptotic factors, respectively, both of which have been characterized as important players in determining cell fate. During the early steps of caspase-mediated apoptosis, however, the level of caspase-9 protein increases, while ProT remains unchanged. Under these conditions, the two HuR-CPs selectively bind to and stabilize caspase-9 mRNA, but do not bind to ProT. Hence, taken together, our data show that by maintaining a threshold of expression of proapoptotic factors such as caspase-9 in response to a lethal stress, the HuR-CPs help a cell to switch from resisting death to undergoing apoptosis.

Cellular senescence is a physiological response by which an organism halts the proliferation of potentially harmful and damaged cells. However, the accumulation of senescent cells over time can become deleterious leading to diseases and physiological decline. Our data reveal a novel interplay between senescence and the stress response that affects both the progression of senescence and the behavior of senescent cells. We show that constitutive exposure to stress induces the formation of stress granules (SGs) in proliferative and presenescent cells, but not in fully senescent cells. Stress granule assembly alone is sufficient to decrease the number of senescent cells without affecting the expression of bona fide senescence markers. SG‐mediated inhibition of senescence is associated with the recruitment of the plasminogen activator inhibitor‐1 (PAI‐1), a known promoter of senescence, to these entities. PAI‐1 localization to SGs increases the translocation of cyclin D1 to the nucleus, promotes RB phosphorylation, and maintains a proliferative, non‐senescent state. Together, our data indicate that SGs may be targets of intervention to modulate senescence in order to impair or prevent its deleterious effects.

Debilitating cancer-induced muscle wasting, a syndrome known as cachexia, is lethal. Here we report a posttranscriptional pathway involving the RNA-binding protein HuR as a key player in the onset of this syndrome. Under these conditions, HuR switches its function from a promoter of muscle fiber formation to become an inducer of muscle loss. HuR binds to the STAT3 (signal transducer and activator of transcription 3) mRNA, which encodes one of the main effectors of this condition, promoting its expression both in vitro and in vivo. While HuR does not affect the stability and the cellular movement of this transcript, HuR promotes the translation of the STAT3 mRNA by preventing miR-330 (microRNA 330)–mediated translation inhibition. To achieve this effect, HuR directly binds to a U-rich element in the STAT3 mRNA-3′untranslated region (UTR) located within the vicinity of the miR-330 seed element. Even though the binding sites of HuR and miR-330 do not overlap, the recruitment of either one of them to the STAT3-3′UTR negatively impacts the binding and the function of the other factor. Therefore, together, our data establish the competitive interplay between HuR and miR-330 as a mechanism via which muscle fibers modulate, in part, STAT3 expression to determine their fate in response to promoters of muscle wasting.

Cellular senescence is a known driver of carcinogenesis and age-related diseases, yet senescence is required for various physiological processes. However, the mechanisms and factors that control the negative effects of senescence while retaining its benefits are still elusive. Here, we show that the rasGAP SH3-binding protein 1 (G3BP1) is required for the activation of the senescent-associated secretory phenotype (SASP). During senescence, G3BP1 achieves this effect by promoting the association of the cyclic GMP-AMP synthase (cGAS) with cytosolic chromatin fragments. In turn, G3BP1, through cGAS, activates the NF-κB and STAT3 pathways, promoting SASP expression and secretion. G3BP1 depletion or pharmacological inhibition impairs the cGAS-pathway preventing the expression of SASP factors without affecting cell commitment to senescence. These SASPless senescent cells impair senescence-mediated growth of cancer cells in vitro and tumor growth in vivo. Our data reveal that G3BP1 is required for SASP expression and that SASP secretion is a primary mediator of senescence-associated tumor growth.