The active thyroid hormone 3,5,3′ triiodothyronine (T3) is a major regulator of skeletal muscle function. The deiodinase family of enzymes controls the tissue-specific activation and inactivation of the prohormone thyroxine (T4). Here we show that type 2 deiodinase (D2) is essential for normal mouse myogenesis and muscle regeneration. Indeed, D2-mediated increases in T3 were essential for the enhanced transcription of myogenic differentiation 1 (MyoD) and for execution of the myogenic program. Conversely, the expression of T3-dependent genes was reduced and after injury regeneration markedly delayed in muscles of mice null for the gene encoding D2 (Dio2), despite normal circulating T3 concentrations. Forkhead box O3 (FoxO3) was identified as a key molecule inducing D2 expression and thereby increasing intracellular T3 production. Accordingly, FoxO3-depleted primary myoblasts also had a differentiation deficit that could be rescued by high levels of T3. In conclusion, the FoxO3/D2 pathway selectively enhances intracellular active thyroid hormone concentrations in muscle, providing a striking example of how a circulating hormone can be tissue-specifically activated to influence development locally. IntroductionThe active thyroid hormone 3,5,3′ triiodothyronine (T3) derives either directly from thyroid secretion or by the monodeiodination of the prohormone thyroxine (T4) by one of two iodothyronine selenodeiodinases. Type 1 deiodinase is expressed in the liver, kidney, and thyroid but not skeletal muscle of vertebrates, and T3 produced from T4 by this enzyme is largely released into the plasma. On the other hand, type 2 deiodinase (D2) is specifically expressed in the central nervous system, pituitary, thyroid gland, brown adipose tissue, retina, and skeletal muscle. Much of the T3 derived from D2-mediated deiodination remains within the cell (1). Thus, this deiodinase provides a mechanism by which thyroid hormone (TH) can be activated in a tissue-specific chronologically programmed fashion, such as during development, or in circumstances where there is a requirement for rapid increase in active TH in a specific tissue. The effectiveness of this mechanism has been shown in the D2-dependent feedback regulation by T4 of the thyrotropin-releasing hormone and thyroid-stimulating hormone (TSH) secretion by the hypothalamus and pituitary (2). A programmed transient increase in D2 is required for the proper increase in T3 at a critical time in the embryonic mouse brain (3) to allow normal development of the cochlea (4, 5), and a sympathetic nervous system-induced increase in D2 provides the cellular T3

We identify a new enzymatic activity underlying metastasis in breast cancer and describe its susceptibility to therapeutic inhibition. We show that human prune (h-prune), a phosphoesterase DHH family appertaining protein, has a hitherto unrecognized cyclic nucleotide phosphodiesterase activity effectively suppressed by dipyridamole, a phosphodiesterase inhibitor. h-prune physically interacts with nm23-H1, a metastasis suppressor gene. The h-prune PDE activity, suppressed by dipyridamole and enhanced by the interaction with nm23-H1, stimulates cellular motility and metastasis processes. Out of 59 metastatic breast cancer cases analyzed, 22 (37%) were found to overexpress h-prune, evidence that this novel enzymatic activity is involved in promoting cancer metastasis.

Increasing evidence indicates that metabolism is implicated in the control of stem cell identity. Here, we demonstrate that embryonic stem cell (ESC) behaviour relies on a feedback loop that involves the non-essential amino acid L-Proline (L-Pro) in the modulation of the Gcn2-Eif2α-Atf4 amino acid starvation response (AAR) pathway that in turn regulates L-Pro biosynthesis. This regulatory loop generates a highly specific intrinsic shortage of L-Pro that restricts proliferation of tightly packed domed-like ESC colonies and safeguards ESC identity. Indeed, alleviation of this nutrient stress condition by exogenously provided L-Pro induces proliferation and modifies the ESC phenotypic and molecular identity towards that of mesenchymal-like, invasive pluripotent stem cells. Either pharmacological inhibition of the prolyl-tRNA synthetase by halofuginone or forced expression of Atf4 antagonises the effects of exogenous L-Pro. Our data provide unprecedented evidence that L-Pro metabolism and the nutrient stress response are functionally integrated to maintain ESC identity.

SummaryPrecise control of the thyroid hormone (T3)-dependent transcriptional program is required by multiple cell systems, including muscle stem cells. Deciphering how this is achieved and how the T3 signal is controlled in stem cell niches is essentially unknown. We report that in response to proliferative stimuli such as acute skeletal muscle injury, type 3 deiodinase (D3), the thyroid hormone-inactivating enzyme, is induced in satellite cells where it reduces intracellular thyroid signaling. Satellite cell-specific genetic ablation of dio3 severely impairs skeletal muscle regeneration. This impairment is due to massive satellite cell apoptosis caused by exposure of activated satellite cells to the circulating TH. The execution of this proapoptotic program requires an intact FoxO3/MyoD axis, both genes positively regulated by intracellular TH. Thus, D3 is dynamically exploited in vivo to chronically attenuate TH signaling under basal conditions while also being available to acutely increase gene programs required for satellite cell lineage progression.

Background: Quiescent/slow cycling cells have been identified in several tumors and correlated with therapy resistance. However, the features of chemoresistant populations and the molecular factors linking quiescence to chemoresistance are largely unknown. Methods: A population of chemoresistant quiescent/slow cycling cells was isolated through PKH26 staining (which allows to separate cells on the basis of their proliferation rate) from colorectal cancer (CRC) xenografts and subjected to global gene expression and pathway activation analyses. Factors expressed by the quiescent/slow cycling population were analyzed through lentiviral overexpression approaches for their ability to induce a dormant chemoresistant state both in vitro and in mouse xenografts. The correlation between quiescence-associated factors, CRC consensus molecular subtype and cancer prognosis was analyzed in large patient datasets. Results: Untreated colorectal tumors contain a population of quiescent/slow cycling cells with stem cell features (quiescent cancer stem cells, QCSCs) characterized by a predetermined mesenchymal-like chemoresistant phenotype. QCSCs expressed increased levels of ZEB2, a transcription factor involved in stem cell plasticity and epithelial-mesenchymal transition (EMT), and of antiapototic factors pCRAF and pASK1. ZEB2 overexpression upregulated pCRAF/pASK1 levels resulting in increased chemoresistance, enrichment of cells with stemness/EMT traits and proliferative slowdown of tumor xenografts. In parallel, chemotherapy treatment of tumor xenografts induced the prevalence of QCSCs with a stemness/EMT phenotype and activation of the ZEB2/pCRAF/pASK1 axis, resulting in a chemotherapy-unresponsive state. In CRC patients, increased ZEB2 levels correlated with worse relapse-free survival and were strongly associated to the consensus molecular subtype 4 (CMS4) characterized by dismal prognosis, decreased proliferative rates and upregulation of EMT genes.