Muscle fibers show great differences in their contractile and metabolic properties. This diversity enables skeletal muscles to fulfill and adapt to different tasks. In this report, we show that the Six/Eya pathway is implicated in the establishment and maintenance of the fast-twitch skeletal muscle phenotype. We demonstrate that the MEF3/Six DNA binding element present in the aldolase A pM promoter mediates the high level of activation of this promoter in fast-twitch glycolytic (but not in slow-twitch) muscle fibers. We also show that among the Six and Eya gene products expressed in mouse skeletal muscle, Six1 and Eya1 proteins accumulate preferentially in the nuclei of fast-twitch muscles. The forced expression of Six1 and Eya1 together in the slow-twitch soleus muscle induced a fiber-type transition characterized by the replacement of myosin heavy chain I and IIA isoforms by the faster IIB and/or IIX isoforms, the activation of fast-twitch fiber-specific genes, and a switch toward glycolytic metabolism. Collectively, these data identify Six1 and Eya1 as the first transcriptional complex that is able to reprogram adult slow-twitch oxidative fibers toward a fast-twitch glycolytic phenotype.

Polyploidy is a general physiological process indicative of terminal differentiation. During liver growth, this process generates the appearance of tetraploid (4n) and octoploid (8n) hepatocytes with one or two nuclei. The onset of polyploidy in the liver has been recognized for quite some time; however, the cellular mechanisms that govern it remain unknown. In this report, we observed the sequential appearance during liver growth of binuclear diploid (2 ؋ 2n) and mononuclear 4n hepatocytes from a diploid hepatocyte population. To identify the cell cycle modifications involved in hepatocyte polyploidization, mitosis was then monitored in primary cultures of rat hepatocytes. Twenty percent of mononuclear 2n hepatocytes failed to undergo cytokinesis with no observable contractile movement of the ring. This process led to the formation of binuclear 2 ؋ 2n hepatocytes. This tetraploid condition following cleavage failure did not activate the p53-dependent checkpoint in G 1 . In fact, binuclear hepatocytes were able to proceed through S phase, and the formation of a bipolar spindle during mitosis constituted the key step leading to the genesis of two mononuclear 4n hepatocytes. Finally, we studied the duplication and clustering of centrosomes in the binuclear hepatocyte. These cells exhibited two centrosomes in G 1 that were duplicated during S phase and then clustered by pairs at opposite poles of the cell during metaphase. This event led only to mononuclear 4n progeny and maintained the tetraploidy status of hepatocytes.Polyploidy is a general physiological process that prevails in many cellular systems including plants, insects, and mammals (1). The onset of cellular polyploidization is associated with late fetal development and postnatal maturation. Advanced polyploidy in mammalian cells is indicative of terminal differentiation and senescence (2). Hepatocytes come under the former category. During growth, the liver parenchyma undergoes dramatic changes characterized by gradual polyploidization during which hepatocytes of several ploidy classes emerge as a result of modified cell division cycles. This process generates the successive appearance of tetraploid and octoploid cell classes with one or two nuclei. Thus, in the liver of a newborn rat, hepatocytes are exclusively diploid (2n), 1 and polyploidization starts after weaning. In adult rats, about 10% of hepatocytes are diploid, 70% are tetraploid, and 20% octoploid. If we consider the polyploid fraction, 20 -30% of hepatocytes are binuclear (either 2 ϫ 2n or 2 ϫ 4n) (3, 4). The degree of polyploidization varies among mammals (5) and particularly in humans, where the number of polyploid cells averages 20 -30% in the adult liver (6, 7). Interestingly, in different liver pathologies, hepatocarcinoma for example, hepatocellular growth shifts to a nonpolyploidizing growth pattern, and expansion of the diploid hepatocyte population has been found to take place (4, 7).Polyploidization is a general strategy of cell growth that enables an increase in metabolic output,...

Loss of normal primary cilia function in mammals is linked to proliferative diseases, such as polycystic kidney disease, suggesting a regulatory relationship between cilia and cell cycle. The primary cilium expressed by most mammalian cells is nucleated from the elder centriole of the centrosome. The relationship between centrosome and cilia suggests that these structures share functions and components. We now show that IFT88/polaris, a component of the intraflagellar transport, remains associated to the centrosome in a proliferative state. IFT88/polaris is tightly associated with the centrosome throughout the cell cycle in a microtubule- and dynein-independent manner. IFT88/polaris tetratricopeptide repeat motifs are essential for this localization. Overexpression of IFT88/polaris prevents G1-S transition and induces apoptotic cell death. By contrast, IFT88/polaris depletion induced by RNA interference promotes cell-cycle progression to S, G2, and M phases. Finally, we demonstrate that IFT88/polaris interacts with Che-1, an Rb-binding protein that inhibits the Rb growth suppressing function. We propose that IFT88/polaris, a protein essential for ciliogenesis, is also crucial for G1-S transition in non-ciliated cells.

Polyploidy is a general physiological process indicative of terminal differentiation. During liver growth, this process generates the appearance of tetraploid (4n) and octoploid (8n) hepatocytes with one or two nuclei. The onset of polyploidy in the liver has been recognized for quite some time; however, the cellular mechanisms that govern it remain unknown. In this report, we observed the sequential appearance during liver growth of binuclear diploid (2 ؋ 2n) and mononuclear 4n hepatocytes from a diploid hepatocyte population. To identify the cell cycle modifications involved in hepatocyte polyploidization, mitosis was then monitored in primary cultures of rat hepatocytes. Twenty percent of mononuclear 2n hepatocytes failed to undergo cytokinesis with no observable contractile movement of the ring. This process led to the formation of binuclear 2 ؋ 2n hepatocytes. This tetraploid condition following cleavage failure did not activate the p53-dependent checkpoint in G 1 . In fact, binuclear hepatocytes were able to proceed through S phase, and the formation of a bipolar spindle during mitosis constituted the key step leading to the genesis of two mononuclear 4n hepatocytes. Finally, we studied the duplication and clustering of centrosomes in the binuclear hepatocyte. These cells exhibited two centrosomes in G 1 that were duplicated during S phase and then clustered by pairs at opposite poles of the cell during metaphase. This event led only to mononuclear 4n progeny and maintained the tetraploidy status of hepatocytes.Polyploidy is a general physiological process that prevails in many cellular systems including plants, insects, and mammals (1). The onset of cellular polyploidization is associated with late fetal development and postnatal maturation. Advanced polyploidy in mammalian cells is indicative of terminal differentiation and senescence (2). Hepatocytes come under the former category. During growth, the liver parenchyma undergoes dramatic changes characterized by gradual polyploidization during which hepatocytes of several ploidy classes emerge as a result of modified cell division cycles. This process generates the successive appearance of tetraploid and octoploid cell classes with one or two nuclei. Thus, in the liver of a newborn rat, hepatocytes are exclusively diploid (2n), 1 and polyploidization starts after weaning. In adult rats, about 10% of hepatocytes are diploid, 70% are tetraploid, and 20% octoploid. If we consider the polyploid fraction, 20 -30% of hepatocytes are binuclear (either 2 ϫ 2n or 2 ϫ 4n) (3, 4). The degree of polyploidization varies among mammals (5) and particularly in humans, where the number of polyploid cells averages 20 -30% in the adult liver (6, 7). Interestingly, in different liver pathologies, hepatocarcinoma for example, hepatocellular growth shifts to a nonpolyploidizing growth pattern, and expansion of the diploid hepatocyte population has been found to take place (4, 7).Polyploidization is a general strategy of cell growth that enables an increase in metabolic output,...

Castration-resistant prostate cancer is a lethal disease. The cell type(s) that survive androgen deprivation remain poorly described, despite global efforts to understand the various mechanisms of therapy resistance. We recently identified in wild-type (WT) mouse prostates a rare population of luminal progenitor cells that we called LSC according to their FACS profile (Lin /Sca-1 /CD49f ). Here, we investigated the prevalence and castration resistance of LSC in various mouse models of prostate tumourigenesis (Pb-PRL, Pten , and Hi-Myc mice). LSC prevalence is low (∼8%, similar to WT) in Hi-Myc mice, where prostatic androgen receptor signalling is unaltered, but is significantly higher in the two other models, where androgen receptor signalling is decreased, rising up to more than 80% in Pten prostates. LSC tolerate androgen deprivation and persist or are enriched 2-3 weeks after castration. The tumour-initiating properties of LSC from Pten mice were demonstrated by regeneration of tumours in vivo. Transcriptomic analysis revealed that LSC represent a unique cell entity as their gene expression profile is different from luminal and basal/stem cells, but shares markers of each. Their intrinsic androgen signalling is markedly decreased, explaining why LSC tolerate androgen deprivation. This also illuminates why Pten tumours are castration-resistant since LSC represent the most prevalent cell type in this model. We validated CK4 as a specific marker for LSC on sorted cells and prostate tissues by immunostaining, allowing for the detection of LSC in various mouse prostate specimens. In castrated Pten prostates, there was significant proliferation of CK4 cells, further demonstrating their key role in castration-resistant prostate cancer progression. Taken together, this study identifies LSC as a probable source of prostate cancer relapse after androgen deprivation and as a new therapeutic target for the prevention of castrate-resistant prostate cancer. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Hepatocyte transplantation might represent a potential therapeutic alternative to liver transplantation in the future; however, transplanted cells have a limited capacity to repopulate the liver, as they do not proliferate under normal conditions. Recently, studies in urokinase (uPA) transgenic mice and in fumarylacetoacetate hydrolase (FAH)-deficient mice have shown that the liver can be repopulated by genetically engineered hepatocytes harboring a selective advantage over resident hepatocytes. We have reported that transgenic mice expressing human Bcl-2 in their hepatocytes are protected from Fas/CD95-mediated liver apoptosis. We now show that Bcl-2 transplanted hepatocytes selectively repopulate the liver of mice treated with nonlethal doses of the anti-Fas antibody Jo2. FK 506 immunosuppressed mice were transplanted by splenic injection with Bcl-2 hepatocytes. The livers of female recipients were repopulated by male Bcl-2 transgenic hepatocytes, as much as 16%, after 8 to 12 administrations of Jo2. This only occurred after anti-Fas treatment, confirming that resistance to Fas-induced apoptosis constituted the selective advantage of these transplanted hepatocytes. Thus, we have demonstrated a method for increasing genetic reconstitution of the liver through selective repopulation with modified transgenic hepatocytes, which will allow optimization of cell and gene therapy in the liver.

Cell-based therapy may some day be a therapeutic alternative to liver transplantation. Recent observations indicating that hematopoietic stem cells can differentiate into hepatocytes have opened new therapeutic prospects. However, the clinical relevance of this phenomenon is unknown. We have previously developed a strategy based on the protective effect of Bcl-2 against Fas-mediated apoptosis to selectively amplify a small number of hepatocytes in vivo. We now show that this approach can be used to amplify a minor population of bone marrow-derived hepatocytes. Normal mice were transplanted with unfractionated bone marrow cells from transgenic animals expressing Bcl-2 under the control of a liver-specific promoter. Recipients were then submitted to weekly injections of the anti-Fas antibody, Jo2. Upon sacrifice, the liver of the recipients showed bone marrow-derived clusters of mature hepatocytes expressing Bcl-2, which showed that the hepatocyte progeny of a genetically modified bone marrow can be selectively expanded in vivo. In contrast, no Bcl-2 expression could be detected without the selective pressure of Jo2, suggesting that differentiation of bone marrow cells into mature hepatocytes is very inefficient under physiologic conditions. We conclude that a selection strategy will be required to achieve a therapeutic level of liver repopulation with bone marrow-derived hepatocytes. (HEPATOLOGY 2002;35:799-804.)

GH is a pleiotropic hormone that plays a major role in proliferation, differentiation, and metabolism via its specific receptor. It has been previously suggested that GH signaling pathways are required for normal liver regeneration but the molecular mechanisms involved have yet to be determined. The aim of this study was to identify the mechanisms by which GH controls liver regeneration. We performed two thirds partial hepatectomies in GH receptor (GHR)-deficient mice and wild-type littermates and showed a blunted progression in the G(1)/S transition phase of the mutant hepatocytes. This impaired liver regeneration was not corrected by reestablishing IGF-1 expression. Although the initial response to partial hepatectomy at the priming phase appeared to be similar between mutant and wild-type mice, cell cycle progression was significantly blunted in mutant mice. The main defect in GHR-deficient mice was the deficiency of the epidermal growth factor receptor activation during the process of liver regeneration. Finally, among the pathways activated downstream of GHR during G(1) phase progression, namely Erk1/2, Akt, and signal transducer and activator of transcription 3, we only found a reduced Erk1/2 phosphorylation in mutant mice. In conclusion, our results demonstrate that GH signaling plays a major role in liver regeneration and strongly suggest that it acts through the activation of both epidermal growth factor receptor and Erk1/2 pathways.