Skeletal muscle stem cell–derived myoblasts are mainly responsible for postnatal muscle growth and injury-induced muscle regeneration. However, the cellular signaling pathways controlling the proliferation and differentiation of myoblasts are not fully understood. We demonstrate that Janus kinase 1 (JAK1) is required for myoblast proliferation and that it also functions as a checkpoint to prevent myoblasts from premature differentiation. Deliberate knockdown of JAK1 in both primary and immortalized myoblasts induces precocious myogenic differentiation with a concomitant reduction in cell proliferation. This is caused, in part, by an accelerated induction of MyoD, myocyte enhancer–binding factor 2 (MEF2), p21Cip1, and p27Kip1, a faster down-regulation of Id1, and an increase in MEF2-dependent gene transcription. Downstream of JAK1, of all the signal transducer and activator of transcriptions (STATs) present in myoblasts, we find that only STAT1 knockdown promotes myogenic differentiation in both primary and immortalized myoblasts. Leukemia inhibitory factor stimulates myoblast proliferation and represses differentiation via JAK1–STAT1–STAT3. Thus, JAK1–STAT1–STAT3 constitutes a signaling pathway that promotes myoblast proliferation and prevents premature myoblast differentiation.

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Skeletal muscle satellite cell-derived myoblasts are mainly responsible for postnatal muscle growth and injury-induced regeneration. However, the cellular signaling pathways that control proliferation and differentiation of myoblasts remain poorly defined. Recently, we found that JAK1/STAT1/STAT3 not only participate in myoblast proliferation but also actively prevent them from premature differentiation. Unexpectedly, we found that a related pathway consisting of JAK2, STAT2, and STAT3 is required for early myogenic differentiation. Interference of this pathway by either a small molecule inhibitor or small interfering RNA inhibits myogenic differentiation. Consistently, all three molecules are activated upon differentiation. The pro-differentiation effect of JAK2/STAT2/STAT3 is partially mediated by MyoD and MEF2. Interestingly, the expression of the IGF2 gene and the HGF gene is also regulated by JAK2/STAT2/STAT3, suggesting that this pathway could also promote differentiation by regulating signaling molecules known to be involved in myogenic differentiation. In summary, our current study reveals a novel role for the JAK2/STAT2/ STAT3 pathway in myogenic differentiation.

CFTR has been recognized to function as both an anion channel and a key regulator of Slc26 anion transporters in heterologous expression systems. Whether this regulatory relationship between CFTR and Slc26 transporters is seen in native intestine, and whether this effect is coupled to CFTR transport function or other features of this protein, has not been studied. The duodena of anesthetized CFTR-, NHE3-, Slc26a6-, and Scl26a3-deficient mice and wild-type (WT) littermates were perfused, and duodenal bicarbonate (HCO(3)(-)) secretion (DBS) and fluid absorptive or secretory rates were measured. The selective NHE3 inhibitor S1611 or genetic ablation of NHE3 significantly reduced fluid absorptive rates and increased DBS. Slc26a6 (PAT1) or Slc26a3 (DRA) ablation reduced the S1611-induced DBS increase and reduced fluid absorptive rates, suggesting that the effect of S1611 or NHE3 ablation on HCO(3)(-) secretion may be an unmasking of Slc26a6- and Slc26a3-mediated Cl(-)/HCO(3)(-) exchange activity. In the absence of CFTR expression or after application of the CFTR(inh)-172, fluid absorptive rates were similar to those of WT, but S1611 induced virtually no increase in DBS, demonstrating that CFTR transport activity, and not just its presence, is required for Slc26-mediated duodenal HCO(3)(-) secretion. A functionally active CFTR is an absolute requirement for Slc26-mediated duodenal HCO(3)(-) secretion, but not for Slc26-mediated fluid absorption, in which these transporters operate in conjunction with the Na(+)/H(+) exchanger NHE3. This suggests that Slc26a6 and Slc26a3 need proton recycling via NHE3 to operate in the Cl(-) absorptive mode and Cl(-) exit via CFTR to operate in the HCO(3)(-) secretory mode.

Asthma is a chronic condition with unknown pathogenesis, and recent evidence suggests that enhanced airway epithelial chloride (Cl -) secretion plays a role in the disease. However, the molecular mechanism underlying Cl -secretion and its relevance in asthma pathophysiology remain unknown. To determine the role of the solute carrier family 26, member 9 (SLC26A9) Cl -channel in asthma, we induced Th2-mediated inflammation via IL-13 treatment in wild-type and Slc26a9-deficient mice and compared the effects on airway ion transport, morphology, and mucus content. We found that IL-13 treatment increased Cl -secretion in the airways of wildtype but not Slc26a9-deficient mice. While IL-13-induced mucus overproduction was similar in both strains, treated Slc26a9-deficient mice exhibited airway mucus obstruction, which did not occur in wild-type controls. In a study involving healthy children and asthmatics, a polymorphism in the 3′ UTR of SLC26A9 that reduced protein expression in vitro was associated with asthma. Our data demonstrate that the SLC26A9 Cl -channel is activated in airway inflammation and suggest that SLC26A9-mediated Cl -secretion is essential for preventing airway obstruction in allergic airway disease. These results indicate that SLC26A9 may serve as a therapeutic target for airway diseases associated with mucus plugging.

Deletion of DRA results in severely reduced colonic HCO3 (-) secretory rate, a loss of colonic fluid absorption, a lack of a firmly adherent mucus layer and a severely reduced colonic mucosal resistance to DSS damage. These data provide potential pathophysiological explanations for the increased susceptibility of CLD patients to intestinal inflammation.

Abstract-After earlier studies in which secretion of aldosterone was demonstrated to be important in rat arterial smooth muscle cell (RASMC) proliferation in vitro, the presence of both 11␤-hydroxylase (CYP11B1) and aldosterone synthase (CYP11B2) gene transcription were shown in these cells by real-time reverse transcription-polymerase chain reaction (RT-PCR). In proliferation studies, tritiated thymidine incorporation into RASMC and RASMC cell number were both significantly increased by angiotensin II (Ang II) (10 Ϫ7 mol/L) compared with controls (PϽ0.01), but this effect was inhibited by the 3␤-hydroxysteroid-dehydrogenase inhibitor trilostane (10 Ϫ6 mol/L and 10 Ϫ5 mol/L, PϽ0.05). Aldosterone alone added to RASMC did not significantly change tritiated thymidine incorporation when compared with controls, but the Ang II-induced increase was significantly enhanced by aldosterone at 10 Ϫ10 mol/L and 10 Ϫ8 mol/L (PϽ0.05). Neither corticosterone nor 18-hydroxydeoxycorticosterone had any such potentiating effect. RT-PCR analysis and real-time quantitative RT-PCR revealed an increase of Ang II type-1 (AT 1 ) receptor mRNA in RASMC treated by aldosterone (10 Ϫ8 mol/L) compared with untreated controls, and this was correlated with a small but significant increase in AT 1 receptor protein (PϽ0.05), as assessed by immunoblotting analysis. These data confirm that steroid production by RASMC is critical in the response to Ang II, and the data support the view that aldosterone specifically is required for the full proliferative response to Ang II in RASMC. One way it may act is by modulating the expression and functions of the AT 1 receptor.