JAK1–STAT1–STAT3, a key pathway promoting proliferation and preventing premature differentiation of myoblasts

Abstract

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.

Oncostatin M inhibits myoblast differentiation and regulates muscle regeneration

“…The supershift assays with STAT1-, STAT2-, or STAT3-specific antibodies revealed that the complexes were mainly composed of STAT3 homodimers, STAT1/STAT3 heterodimers, and a small amount of STAT1 homodimers (lanes 5-7). These STAT complexes were similar to those induced by LIF or IL-6 [41,48].…”

“…The Notch pathway has been shown to function during this period [25]. In addition, our recent studies showed that a pathway consisting of JAK1, STAT1, and STAT3 in myoblasts also functions at this stage, as the pathway inhibits myoblast differentiation and is activated at 1 day after injury [41]. Moreover, maximal induction of the OSM gene at 1 day after the injury perfectly correlates with the timing of activation of the JAK1/STAT1/ STAT3 pathway in regenerating muscles.…”

“…Both age-matched (i.e., 6-to 8-week-old) and sex-matched mice were used for this experiment, and CTX was applied to both left and right legs of each mouse. In this experimental scheme, the myogenin gene was consistently induced to the maximal level at day 3 after injury ( Figure 7A) [41,63]. The expression of the OSM mRNA was undetectable before injury.…”

“…It is generally believed that the infiltrating inflammatory cells, especially macrophages, secret diffusible factors (e.g., cytokines or chemokines) that in turn bind to receptors on the surface of quiescent MSCs to trigger their activation and proliferation [14,15]. In the CTX-induced muscle regeneration model employed in this study, the majority of the satellite cellderived myoblasts start to differentiate on day 3 after CTX injection, which was evident by a maximal expression of the myogenin gene ( Figure 7A) [41,63]. This suggests that most of the satellite cells undergo activation and proliferation within the first 2 days after injury.…”

“…We recently showed that LIF potently represses myogenic differentiation by activating the JAK1/STAT1/ STAT3 pathway [41]. As both OSM and LIF are members of the IL-6 family, we hypothesized that OSM may also exert its inhibitory effect on myoblasts via the same pathway.…”