Phosphoinositide 3-Kinase Induces the Transcriptional Activity of MEF2 Proteins during Muscle Differentiation

Abstract

The activity of phosphoinositide 3-kinase (PI3-K) is essential for the differentiation of skeletal muscle cells by largely unknown mechanisms. Here we show that inhibition of PI3-K activity by the pharmacological agent LY294002 affects early processes of myoblast differentiation including the transcriptional activation of myogenin. Previous studies indicated that transcription of myogenin was dependent on MyoD and MEF2 proteins. We find that expression of a dominant negative form of PI3-K or growth in the presence of LY294002 inhibits cellular activity of MEF2 but not of MyoD. Evidence reveals that whereas MEF2 transcriptional activity is inhibited, its DNA binding activity remains unaffected. Recent studies demonstrated that phosphorylation by p38 mitogen-activated protein kinase (MAPK) induced transcriptional activity of MEF2 proteins. We show that the phosphorylation of MEF2 occurring during muscle differentiation is prevented if the activity of PI3-K is inhibited. However, our results also indicate that p38 MAPK is not affected by PI3-K in muscle cells. Nevertheless, p38 MAPK can substitute for PI3-K in the induction of MEF2 and muscle transcription. Together, these findings indicate that PI3-K affects skeletal muscle differentiation by inducing phosphorylation and transcriptional activity of MEF2 proteins in a parallel but distinct route from p38 MAPK.

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“…In addition, p38 has been shown to phosphorylate, and activate, some members of the MEF2 family (Zhao et al, 1999), and MEF2 binding sites in the BZLF1 promoter are also essential for BZLF1 transcriptional activation following various physiologic stimuli . Likewise, PI3 kinase has been shown to increase the transcriptional activity of at least some MEF2 family members (Tamir and Bengal, 2000), and can activate promoters through CRE sites (Wang et al, 1999). Finally, both gamma irradiation and chemotherapy induce activation of the cellular immediate-early protein EGR-1 (Datta et al, 1993;Lim et al, 1998), at least partially through a p38 kinase-dependent mechanism, and EGR-1 has been shown to bind and activate the BRLF1 promoter (Zalani et al, 1995).…”

Phosphatidylinositol 3-Kinase Regulates Bone Morphogenetic Protein-2 (BMP-2)-induced Myocyte Enhancer Factor 2A-dependent Transcription of BMP-2 Gene in Cardiomyocyte Precursor Cells

“…For example, activation of insulin-like growth factor-1 receptor tyrosine kinase induces myogenic differentiation via PI 3-kinase-dependent increase in MEF-2 transcriptional activity (12). To directly examine the effect of PI 3-kinase on MEF-2-dependent transcription in BMP-2 receptor serine threonine kinase activation, we constructed a reporter plasmid in which four copies of the MEF-2 DNA binding element were cloned upstream of the SV-40 basal promoter driving luciferase cDNA (Fig.…”

“…One of the major kinases that regulate MEF-2 transcriptional activity is the p38 mitogen-activated protein kinase (10). More recently, PI 3-kinase, which is activated by the receptor and nonreceptor tyrosine kinases, has been shown to regulate MEF-2 transcriptional activity and myogenesis (11,12). Thus PI 3-kinase is required for insulinlike growth factor-1 receptor tyrosine kinase-induced myogenesis in culture and during mouse embryogenesis (13,14).…”

Amphoterin Stimulates Myogenesis and Counteracts the Antimyogenic Factors Basic Fibroblast Growth Factor and S100B via RAGE Binding

“…RAGE engagement on myoblasts activates p38 MAPK via Cdc42-Rac-1-MKK6. Myogenic differentiation and myotube formation were shown to depend on activation of p38 MAPK and Akt (5,10,17,55,62,66). Treatment of WT myoblasts and L6/RAGE, L6/RAGE⌬cyto, and mock-transfected myoblasts with either SB203580 (an inhibitor of p38 MAPK) or LY294002 (an inhibitor of the Akt-activating kinase phosphatidylinositol 3-kinase) resulted in the blockade of myogenic differentiation and myotube formation (data not shown), in accordance with the crucial role played by these kinases in myogenesis.…”

“…However, IGF I and IGF II were reported to promote or inhibit myogenic differentiation depending on the absence or presence of TNF-␣, respectively (16), and down-regulation of nerve growth factor low-affinity receptor was shown to be required for myoblast terminal differentiation (12). Signaling pathways implicated in the transduction of the effects of these agents acting on myoblasts include (i) the mitogen-activated protein (MAP) kinase (MAPK) p38 and Akt, the activation of which is required for myogenesis (5,9,10,17,32,44,55,57,62,66); (ii) an NF-B-dependent pathway activated by cytokines such as TNF-␣, which interferes with myogenesis (30); (iii) a PW1-dependent, NF-Bindependent activation of caspases in the absence of apoptosis (8); (iv) the Ras-MEK-extracellular signal-regulated kinase (ERK) pathway, which suppresses myogenesis (4,42,43,61) but is required at a later stage of muscle differentiation (4); and (v) activation of inducible nitric oxide synthase via NF-B, which results in stimulation of myogenesis (25).Recently, we found that S100B, a member of a multigenic family of Ca 2ϩ -modulated proteins of the EF-hand type with both intracellular and extracellular regulatory activities (11,19), inhibited myoblast differentiation and myotube formation when administered to the rat myoblast cell line L6 (51). Inhibition of myogenesis was registered at picomolar doses of S100B and was reversible, pointing to S100B binding to a cell surface receptor with a relatively high affinity.…”