S100B inhibits myoblast differentiation via activation of a Ras-MEK-ERK1/2 signaling pathway

We have recently reported that the Ca2+-modulated protein of the EF-hand type, S100B, inhibits rat L6 myoblast differentiation by interacting with an unknown receptor with a high affinity (Kd ~40 pM) and causing inactivation of p38 MAPK (1) and counteracts the promyogenic effect of RAGE activation by amphoterin (2). We show here that the S100B’s ability to inhibit myoblast differentiation depends on activation of the Ras- MEK-ERK1/2 pathway as S100B is unable to inhibit myogenesis in the presence of the MEK inhibitor, PD98059, or in myoblasts transfected with a dominant-negative mutant of Ras. Also, S100B stimulates NF-κB transcriptional activity dose-dependently up to 200 pM, this latter effect being negated in the presence of PD98059, NF-κB inhibitors (PDTC or the NF-κB super-repressor, I-κBαSR) or the antioxidant N-acetylcysteine. Yet, no obvious correlation could be ascertained between S100B effects on NF-κB activity and the S100B inhibitory effect on myogenesis. In fact, S100B is unable to stimulate NF-κB transcriptional activity at >200 pM (i.e., at doses that are still antimyogenic), suggesting that S100B mainly inhibits myogenesis via inactivation of p38 MAPK likely through persistent stimulation of Ras-MEK-ERK1/2. Finally, extracellular S100B inhibits the induction of the anti-proliferative factor, p21WAF1, the activity of which is required for myoblasts to exit from the cell cycle (3). Analyses are in progress to identify the factor(s) linking inhibition of p38 MAPK to inhibition of p21WAF1 induction in myoblasts exposed to pM doses of S100B under differentiation conditions, and functional correlates of stimulation of NF-κB transcriptional activity in myoblasts by low pM levels of S100B. 1. Sorci G et al (2003) Mol Cell Biol 23:4870-4881; 2. Sorci G et al (2004) Mol Cell Biol 24, 4880-4894; 3. Lee J et al (2002) BBRC 298:765-771.