The small molecule SB an inhibitor of p mitogen activated
The small molecule SB203580, an inhibitor of p38α mitogen activated protein kinase (MAPK) , has also been reported to increase cardiomyocyte yield by over 2-folds . Thus, the p38α MAPK pathway has been proposed as a second signaling cascade that TP-0903 can be manipulated to improve cardiac differentiation protocols.
Recently we investigated the cardiomyogenic activities of a series of 2,4,5-trisubstituted imidazole analogues of SB203580 . In that study, we demonstrated that the cardiomyogenic activities of 2,4,5-trisubstituted imidazoles are sensitive to changes in the substituent at the C2 carbon position on imidazole. However, in our report we were unable to establish a correlation between p38α MAPK inhibition and cardiomyogenesis. This may be due to the fact that major off targets of SB203580 are the JNKs pathway kinases . A clear relationship between p38/MAPK, JNK and cardiomyogenesis could not be established either . More importantly, the SB203580 molecule has been demonstrated to inhibit the kinase activity of casein kinase 1 (CK1) , a known participant in the WNT signaling cascade .
Material and methods
Discussion and conclusion SB203580 is a 2,4,5-trisubstituted azole and a potent stress pathway inhibitor. Its cardiogenic potential in hESC differentiation suggested that timed inhibition of the p38α MAPK stress pathway is important for cardiomyocyte differentiation . However our studies on the differentiation of several hESC lines with a series of 2,4,5-trisubstituted azoles showed that cardiogenesis did not correlate with their p38α MAPK inhibitory activities. Furthermore, non-2,4,5-trisubstituted azole-based p38α MAPK inhibitors failed to induce cardiogenesis, casting further doubt on the importance of p38α MAPK inhibition in cardiogenesis. Other studies have shown that p38α MAPK directly phosphorylates substrates such as MAPKAPK2, ATF-2 and MEF2C , , . We showed that the 2,4,5-trisubstituted azole TA-02 at 5μM concentration induced cardiogenesis, but also increased ATF-2 phosphorylation and MEF2C õexpression in contrast to what would be expected with a mechanism dependent on p38α MAPK inhibition. Hence, we postulated that another mechanism other than p38α MAPK inhibition was the driving force behind cardiogenesis. Consistent with this hypothesis, other studies have shown that p38α MAPK activity is necessary to generate cardiomyocytes during differentiation . A study with p38α MAPK mouse knock-out models showed inhibition of cardiogenesis and direction towards neural differentiation . p38α MAPK activity is also required for mesoderm development , and the development of the cardiac phenotype is further channeled via ATF-2 signaling  and MEF2C expression . In fact, it is likely that cardiogenic p38α MAPK inhibitors such as SB203580 do not completely inhibit p38α MAPK at the lower concentrations found to promote cardiogenesis. It has been shown that p38α MAPK inhibition by SB203580 is dose-dependent and incomplete when applied at 5μM in fibroblast and stem cell cultures , . In our previous study , we showed that higher concentrations of SB203580 (>15μM) inhibit cardiac development, suggesting that a concentration of SB203580 sufficient to completely inhibit p38α MAPK also blocks cardiogenesis. In support of this idea, our current study showed that more potent p38α MAPK inhibitors (e.g. TA-01) inhibit cardiac development when applied at 5μM from days 1–8. Further, we were able to demonstrate that mesoderm development was impaired when 2,4,5-trisubstituted azoles were applied during early EB development. Thus, it is likely that complete p38α MAPK inhibition restricts cardiac differentiation, especially when applied during the early mesoderm stage.