Several studies have indicated that ICT induces activation o

Several studies have indicated that ICT induces activation of ERK and p38 kinase in a variety of cells. The ERK pathway was involved in and partly contributed to the neuroprotective effects in rat neuronal vps34 [14] and anticancer effects in several cancer cells [5], [7], [8]. ICT could also simultaneously affect the activity of ERK and p38 in embryonic stem cells (ESC) [16], [35]. In this study, we found that ICT could induce the differentiation of MC3T3-E1 subclone14 cells with p38 and ERK1/2 phosphorylation but not JNK phosphorylation at the same time point. The ERK1/2 antagonist PD98059 and the p38 antagonist SB203580 could significantly block MC3T3-E1 subclone14 cell differentiation, which demonstrated that the MAPK (p38 and ERK1/2) signal was involved in the ICT-induced osteoblastic differentiation of MC3T3 E1 cells. Crosstalk between the ER and the MAPK signaling pathway has been described in various experimental models [36], [37]. In mouse ESC, ICT can suppress p38 phosphorylation and sustain ERK phosphorylation simultaneously in an ER-independent manner [16]. ICT also stimulates the ERK signaling pathway without involvement of ERs to induce growth inhibition and apoptosis in human prostatic smooth muscle cells [10]. A potential neuroprotective effect of ICT against Abeta (25–35) may partially contribute to ERK phosphorylation in an ER-dependent manner [14]. We investigated whether the ICT-induced MAPK activation occurs downstream or upstream of the ER pathway, or if the two pathways are independently stimulated. We found that ER antagonist ICI182780 could significantly reduce ICT-induced ERK and p38 phosphorylation and activation; therefore, we suggest that MAPK pathways may be downstream of the ER pathway in the differentiation of MC3T3 E1 cells. Further experiments are required to distinguish which receptor (membrane receptor or nuclear receptor) is more important in this process. Interestingly, we found that ICI182780 cannot completely block ICT-induced ERK1/2 or p38 phosphorylation; whether MAPK is also activated by other signaling pathways is still unknown.
Conflict of interest
Acknowledgements This work was supported by the National Natural Science Foundation of China (81673837), the Natural Science Foundation of Guangdong Province (S2012040007531) and Science and Technology Planning Project of Guangdong Province (2016A020226039). We also thank American Journal Experts for English expression polished.
Introduction Aberrations in post-translational modifications by ubiquitin or ubiquitin-like proteins (Ubl), such as the small ubiquitin-like modifiers (SUMO), are associated with the pathogenesis of life-threatening diseases, such as cancer (Sarge and Park-Sarge, 2011, Zhu et al., 2010), neurodegenerative disorders (Steffan et al., 2004, Subramaniam et al., 2009), and viral infection (Jaber et al., 2009, Kim et al., 2010). For example, multiple studies indicate that SUMOylation is dysregulated in many types of cancers and that the SUMO-activating enzyme (SAE, SUMO E1) could be a potential target to inhibit c-Myc- and KRas-dependent oncogenesis (He et al., 2017, Kessler et al., 2012, Luo et al., 2009, Yu et al., 2015) and reduce cancer cell stemness and resistance (Bogachek et al., 2016, Du et al., 2016). The activating enzyme catalyzing ubiquitin-like Atg8 and Atg12 modifications in autophagy, known as Atg7, has been shown as an indirect target for KRas-dependent oncogenesis (Guo et al., 2013, Rosenfeldt et al., 2013). Despite the importance of Ubl modifications in dysregulated signaling pathways in diseased cells, only a handful of U.S. Food and Drug Administration-approved drugs targeting this type of post-translational modifications have been developed. This deficiency illustrates knowledge gaps in targeting these enzymes by small molecules and underscores the need to discover novel chemotypes and mechanisms to inhibit Ubl modifications. An Ubl modification requires several steps that are catalyzed by three enzymes, referred to as E1 (activating enzyme), E2 (conjugation enzyme), and E3 (ligase). The SUMO E1 is a heterodimer of SAE1 and Uba2 (also known as SAE2). In brief, an Ubl is first activated by E1 through ATP hydrolysis and forms a thioester conjugate with E1. The Ubl is then transferred to E2, forming a thioester conjugate with E2. Finally, the Ubl is transferred to target proteins, a step usually catalyzed by an E3. Usually, Ubl modifications add new docking sites to target proteins. For example, SUMO modifications enables new protein-protein interactions through the SUMO-interacting motif in receptor proteins (Song et al., 2004, Song et al., 2005). At least three members of the SUMO family (SUMO1, 2, and 3) are ubiquitin-like proteins that can conjugate to other cellular proteins by a biochemical mechanism similar to ubiquitylation (Hay, 2005, Sarge and Park-Sarge, 2009, Yeh, 2009). Currently, the only known mechanism to inhibit the E1 enzymes targets their ATP-binding sites (Brownell et al., 2010, Soucy et al., 2009).