Figure 2.

The MYC-driven self-reinforcing loop in ESCs. (A) MYC-dependent ESCs are maintained in presence of OHT, which activate MYC-ER, finally leading to its nuclear localization. In the nucleus, MYC-ER stimulates a self-reinforcing positive feedback loop. Together with PRC2, MYC represses antagonist of the WNT pathway, such as DKK and SFRP family members, thus resulting in activation of the WNT pathway. Consequently, active β-catenin translocates into the nucleus, where, among its targets, it transcriptionally activates both myc and mycn. Endogenous MYC and MYCN, in turn, reinforce this mechanism by contributing to the repression of WNT antagonists. The feedback positive loop is also sustained by the fact that MYC mediates the inhibition of the destruction complex (AXIN, APC, GSK3β) function, resulting in reduction of GSK3β-dependent degradation of both MYC/MYCN and β-catenin (Fagnocchi et al., 2016a). (B) Role of MYC in sustaining mESCs identity in MYC-dependent vs. 2i/LIF maintained cells. MYC-dependent cells rely on MYC for the concomitant down-regulation of MAPK/ERK and up-regulation of WNT pathways (Chappell et al., 2013; Fagnocchi et al., 2016a). In addition, it also sustains proliferation by inducing the biosynthetic machinery. Once MYC activity is lost, by either knocking down myc/mycn or by inactivating MYC-ER after short-term exposure to OHT, mESCs differentiate spontaneously as both the proliferation and the maintenance of pluripotency gene expression are affected (Cartwright et al., 2005; Fagnocchi et al., 2016a). On the contrary, in 2i/LIF cultured cells, chemical inhibitors bypass the role of MYC in modulating signaling pathways. Indeed, CHIR and PD activate and inhibit WNT and MAPK/ERK pathways, respectively (Cole et al., 2008; Ying et al., 2008). Accordingly, MYC is less expressed in 2i/LIF mESCs (Marks et al., 2012). Nonetheless, MYC still have a key function in 2i/LIF medium, as its deletion leads to impairment of the biosynthetic machinery and entrance of ESCs in a reversible dormant state (Scognamiglio et al., 2016). Concomitantly, the pluripotency is not affected as chemical inhibitors still modulate signaling to the core TRN even in the absence of MYC. Solid green and red arrows indicate high or low expression on nearby genes, respectively. Solid black arrows and flat lines indicate activation or repression, respectively. Red crosses indicate inhibition of related functions.