Regardless of the detailed mechanisms of metabolic regulatio
Regardless of the detailed mechanisms of metabolic regulation in ESCs, recent studies indicate that reprogramming MEFs into iPSCs involves dramatic changes in cell metabolism from oxidative phosphorylation to glycolysis (Folmes et al., 2011). Somewhat paradoxically, however, the early phase of the metabolic shift involves a burst in oxidative phosphorylation (Hawkins et al., 2016; Kida et al., 2015), which is mediated by estrogen-related receptors, when cell proliferation increases dramatically (Kida et al., 2015). This burst in oxidative phosphorylation generates reactive oxygen species (ROS), which in turn leads to the increased expression of nuclear factor erythroid-derived 2-like 2, nuclear factor κB, and AP-1, eventually increasing the level of HIF1α. Sequential activation of these transcription factors gradually diminishes oxidative phosphorylation and enhances glycolysis (Hawkins et al., 2016).
The metabolic shift elicited by KLF4-induced TCL1 during the transition from iPSCs(Low-K) to iPSCs(High-K) is probably distinct from, and preceded by, the ROS-induced changes in metabolism (Hawkins et al., 2016; Kida et al., 2015). Indeed, no difference in the cell proliferation rate was observed between iPSCs(Low-K) and iPSCs(High-K). Moreover, the protein level of HIF1α, which is regulated by proteolysis (Courtnay et al., 2015), and the mRNA levels of Pdk1 and Hk2, which are regulated by HIF1α (Semenza, 2010), remain essentially unchanged during this transition (Figure 6F and data not shown). Thus, the metabolic shift may involve a series of events, consisting of at least two phases, and LY2584702 respond appropriately to the ongoing metabolic demand in each phase of reprogramming. Notably, the latter metabolic shift probably initiates slightly earlier but largely concurs with the acquisition of full pluripotency when the metabolism in reprogrammed cells becomes indistinguishable from that of ESCs.
In this article, we revealed that one of the key functions of KLF4-induced TCL1 during reprogramming is to promote the metabolic shift from oxidative phosphorylation to glycolysis. Despite its role in a late stage of reprogramming, expression of TCL1 with the four reprogramming factors from the beginning of reprogramming results in faster and higher induction of pluripotency markers (Figures 3C–3E). Indeed, TCL1b1, another member of the TCL family (Hallas et al., 1999), also improves the efficiency of reprogramming (Khaw et al., 2015). Given that the metabolic shift in the earlier stage occurs via ROS production (Hawkins et al., 2016), it is tempting to hypothesize that expression of TCL1 throughout reprogramming may permit cells with lower ROS production to undergo the metabolic shift. This might minimize the exposure of cells to ROS during reprogramming, potentially reducing damage to the genome in the generated iPSCs.
Acknowledgments We thank T. Mizutani for providing pMCsΔYY1-IRES-GFP plasmid. We also thank T. Nishimura for technical assistance. This work was supported by JSPS KAKENHI grant numbers JP26870074, JP16K08610, and JP25460354 (to K.N., A.F., and K.H., respectively), Kato Memorial Bioscience Foundation (to K.N.), Inamori Foundation (to K.N.), and the Program to Disseminate Tenure Tracking System by MEXT (to K.N.).