SOX2 represses c-MYC transcription by altering the co-activator landscape of the c-MYC super-enhancer and promoter regions.

Our previous studies determined that elevating SOX2 in a wide range of tumor cells leads to a reversible state of tumor growth arrest. Efforts to understand how tumor cell growth is inhibited led to the discovery of a SOX2:MYC axis that is responsible for downregulating c-MYC (MYC) when SOX2 is elevated. Although we had determined that elevating SOX2 downregulates MYC transcription, the mechanism responsible was not determined. Given the challenges of targeting MYC clinically, we set out to identify how elevating SOX2 downregulates MYC transcription. In this study, we focused on the MYC promoter region and an upstream region of the MYC locus that contains a MYC super-enhancer encompassing five MYC enhancers and which is associated with several cancers. Here we report that BRD4 and p300 associate with each of the MYC enhancers in the upstream MYC super-enhancer as well as the MYC promoter region and that elevating SOX2 decreases the recruitment of BRD4 and p300 to these sites. Additionally, we determined that elevating SOX2 leads to increases in the association of SOX2 and H3K27me3 within the MYC super-enhancer and the promoter region of MYC. Importantly, we conclude that the increases in SOX2 within the MYC super-enhancer precipitate a cascade of events that culminates in the repression of MYC transcription. Together, our studies identify a novel molecular mechanism able to regulate MYC transcription in two distinctly different tumor types and provide new mechanistic insights into the molecular interrelationships between two master regulators, SOX2 and MYC, widely involved in multiple cancers.

Sox2 expression is regulated by a negative feedback loop in embryonic stem cells that involves AKT signaling and FoxO1.

The self-renewal and pluripotency of embryonic stem cells (ESC) is regulated by a highly integrated network of essential transcription factors, which includes Sox2. Previous studies have shown that elevating Sox2 on its own in mouse ESC induces differentiation and inhibits the expression of endogenous Sox2 at the protein and mRNA level. These findings led us to hypothesize that increases in Sox2 activate a negative feedback loop that inhibits the transcription of the endogenous Sox2 gene. To test this hypothesis, we used i-OSKM-ESC, which elevate Sox2 in conjunction with Oct4, Klf4, and c-Myc when treated with doxycycline (Dox). Elevating the expression of these four transcription factors in i-OSKM-ESC does not induce differentiation, but it represses expression of endogenous Sox2. We determined that increases of Sox2 in i-OSKM-ESC lead to increases in activated AKT and inactivation of FoxO1 (an activator of Sox2), as well as decreases in binding of FoxO1 to the 5'flanking region of Sox2. Importantly, we determined that inhibition of AKT in Dox-treated i-OSKM-ESC leads to re-expression of endogenous Sox2 at the mRNA and protein level and reactivation of FoxO1. These findings argue that AKT signaling is part of the negative feedback loop that helps carefully control the transcription of Sox2 in ESC by modulating the binding of FoxO1 to the Sox2 gene. Collectively, our findings provide new insights into the mechanisms that enable ESC to carefully regulate the levels of Sox2 and retain their stem cell properties.