RELB Alters Proliferation of Human Pluripotent Stem Cells via IMP3- and LIN28-Mediated Modulation of the Expression of IGF2 and Other Cell-Cycle Regulators

Nilay Yogeshkumar Thakar, Dmitry Alexander Ovchinnikov, Marcus Lachlan Hastie, Jeffrey Gorman, Ernst Jurgen Wolvetang

The molecular mechanisms that orchestrate the exit from pluripotency, cell cycle progression and lineage-specific differentiation in human pluripotent stem cells (hPSCs) are poorly understood. RELB, a key protein in the non-canonical NFκB signalling pathway, was previously implicated in controlling the switch between human embryonic stem cell (hESC) proliferation and differentiation. Here we show that RELB enhances the proliferation of hESCs and human induced pluripotent stem cells (hiPSCs) without affecting their pluripotency. We demonstrate that RELB does this by interacting with 2 RNA-binding proteins LIN28A and IMP3 (IGF-2 mRNA-binding protein 3); further, these interactions control mRNA levels and protein expression of IGF-2 and key cell-cycle genes. Finally, following stress, these proteins co-localize in stress granules in hESCs and iPSCs. Our data identify RELB as a novel regulator of hPSC proliferation, and suggest a new function for RELB, additional to its widely-accepted role as a transcription factor, that involves recruitment of IMP3 and LIN28 to the cytosolic mRNA translation-control domains for post-transcriptional modulation of IGF2 and cell cycle gene expression.