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Link between the Proteasome and Pluripotency Further Strengthened

Review of “Nrf2, a regulator of the proteasome, controls self-renewal and pluripotency in human embryonic stem cells” from Stem Cells by Stuart P. Atkinson

A link between the proteasome and pluripotency has recently been forged [1, 2], with elevated proteasomal activity in embryonic stem cells (ESCs) linked to both maintenance of the pluripotent state and cell fate decisions. The Nrf2 transcription factor, known to activate cellular defense against endogenous and exogenous stresses [3, 4], can also modulate proteasome subunit gene expression [5, 6], but any links between Nrf2 and ESCs still lie undiscovered. Now, in a study published in Stem Cells, researchers from the laboratory of Kenneth S. Kosik (University of California at Santa Barbara, USA) have identified Nrf2 as a novel pluripotency gene, through transcriptional regulation of proteasome subunits and, whilst doing so, have further strengthened the link between the proteasome and pluripotency [7].

Initial experiments showed that high levels of Nrf2 protein in human (h)ESCs and hiPSCs declined drastically during differentiation (See Figure), and downregulation of Nrf2 activity by siRNA or overexpression of KEAP1 (binds to Nrf2 and induces degradation) reduced the self-renewing activity of ESCs, while inhibition of Nrf2 repression during differentiation of ESCs led to the maintenance of a stem cell-like state. Additionally, Nrf2 siRNA treatment of somatic cells undergoing reprogramming reduced the efficiency of reprogramming, all in all, suggesting a potent link between Nrf2 and pluripotency. In relation to the proteasome, Nrf2 activation increased proteasome activity in hESCs and iPSCs, while inhibition of the proteasome itself led to an increased level of Nrf2 protein suggesting reciprocal regulation. Differentiation of hESCs leads to a downregulation in proteasomal activity, in line with Nrf2 downregulation, with proteasome-associated gene POMP, an essential molecular chaperone for proteasome assembly [8], being the most downregulated gene upon differentiation with kinetics similar to that of OCT4. POMP repression by siRNA in hESCs decreased proteasome activity, while POMP overexpression induced activity.  Nrf2 is known to bind the regulatory regions on the POMP gene [9], and Nrf2 activation during differentiation inhibited the observed downregulation of POMP expression. With these experiments, the authors have shown a highly interdependence between Nrf2, pluripotency and the proteasome.

Finally, the effects of proteasomal activity on self-renewal and pluripotency were assessed using specific proteasome inhibitors, which decreased pluripotency-associated gene expression, induced G2/M arrest and depletion of cells in the S phase, while decreasing levels of CyclinD1, which has been recently described as being important in determining the differentiation capacity of hESCs [10]. Inhibition of the proteasome during differentiation inhibited OCT4 and NANOG gene expression downregulation, while differentiation-associated gene expression was repressed. POMP downregulation was also assessed for its effect on reprogramming, finding that POMP inhibition reduced the amount of colonies formed, and cells of these colonies displayed characteristics of partially reprogrammed cells.

Overall this study suggests that the control of cellular stress, proteostasis and stemness all converge, with Nrf2 as a master regulator. Furthermore, it suggests that modulation of the proteasome may be a useful tool to generate specific therapeutically relevant cell types from pluripotent cells, and also may provide mechanism by which the protein homeostasis can be altered in diseases/disorders in which protein accumulation is a major problem.

 

References

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