GSK3β promotes p53/Nrf2-dependent expression of the stress response protein REDD2 in retinal Müller glia exposed to hyperlipidemic conditions.

The stress response proteins regulated in development and DNA damage (REDD)1 and REDD2 act as negative regulators of mechanistic target of rapamycin complex 1 (mTORC1). While the role of REDD1 in diabetes complications in the retina has been well-explored, the potential contribution of REDD2 has not been previously examined. In mice fed a pro-diabetogenic high-fat diet, REDD2 mRNA ribosome-association was increased in retinal Müller glia. Hyperlipidemic culture conditions also increased both REDD1 and REDD2 mRNA expression in human Müller cell cultures. Mechanistic studies identified key regulatory residues in REDD2 at P100 and K179/Y182 that were necessary for mTORC1 suppression. In Müller cells exposed to hyperlipidemic conditions, REDD1 and REDD2 mRNA expression were upregulated in coordination with markers of ER stress. However, chemical induction of ER stress with tunicamycin increased REDD1, but not REDD2. Rather, increased REDD2 mRNA expression in Müller cells exposed to hyperlipidemic conditions required the transcription factors p53 and nuclear factor erythroid 2-related factor 2 (Nrf2). Unlike the Nrf2-target heme oxygenase 1 (HO-1), the effect of Nrf2 on REDD2 was redox-independent, as REDD2 expression was insensitive to the antioxidant N-acetylcysteine, the Nrf2 agonist sulforaphane, or oxidant stress. Hyperlipidemic conditions attenuated the inhibitory phosphorylation of glycogen synthase kinase 3β (GSK3β) and GSK3β inhibition suppressed REDD2 mRNA expression under hyperlipidemic conditions. Expression of a constitutively active GSK3β variant also promoted REDD2 mRNA expression in a manner that required both p53 and Nrf2. The findings support that GSK3β promotes REDD2 mRNA transcription in Müller glia under hyperlipidemic conditions via activation of p53/Nrf2.