Franny Jongbloed

130 CHAPTER 5 First, cell metabolism is altered as indicated by the nuclear receptor signaling response via the activation of the retinol pathway. Nuclear receptors are transcription factors that are known to be activated by steroid hormones and lipi-soluble agents, such as the retinoid acids (RAs) 20 . As previously shown, RAs are able to induce many of the beneficial effects observed after DR. RAs are able to protect from ischemic stroke in the brain and have a positive effect on insulin resistance 21-24 . This activation is in line with our previous results, where nuclear receptor signaling activation was proposed as one of the main contributors to the beneficial effects of DR against renal IRI 17 . In our renal IRI model, upstream transcription factor FOXO3 was identified as a main player of the activation of the nuclear receptor response. FOXO3 was upregulated after all three EAA-free diets, however only reached statistical significance after the Trp-free diet. Besides cell metabolism, FOXO3 phosphorylation and activation is at the basis of many processes, most importantly stress resistance and autophagy 25,26 . These data strengthen the role of nuclear receptor signaling mediated by FOXO3, however excludes FOXO3 as the only contributor for the beneficial effects. For instance, the LXR/RXR pathway has been implicated in the response as well 27 . The liver X receptor (LXR) is mainly expressed in the liver and is an important factor of its lipid metabolism 28 . However, we found an inhibition of this pathway in our EAA-free analysis. The downregulation of the LXR/RXR pathway might in part be a response to the massive downregulation of SREBF2 by all EAA-free diets, which is a main regulator of the pathway 29 . Blocking SREBF2 in a mouse model showed to be effective in preventing the development of hepatic steatosis and insulin resistance, and might be more important than the activation of the LXR/RXR pathway itself 27 . Our results suggest that inhibition of SREBF2 might play a role in the protective effect on hepatic IRI. Future studies with for instance SREBF2 -/- mouse models could highlight the effects of SREBF-2 deficiency on IRI. Second, we found two transcription factors involved with inhibition of cellular proliferation. FOXM1, which increases G2/M DNA damage checkpoint activity 30 , and SMARCB1 which inhibits cellular proliferation by mediating between various signaling pathways as part of the SWI/SNF complex. In recent work, SMARCB1 has been named often as a tumor suppressor via inhibition of cell growth 31,32 . Inhibition of cell growth is a common feature induced by DR regimens. We propose that the evolutionary adaptive response to DR that shifts resources from growth (metabolism and temporary withdrawal from the cell cycle) to maintenance occurs already after three days of essential amino acid deprivation 33 . The associated increase in stress resistance is rapidly induced and offers robust protection against hepatic IRI.