Nutritional and hormonal factors control the gene expression of FoxOs, the mammalian homologues of DAF-16
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Transcription factors of the FoxO family in mammals are orthologues of the Caenorhabditis elegans forkhead factor DAF-16, which has been characterized as a target of insulin-like signalling. Three members of this family have been identified in rodents: FoxO1, FoxO3 and FoxO4, originally termed FKHR, FKHRL1 and AFX respectively. A number of in vitro studies have revealed that FoxOs are regulated through phosphorylation in response to insulin and related growth factors, resulting in their nuclear exclusion and inactivation. To clarify the mechanisms involved in the regulation of these factors in vivo, we investigated in the present study whether or not, and if so how, their mRNA levels in rat liver respond to the stimuli of several nutritional and hormonal factors. Imposed fasting for 48 h significantly elevated mRNA levels of FoxO1 (1.5-fold), FoxO3 (1.4-fold), and FoxO4 (1.6-fold). Refeeding for 3 h recovered the induced mRNA levels of FoxO1 and FoxO3 to the control levels, but did not affect that of FoxO4. FoxO1 and FoxO4 mRNA levels were proved to be highly reflective of their protein levels measured by Western immunoblotting. Of the three FoxO genes, FoxO4 only showed altered levels of mRNA (a 1.5-fold increase) in response to a protein-free diet. Streptozotocin-induced diabetes for 28 days decreased hepatic mRNA levels of FoxO1 and FoxO3 and increased the level of FoxO4 mRNA, but short-term (7 days) diabetes had fewer effects on the expression of these genes. Insulin replacement partially restored the FoxO1 and FoxO4 mRNA levels, but had no effect on the FoxO3 mRNA level. Daily administration for 1 week of dexamethasone, a synthetic glucocorticoid, increased the mRNA levels of FoxO1 (1.8-fold) and FoxO3 (2.4-fold). These results show that the FoxO genes respond differently to nutritional and hormonal factors, suggesting a new mechanism for the regulation of FoxO-dependent gene expression by these factors. Moreover, changes of FoxO1 and FoxO4 in the nucleus in response to fasting also suggest that the regulation of nucleus/cytoplasm translocation actually functions in vivo.
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