Forkhead Transcription Factor FOXO3a Research Articles

Regulation of sterol carrier protein gene expression by the Forkhead transcription factor FOXO3a

The SCP gene encodes two proteins, sterol carrier protein X (SCPx) and SCP2, that are independently regulated by separate promoters. SCPx has been shown to be the thiolase involved in the breakdown of branched-chain fatty acids and in the biosynthesis of bile acids. The in vivo function of SCP2 however remains to be established. The transcriptional regulation of SCPx and SCP2 is unclear, but their promoter regions contain several putative regulatory domains. We show here that both SCPx and SCP2 are upregulated by the daf-16-like Forkhead transcription factor FOXO3a (also known as FKHRL1) on the level of promoter activity. It was recently described that Forkheads regulate protection against (oxidative) stress in both Caenorhabditis elegans and mammalian cells. We looked into a role for SCP2 in the cellular defense against oxidative damage and found that a fluorescent fatty acid analog bound to SCP2 is protected against H2O2/Cu2+-induced oxidative damage. We propose a model for the way in which SCP2 could protect fatty acids from peroxidation.

The Akt-regulated Forkhead Transcription Factor FOXO3a Controls Endothelial Cell Viability through Modulation of the Caspase-8 Inhibitor FLIP

FLICE-inhibitory protein (FLIP) is a homolog of caspase-8 that lacks catalytic activity and has been shown to be important in protecting endothelial cells from apoptosis. The serine/threonine kinase Akt/PKB was recently reported to promote FLIP expression in endothelial and tumor cells. Here we examined the role of the forkhead transcription factor FOXO3a, a downstream target of Akt, in controlling FLIP regulation in endothelial cells. FOXO3a nuclear translocation was regulated by Akt in human umbilical vein endothelial cells. Transduction of a nonphosphorylatable, constitutively active mutant of FOXO3a (TM-FOXO3a) led to the down-regulation of FLIP levels. Transduction with TM-FOXO3a also increased caspase-8 activity and promoted apoptosis in endothelial cells. Conversely, transduction of a dominant-negative mutant of FOXO3a up-regulated FLIP levels and protected endothelial cells from apoptosis under serum deprivation conditions. Restoration of intracellular FLIP blocked caspase-8 activation and inhibited apoptosis in TM-FOXO3a-transduced cells. These data suggest that FOXO3a is a downstream target of Akt in endothelial cells that can promote apoptosis via FLIP down-regulation and activation of the extrinsic apoptotic pathway.

Proteolytic regulation of Forkhead transcription factor FOXO3a by caspase-3-like proteases.

Forkhead family transcription factors are critical regulators of cell cycle progression and apoptosis in hematopoietic cells. Here, we show that FOXO3a (also known as FKHRL1) is a new substrate of caspase-3-like proteases during apoptosis in T lymphocytes. FOXO3a was cleaved in vivo by caspases in leukemic Jurkat cells following engagement of Fas (CD95) receptor, staurosporine, and etoposide treatment, but not following engagement of CD99, a caspase-independent cell death inducer. Caspase-mediated cleavage of FOXO3a was also observed in CD4+ peripheral T cells subjected to activation-induced cell death. The expression of the death adapter FADD and caspase-8 was required for Fas-induced FOXO3a cleavage, but activation of survival pathways by overexpression of FLICE-inhibitory protein or phorbol myristate acetate treatment prevented it. FOXO3a was cleaved in vitro by caspase-3-like proteases at the consensus sequence DELD304A, releasing the N-terminal DNA-binding domain of FOXO3a from its C-terminal transactivating domain. Whereas full-length FOXO3a enhanced Forkhead response element-dependent transcription and apoptosis in Jurkat cells, both fragments were inactive to promote gene activation and cell death. In contrast, a caspase-resistant FOXO3a mutant exhibited enhanced transcriptional and proapoptotic activities. Together, these results indicate that the proteolytic cleavage of FOXO3a by caspases may represent a novel regulatory mechanism of FOXO3a activity during death receptors signaling.

Too Much, Too Soon

In female mammals, the initiation of ovarian follicular growth is somehow regulated to ensure that mature follicles are produced at each cycle. Castrillon et al. now show that female mice deficient in the Forkhead transcription factor Foxo3a displayed an age-related loss of fertility that was caused by extensive activation of ovarian follicles prior to sexual maturity. These results raise the possibility that accelerated follicular initiation contributes to premature ovarian failure, a common cause of infertility in women. D. H. Castrillon, L. Miao, R. Kollipara, J. W. Horner, R. A. DePinho, Suppression of ovarian follicle activation in mice by the transcription factor Foxo3a. Science 301 , 215-218 (2003). [Abstract] [Full Text]

Cell cycle inhibition by FoxO forkhead transcription factors involves downregulation of cyclin D.

The FoxO forkhead transcription factors FoxO4 (AFX), FoxO3a (FKHR.L1), and FoxO1a (FKHR) represent important physiological targets of phosphatidylinositol-3 kinase (PI3K)/protein kinase B (PKB) signaling. Overexpression or conditional activation of FoxO factors is able to antagonize many responses to constitutive PI3K/PKB activation including its effect on cellular proliferation. It was previously shown that the FoxO-induced cell cycle arrest is partially mediated by enhanced transcription and protein expression of the cyclin-dependent kinase inhibitor p27(kip1) (R. H. Medema, G. J. Kops, J. L. Bos, and B. M. Burgering, Nature 404:782-787, 2000). Here we have identified a p27(kip1)-independent mechanism that plays an important role in the antiproliferative effect of FoxO factors. Forced expression or conditional activation of FoxO factors leads to reduced protein expression of the D-type cyclins D1 and D2 and is associated with an impaired capacity of CDK4 to phosphorylate and inactivate the S-phase repressor pRb. Downregulation of D-type cyclins involves a transcriptional repression mechanism and does not require p27(kip1) function. Ectopic expression of cyclin D1 can partially overcome FoxO factor-induced cell cycle arrest, demonstrating that downregulation of D-type cyclins represents a physiologically relevant mechanism of FoxO-induced cell cycle inhibition.

Forkhead transcription factor FOXO3a protects quiescent cells from oxidative stress.

Reactive oxygen species are required for cell proliferation but can also induce apoptosis. In proliferating cells this paradox is solved by the activation of protein kinase B (PKB; also known as c-Akt), which protects cells from apoptosis. By contrast, it is unknown how quiescent cells that lack PKB activity are protected against cell death induced by reactive oxygen species. Here we show that the PKB-regulated Forkhead transcription factor FOXO3a (also known as FKHR-L1) protects quiescent cells from oxidative stress by directly increasing their quantities of manganese superoxide dismutase (MnSOD) messenger RNA and protein. This increase in protection from reactive oxygen species antagonizes apoptosis caused by glucose deprivation. In quiescent cells that lack the protective mechanism of PKB-mediated signalling, an alternative mechanism is induced as a consequence of PKB inactivity. This mechanism entails the activation of Forkhead transcription factors, the transcriptional activation of MnSOD and the subsequent reduction of reactive oxygen species. Increased resistance to oxidative stress is associated with longevity. The model of Forkhead involvement in regulating longevity stems from genetic analysis in Caenorhabditis elegans, and we conclude that this model also extends to mammalian systems.

Other Noteworthy Papers This Week (24 April 2002)

The CHEK2 -Breast Cancer Consortium, Low-penetrance susceptibility to breast cancer due to CHEK2 * 1100delC in noncarriers of BRCA1 or BRCA2 mutations. Nat. Genet. , 22 April 2002 [e-pub ahead of print]. [Abstract] [Full Text] H. Tran, A. Brunet, J. M. Grenier, S. R. Datta, A. J. Fornace Jr., P. S. DiStefano, L. W. Chiang, M. E. Greenberg, DNA repair pathway stimulated by the forkhead transcription factor FOXO3a through the Gadd45 protein. Science 296 , 530-534 (2002). [Abstract] [Full Text]

Damage Control

In the nematode Caenorhabditis elegans , activation of a member of the so-called "forkhead" family of transcription factors can cause an increase in life-span of the organism, perhaps by promoting resistance to oxidative damage or other forms of stress. Tran et al. examined the role of mammalian forkhead family member FOXO3a in stress responses and found that activation of the transcription factor caused a delay in the cell cycle and promoted repair of damaged DNA. Analysis of the transcriptional profile of cells expressing activated FOXO3a revealed a group of genes implicated in cellular stress responses and DNA repair. The results indicate that forkhead transcription factors are key regulators of stress responses in mammals, as well as in worms, and could conceivably influence organismal life-span in this manner. H. Tran, A. Brunet, J. M. Grenier, S. R. Datta, A. J. Fornace Jr., P. S. DiStefano, L. W. Chiang, M. E. Greenberg, DNA repair pathway stimulated by the Forkhead transcription factor FOXO3a through the Gadd45 protein. Science 296 , 530-534 (2002). [Abstract] [Full Text]

DNA repair pathway stimulated by the forkhead transcription factor FOXO3a through the Gadd45 protein.

The signaling pathway from phosphoinositide 3-kinase to the protein kinase Akt controls organismal life-span in invertebrates and cell survival and proliferation in mammals by inhibiting the activity of members of the FOXO family of transcription factors. We show that mammalian FOXO3a also functions at the G2 to M checkpoint in the cell cycle and triggers the repair of damaged DNA. By gene array analysis, FOXO3a was found to modulate the expression of several genes that regulate the cellular response to stress at the G2-M checkpoint. The growth arrest and DNA damage response gene Gadd45a appeared to be a direct target of FOXO3a that mediates part of FOXO3a's effects on DNA repair. These findings indicate that in mammals FOXO3a regulates the resistance of cells to stress by inducing DNA repair and thereby may also affect organismal life-span.