FoxK2 is required for cellular proliferation and survival

FoxK2 is a forkhead transcription factor expressed ubiquitously in the developing murine central nervous system. Here we investigated the role of FoxK2 in vitro and focused on proliferation and cellular survival. Knockdown of FoxK2 results in a decrease in BrdU incorporation and H3 phosphorylation, suggesting attenuation of proliferation. In the absence of growth factors, FoxK2 knockdown results in a dramatic increase in caspase 3 activity and propidium iodide positive cells, indicative of cell death. Additionally, knockdown of FoxK2 results in an increase in the mRNA of Gadd45α, Gadd45γ, as well as an increase in the phosphorylation of the mTOR dependent kinase p70S6K. Rapamycin treatment completely blocked the increase in p70S6K and synergistically potentiated the decrease in H3 phosphorylation upon FoxK2 knockdown. To gain more insight into the proapoptotic effects upon FoxK2 knockdown we screened for changes in Bcl2 genes. Upon FoxK2 knockdown both Puma and Noxa were significantly upregulated. Both genes were not inhibited by rapamycin treatment, instead rapamycin increased Noxa mRNA. FoxK2 requirement in cellular survival is further emphasized by the fact that resistance to TGFβ-induced cell death was greatly diminished after FoxK2 knockdown. Overall our data suggest FoxK2 is required for proliferation and survival, that mTOR is part of a feedback loop partly compensating for FoxK2 loss, possibly by upregulating Gadd45s, whereas cell death upon FoxK2 loss is induced in a Bcl2 dependent manner via Puma and Noxa.

In control of biology: of mice, men and Foxes

Forkhead proteins comprise a highly conserved family of transcription factors, named after the original forkhead gene in Drosophila. To date, over 100 forkhead genes have been identified in a large variety of species, all sharing the evolutionary conserved 'forkhead' DNA-binding domain, and the cloning and characterization of forkhead genes have continued in recent years. Forkhead transcription factors regulate the expression of countless genes downstream of important signalling pathways in most, if not all, tissues and cell types. Recent work has provided novel insights into the mechanisms that contribute to their functional diversity, including functional protein domains and interactions of forkheads with other transcription factors. Studies using loss- and gain-of-function models have elucidated the role of forkhead factors in developmental biology and cellular functions such as metabolism, cell division and cell survival. The importance of forkhead transcription factors is underlined by the developmental defects observed in mutant model organisms, and multiple human disorders and cancers which can be attributed to mutations within members of the forkhead gene family. This review provides a comprehensive overview of current knowledge on forkhead transcription factors, from structural organization and regulatory mechanisms to cellular and developmental functions in mice and humans. Finally, we will discuss how novel insights gained from involvement of 'Foxes' in the mechanisms underlying human pathology may create new opportunities for treatment strategies.

Identification of forkhead transcription factors in cortical and dopaminergic areas of the adult murine brain

The murine forkhead family of transcription factors consists of over 30 members, the vast majority of which is important in embryonic development. Implicated in processes such as proliferation, differentiation and survival, forkhead factors show highly restricted expression patterns. In search for forkhead genes expressed in specific neural systems, we identified multiple family members. We performed a detailed expression analysis for Foxj2, Foxk1 and the murine orthologue of the human ILF1 gene, which show a remarkable preference for complex cortical structures. In addition, a comprehensive examination of forkhead gene expression in dopamine neurons of the ventral tegmental area and substantia nigra pars compacta, revealed Ilf1 as a novel transcriptional regulator in midbrain dopamine neurons. These forkhead transcription factors may play a role in maintenance and survival of developing and adult neurons.

Cloning and analysis of the murine Foxi2 transcription factor

Forkhead transcription factors comprise a large family of key regulators of embryonic development. Here, we describe the cloning and analysis of the murine Foxi2 gene, coding for a putative 311 amino acid protein resembling Foxi subfamily members in mice and other species. Expression analysis during the final stages of embryonic development revealed that Foxi2 expression is mainly confined to subsets of cells in epithelial structures and particular ducts, in addition to the developing forebrain and neural retina. Since FoxI factors are thought to be implicated in the regulation of cell fate, the highly restricted expression pattern of Foxi2 suggestive of a possible role in controlling cellular identity.

FoxO6, a novel member of the FoxO class of transcription factors with distinct shuttling dynamics

Forkhead transcription factors of the FoxO-group are associated with cellular processes like cell cycle progression and DNA-repair. FoxO function is regulated by protein kinase B (PKB) via the phosphatidylinositol 3-kinase/PKB survival pathway. Phosphorylation of serine and threonine residues in specific PKB phosphorylation motifs leads to exclusion of FoxO-proteins from the nucleus, which excludes them from exerting transactivating activity. Members of the FoxO-group have three highly conserved regions containing a PKB phosphorylation motif. This study describes the cloning and characterization of a novel forkhead domain gene from mouse that appeared to be highly related to the FoxO group of transcription factors and was therefore designated FoxO6. The FoxO6 gene was mapped in region D1 on mouse chromosome 4. In humans, FOXO6 is located on chromosomal region 1p34.1. Embryonic expression of FoxO6 is most apparent in the developing brain, and FoxO6 is expressed in a specific temporal and spatial pattern. Therefore it is probably involved in regulation of specific cellular differentiation. In the adult animal FoxO6 expression is maintained in areas of the nucleus accumbens, cingulate cortex, parts of the amygdala, and in the hippocampus. Structure function analysis of FoxO6 compared with its group members shows that the overall homology is high, but surprisingly a highly conserved region containing multiple phosphorylation sites is lacking. In transfection studies, FoxO6 coupled to GFP showed an unexpected high nuclear localization after stimulation with growth factors, in contrast to the predominant cytosolic localization of FoxO1 and FoxO3. We also show that nuclear export of FoxO6 is mediated through the phosphatidylinositol 3-kinase/PKB pathway. Furthermore, we show using a chimeric approach that we can fully restore the ability of FoxO6 to shuttle between nucleus and cytosol. In conclusion, the data presented here gives a new view on regulation of FoxO-function through multiple phosphorylation events and other mechanisms involved in the nuclear exclusion of FoxO-proteins.