A Modern Topic in Biology

Wnt/β catenin and FoxO Gene Activity

2.1. Wnt/β Catenin Pathway

The Wnt/ β-catenin signaling represents one of the most extremely studied pathways, where this pathway is tightly associated with cancers. When Wnt ligands bind to the receptor Frizzled (Fz) and co-receptor LDL-receptor-related protein (LRP), the extracellular signals are transduced via Dishveled (Dsh) to the destruction complex composed of Axin (Axn), Adenomatous polyposis coli (APC) and glycogen synthase kinase 3 (GSK3), and this will prevent the proteosomal degradation of  the transcriptional coactivator β-catenin. Thus, the stabilized Β-catenin translocates into the nucleus and associates with the T cell factor (TCF)/Lymphoid-enhancing factor(LEF) family of transcription factors to activate transcription of target genes. By this mechanism, Wnt/ β catenin signaling plays a wide range of functions, such as emryogenesis, organ maintenance, cell proliferation and cell fate decisions. In addition, Wnt signaling is also reported to regulate cell death during Drosophila retina and ommatidia development (Swarup, 2012).

Working with Wnt proteins as a biological agents has proved to be a problem. There are many unpublished methods of failed attempts to produce Wnt proteins in a cell culture. Kitajewski et al. (1992) claimed that when the genes in cultured cells are overexpressed, an accumulation of misfolded proteins in the endoplasmic reticulum will be observed. Secreted forms of Wnt proteins can be found in the extracellular matrix of the cell surface. (Bradley & Brown, 1990). Most Wnt protein is cell bound under any circumstances, but then, several systems have more recently been developed that soluble forms are produced. The Drosophila Wg has been recovered from the medium of cultured cells. Wg protein can be tested for the stabilization of the Arm protein. (VanLeeuwen et al., 1994).

2.2. FoxO Gene

The Forkhead box O (FoxO) transcription factor belong to the large forkhead family proteins, which are characterized by a winged helix DNA binding domain called “Forkhead box.” Vertebrates have only one FoxO gene, while mammals have four members including FoxO1, FoxO3a, FoxO4, FoxO6. The activity of FoxO is negatively regulated by the Insulin/PI3K/Akt signaling pathway. Activation of Akt phosphorylates FoxO and results in nuclear exclusion of FoxO and inhibiting its transcriptional activity. In case of stress conditions, such as high levels of reactive oxygen species (ROS) or deprivation of growth factors, promote the nuclear localization of FoxO and then induce its target gene expression (Calnan, 2008).

FoxO is involved in a variety of physiological processes including cell cycle, cell death and differentiation, DNA repair, oxidative stress response and longevity. The dysregulation of FoxO has been associated with many diseases, including immune defects, malignancy, diabetes and Alzheimer’s disease. FoxO transcription factors are regulated by a wide range of external stimuli, such as insulin, insulin-like growth factor (IGF-1), other growth factors, neurotrophins, nutrients, cytokines and oxidative stress stimuli. These stimuli control FoxO protein levels, subcellular localization, DNA- binding  and transcriptional activity. While FoxO transcription factors are relatively stable proteins, they can still be degraded in a proteosome-dependent manner in response to insulin and growth factors ( Matsuzaki et al., 2003).

FoxO transcription factors have a crucial role at the intersection of many signaling pathways. The regulation of FoxO activity is critical for eliciting appropriate response due to the diverse roles of the FoxO factors.

Burgering and Kops  (2002) claimed that Akt regulates transcription through Forkhead-related FoxO family by phosphorylating the protein at their conserved serine/threonine residues. This leads to retention of FoxO transcription factors in the cytoplasm, thereby down-regulating RNA synthesis of specific target genes that affect cell cycle progression, apoptosis and modulate metabolic genes.

2.3. FoxO and β-catenin interaction

The interaction between β-catenin with FoxO gene enhances its transcriptional activity in mammalian cells. However, FoxO inhibits Wnt signaling by diverting the limited pool of β-catenin from Wnt/TCF to FoxO leading to defects on embryogenesis and bone formation (Calnan, 2008).