Amino acid limitation induces down-regulation of WNT5a at transcriptional level

WNT5a in Colorectal Cancer: Research Progress and Challenges

Despite the clinical development of new adjuvant and neoadjuvant chemotherapy drugs, colorectal cancer is still one of the leading causes of cancer-related death in human beings. WNT5a, an autocrine and paracrine β-catenin independent ligand, has been shown to induce tumor inhibition and carcinogenic signals, depending on the type of cancer. In patients with colorectal cancer, WNT5a triggers a variety of downstream signaling pathways, which mainly affect the migration and invasion of tumor cells. This article reviews the mechanism and therapeutic potential of WNT5a in colorectal cancer. In short, an in-depth understanding of the role of WNT5a in colorectal cancer is very helpful to better deal with this disease.

WNT-5A: signaling and functions in health and disease

WNT-5A plays critical roles in a myriad of processes from embryonic morphogenesis to the maintenance of post-natal homeostasis. WNT-5A knock-out mice fail to survive and present extensive structural malformations. WNT-5A predominantly activates β-catenin-independent WNT signaling cascade but can also activate β-catenin signaling to relay its diverse cellular effects such as cell polarity, migration, proliferation, cell survival, and immunomodulation. Moreover, aberrant WNT-5A signaling is associated with several human pathologies such as cancer, fibrosis, and inflammation. Thus, owing to its diverse functions, WNT-5A is a crucial signaling molecule currently under intense investigation with efforts to not only delineate its signaling mechanisms and functions in physiological and pathological conditions, but also to develop strategies for its therapeutic targeting.

Differential regulation of the mouse and human Wnt5a alternative promoters A and B

Wnt5a is an extracellular glycoprotein that activates Wnt signaling pathways, important in development and tissue homeostasis. Wnt5a expression is often misregulated during cancer progression. In this study, we analyzed the transcriptional regulation of two of the Wnt5a alternative promoters, termed A and B. Transient transfection of promoter A and B luciferase reporter constructs in to NIH3T3 and Caco-2 cells indicated that the separated promoters are both functional and that 300-450 base pair (bp) of upstream sequence is sufficient for activity. Promoter B constructs displayed distinct patterns of expression in the two cell types. The endogenous levels of promoter A-derived transcripts were found to be greater than the promoter B transcripts by four- to sixfold in fibroblast cells. Treatment of NIH3T3 cells with tumor necrosis factor (TNF)-alpha leads to an increase in both promoter A and B activities, but promoter B was more responsive. Using inhibitors of TNF-alpha effector proteins, we provide evidence that the transcription factor nuclear factor-kappaB and the MEK1/2 and p38 kinases have distinct roles in determining the activity levels of promoters, A and B. These results support the conclusion that Wnt5a promoters, A and B, are differentially regulated and provide a model for complex transcriptional regulation of Wnt5a.

Dickkopf homolog 1, a Wnt signaling antagonist, is transcriptionally up-regulated via an ATF4-independent and MAPK/ERK-dependent pathway following amino acid deprivation

Amino acid response (AAR) pathway is activated when cells are deprived of amino acids. In the present study, using the human colon cancer cell line SW480, we observed that DKK1, an antagonist of the Wnt pathway, was significantly induced at the mRNA level after the removal of amino acids from the medium. Addition of the amino alcohol histidinol, which prevents the formation of histidinyl-tRNA(His), also increased DKK1 mRNA to a level similar to that observed when cells were deprived of all amino acids. Transcriptional activity and stability of DKK1 mRNA were both increased in the amino acid-deprived condition. The induction of DKK1 gene expression was confirmed by the increased immunofluorescent staining of the DKK1 protein in the amino acid deprived condition. Although chromatin immunoprecipitation assays showed increased RNA Polymerase II binding at the DKK1 promoter in amino acid-limited conditions, ATF4 binding to the promoter is absent. Luciferase reporter assays did not detect any functional AARE within the DKK1 gene structure. Knockdown of ATF4 by siRNA did not affect the increase of DKK1 mRNA during amino acid limitation. Inhibition of ERK phosphorylation abolished the induction of DKK1. Our study revealed that DKK1 is a novel target gene in the response to amino acid deficiency and that the expression of DKK1 is up-regulated through an ATF4-independent and an ERK-dependent pathway.