BackgroundThe Wnt signaling pathway is a prehistoric biological feature that has been conserved throughout evolution. It is known to play a crucial role in the embryonic development of all kinds of animal species; particularly in the regeneration of tissues in adult of these organisms amongst other many processes. It has also been recorded that mutations and/or deregulated expression of mechanisms of the Wnt pathway is liable to induce disease. And of most importance in the various diseases is cancer. About 25 years ago, the first gene that encodes a Wnt signaling component, Int1, was identified and characterized molecularly from mouse tumour cells. In contrast, the homologous gene Wingless in Drosophila melanogaster, which in known to produce developmental defects in embryos, was characterized. These two scientific breakthrough served as springboard leading to identification and illustration epistatic relationships of further components of the Wnt pathway. It is evident today that Wnt, Notch, Hedgehog, TGF? and a handful of other signaling systems are significant molecular machineries responsible for the control embryonic development.
The functions of these signaling systems are beyond cell and tissue boundaries, acting as morphogens that are produced from one cell or tissue type to stimulate surface receptors, signal transduction mechanisms and transcription factors in adjacent cells or tissues, which regulate processes such as cell proliferation, differentiation or survival. As development progresses, the activity of such signaling systems is strongly regulated. In cancer and other diseases however, this regulatory activity could be escaped. In that regard, a signaling cascade supposed to regulate the proliferation, survival of cells might become an oncogene as it undergoes a gain-of-function mutation. Alternatively, an inhibitor might lose its ability to regulate signaling and lose its functions as a tumour suppressor. In either way, as seen in Wnt signaling pathway, are linked to cancer.
Therefore, a great deal of effort has been invested globally in developing therapeutic agents that function to improve the Wnt pathway. In addition, a lot of discoveries with regards to Wnt research has covered the spectrum of model organisms, including worms, frogs, mice and even humans and therefore harnessing successful interdisciplinary research.This review is intended for the historical antecedents leading to crucial discoveries about the components and functions of this essential pathway.
Special attaention shall be given to Beta?catenin, its identification as part of the wnt signaling pathway, how its overlapping interaction domains for various binding partners allow this signaling to occur, our current knowledge of the various mammalian developmental processes regulated by Beta?catenin and how these could be used for therapeutic purposesHistorical antecedents;In 1982, Roel Nusse and Harold Varmus, reported from the University of California, San Francisco, that a tumour virus induced mammary gland tumours in mice by stimulating the expression of a gene that was initially unknown. This gene, they named Int1. They also speculated that a mutation that causes a loss-of-function in the mouse had occurred, which influences an absence of anterior cerebellum 2 this malformation was linked to a mutant allele of Int1 3,4). A Drosophila melanogaster mutant without wings, Wingless (Wg), was pronounced in 1973 5), with a fly gene which was a homologue of mammalian Int1 6,7. The Wg mutation was also associated with segmentation defects in Drosophila embryos. Afterward, the developmental phenotypes were linked to alterations in components of the Wnt signaling pathway. In 1995, the findings on these Drosophila mutants was awarded the Nobel Prize in Physiology. Currently, the term Wnt is an amalgam of Wg and Int10.
Pursuant to these findings however, there had been some earlier work on Wnt signaling, in ‘precloning’ times, during which the underlying mechanisms had not been discovered. In the 1930s, laboratory mice were used to establish that, viral insertion can promote mammary tumours 11. Even before this discovery, Mangold and Spemann’s 1935’s Nobel Prize award experiment which was conducted in 1924 which illustrated that a protein which was later known as wnt which was transplanted in the tissue could induce a second body axis, a phenomenon referred to as twin-headed, in newts embryo 12, 13. Moreover in 1902, Morgan, illustrated that lithium chloride can also induced double axes in frog embryos through activation of a pathway that was later termed the Wnt signaling pathway14,15.The canonical wnt signaling pathwayAlmost a decade after the discovery of Int1, almost all successful research into the Wnt pathway was in the developmental field. In fact, a lot of the genes in the Wnt pathway, that were first discovered to function momentarily in development, turned out to be oncogenes and tumour suppressors when deregulated in human cancer.
This fact was confirmed by studies conducted by Nüsslein-Volhard and Wieschaus’s genetic screen in the late 1970s and early 1980s8,9, using Drosophila mutants to display defects in embryonic segmentation similar to Wingless mutants. These were linked to mutations in armadillo (?-catenin in vertebrates), disheveled or porcupine genes.In addition, McMahon and Moon and several others have employed biochemical analyses demonstrate direct interactions between wnt pathway and ?-catenin. Particular to this is that ?-catenin, which is known to interact with the cell-adhesion molecule E-cadherin, are transported to the nucleus where it interact with the transcription factors lEF1 and TCF.
This converts lEF1 into a transcriptional activator30–32. The foregoing finding was a breakthrough in elucidating the mechanism by which cytoplasmic Wnt signals confer alterations in gene expression in the nucleus. Conversely, regulators that oppose the Wnt pathway, produced a complete loss of the body axis36–38. Genetic analyses in mice has been used to illustrate that Axin1 or ?-catenin control axis formation36,39. It also is as well noteworthy that Costanini and collaborators’ analysis of the Axin1 mutation 36 was the classical mouse mutant Fused, described in 1949 40).
This was subsequently recognized as part of the Wnt pathway. The identification and characterization of another negative regulator of the Wnt pathway. In all, the concluding statement goes to support that, the basic mechanism and the components of Wnt signaling are passed from one generation to another between invertebrates and vertebrates. This therefore suggests that the conservation within and among members of the Wnt pathway and their similar interactions in various organisms suggests that this pathway is paramount to human development and may be implicated in human disease. Dating from the mid-1990s, many more components of the Wnt signaling pathway have been discovered. The known components of the Wnt signaling pathway can be amassed as follows; secreted Wnt proteins bind to Frizzled receptors and lRP5–lRP6 co-receptors in the plasma membrane. Several inhibitors of this interaction were discovered at the end of the 1990s. For instance, during head formation, DKK1 antagonizes Wnt signaling in mice77.
Apart from that, secreted proteins as those of Norrin and R-Spondin has also been shown to be activators of the canonical Wnt pathway. This comes about as a result of their interaction with Frizzled–lRP receptors79,80. In the mid-1990s, it was shown by the groups of Polakis, Nusse, Wieschaus and collaborators81–83 that the control of ?-catenin stability is critical in Wnt signaling. Consequently, in the absence of Wnt ligands, cytoplasmic ?-catenin is recruited into an obliteration complex, where it interacts with APC and the axins, and this is N-terminally phosphorylated by casein kinase 1? (CK1?) and GSK3?84?88. Right after phosphorylation, ?-catenin is targeted for proteasome-dependent degradation which involves interaction with ?-TrCP (?-transducin repeat-containing protein). 89–92 Thus, in the non-stimulated state, there is a down regulation of cytoplasmic ?-catenin levels and lEF and TCF in the nucleus interact with Grouchos to suppress Wnt-specific target genes93–95.
Mutations which occur in genes that regulate ?-catenin stability, such as those that encode members of the destruction complex, or ?-catenin, are known to be significant in cancer progression. Once canonical Wnt ligands are available, lRP5–lRP6 is phosphorylated by CK1? and GSK3?96,97, and Dishevelled is translocated to the plasma membrane, where it cooperates with frizzled receptors and polymerizes with other Dishevelled molecules98,99. Phosphorylation of lRP5 or lRP6 as well as the formation of the Dishevelled polymer, as well as internalization with caveolin100, are used as mediators for the recruitment of axin to the plasma membrane and inactivation of the destruction complex. As soon as they are recruited into the nucleus, ?-catenin forms a transcriptionally active complex with lEF and TCF transcription factors30–32 through the displacement of Grouchos and interacting with other co-activators such as Pygopus, CBP and (CREB-binding protein)101–105. CBP and Hyrax control gene expression through chromatin remodeling and by influencing RNA polymerase II. The discovery of proto-oncogene MYC is a direct transcriptional target of Wnt–?-catenin signaling as the transforming activity in cancer in 1998 was a breakthrough106. It must however be mentioned that, the first direct ?-catenin target gene, lEF, in Drosophila had been identified by Bienz107. Canonical Wnt signaling in cancerBefore 1993, there had not been an overlap between research on Wnt signaling and human cancer.
Then in the end of 1993, Vogelstein, Kinzler and Polakis first published an important biochemical interaction of the tumour suppressor, APC, and the Wnt pathway component ?-catenin 113,114. In that report, they explained; two types of repeat in APC are essential for interaction with ?-catenin, and they consist of three 15-amino-acid and seven 20-amino-acid repeats and these compete with E-cadherin for ?-catenin binding 115. The first clue to the molecular pathogenesis of adenomatous polyposis, a colon cancer, was the 1987 finding that familial adenomatous polyposis (FAP), is associated with deletions of the specific chromosome region 5q21–22 117,118. FAP is known to be associated with the development of hundreds to thousands of adenomatous polyps in the colon, some of which are able to develop into malignant carcinomas if surgical recessions are not performed. Two years down the line, that is in 1989, truncating mutations in APC were characterized in patients with FAP and/or frequent sporadic colorectal cancers; sporadic colorectal cancer is made of approximately 85% of human colorectal cancers119,120 (116). A high rate of APC mutations were frameshift, which resulted in truncations of about 50% of the APC protein 121. However, just a single APC mutation is insufficient to induce adenomatous polyposis, meaning a second mutation is always required.
In fact, several APC mutations accumulate before the region that encodes the SAMP repeats regions regulating the interaction of APC with scaffold proteins of the ??catenin destruction complex 37,122. Following this, an APC mouse mutant with a truncation mutation after the region that encodes the first SAMP repeat would not develop tumours123. It was first reported in a rather narrow sampling that about 10% of sporadic colorectal cancers contain activating mutations in CTNNB1 128–130.
But a more recent and more extensive mutational studies found that the frequency in sporadic colorectal cancers is somewhat closer to 1% 131. The study examined over 3,500 different human cancers to determine the occurrence of CTNNB1 mutations. The study recorded over 700 cases mutations that were predominantly centered in the N terminus of CTNNB1 121,133. These recorded mutations in the N terminus either caused the deletion of an N-terminal fragment or altered the N-terminal phosphorylation sites Ser45, Thr41, Ser37, Ser33. Exon 3 of Ctnnb conditional mutation deletion in mice, adenomatous polyposis or other cancers develop134.
Consequently, the tumour-causing mutations in APC, AXIN1, AXIN2 and CTNNB1 generally will lead to inappropriate stabilization of ?-catenin. 135–137. Loss-of-function mutations can also occur in WTX, which encodes component of the ?-catenin destruction complex that was recently discovered 138,139. Moreover, uncontrolled Wnt–?-catenin signaling caused by elevated ?-catenin levels also induces aggressive fibromatosis and pulmonary fibrosis140,141.
Of significant interest is the recent discovery that, activating LRP5 mutations are also known in thyroid tumours150. It therefore makes sense to predict that inappropriate activation of developmental signaling pathways that are responsible for and promote tumour progression. Wnt and therapeuticsLate in the 1990s, when inappropriate activation of the Wnt pathway had been linked to human cancer, researchers, pharmaceutical and biotechnology companies have been interested in developing Wnt pathway inhibitors. Studies thenceforth have reported that number of existing drugs and derivatives of non-steroidal anti-inflammatory drugs (NSAIDs; and some vitamins operate by directly or indirectly targeting the Wnt pathway, 158–160. Some NSAIDs are also reported to affect the level of ?-catenin and/or its cellular distribution.
It is noteworthy that these drugs were specifically developed for the treatment of other diseases and are approved by various regulatory agencies as pain killers. However, studies have established that NSAIDs can inhibit colorectal tumours in rats and have cancer preventive properties. Of particular importance is that, Celecoxib was approved for the treatment of patients with FAP in 1999 163. Again in 2004, a high-throughput screening method to search for low-molecular-weight antagonists specifically for interaction between ?-catenin and TCF4 was reported 164. Recently however, interest in antibody-based therapies have also received significant attention, and the quest to develop such agents to target molecules of the Wnt pathway that are overexpressed in disease condition165,166. Moreover, proteins such as SFRPs, that inhibit the Wnt pathway, are presently being developed and tested in preclinical tumour models167. Low-molecular-weight compounds that have inhibitory properties against transcriptional co-activators of the Wnt pathway are also under scrutiny for possible therapeutic applications176,177.
Wnt signaling and stem cellsRecent findings have confirmed that the Wnt signaling pathway could be one of the rate limiting in the specification and maintenance of precursor cell and stem cell lineages in several tissues and organs. Clevers laboratory reported in 1998 that mutation of the ?-catenin interaction partner Tcf4 in mice can cause a complete loss of intestinal stem cell compartment 178. Additional research has reported that canonical Wnt signaling work hand-in-hand with BMP and Notch signaling in the intestinal stem cell niche to regulator stem cell self-renewal179–181. Pursuant to that, Wnt signaling has been implicated in the stem cell compartments of several other tissues. There is also evidence that in the skin, loss-of-function mutation of Ctnnb1 prevents the production of hair cell progenitors 182.
This confirms the finding; activating mutations of Ctnnb1 in the skin lead to an enlargement of hair precursor cells and induce ‘hair tumours’: 183,184. Further to this, Wnt–?-catenin signaling has been speculated to increase haematopoietic stem cell renewal58,185–187. regulates precursor cell upkeep in the nervous system189,190.
To confirm the foregoing claim, it was shown that the balance of neuronal progenitor cell division and modeling is disturbed at the level of the dorsal horn of the spinal cord either by loss- and/or gain-of-function Ctnnb1 mutant mice. It has also been reported that Wnt–?-catenin signaling in the dorsal horn of the spinal cord is dependent on BMP and that is responsible for controlling the action of the vital basic helix-loop-helix transcription factor olIG3 in the domain 2 and 3 postmitotic neurons190.Recently, Wnt–?-catenin signaling has been reported to be involved in the control of self-renewal and differentiation of Islet1- precursor cells in neo- and postnatal hearts. This information could be useful in the development of cell-based therapies in regenerative medicine191,192 193. Following these findings, Wnt pathway controls specification and upkeep of particular progenitor and stem cell lines in several tissues and organs in embryonic development and throughout life. 194–199.
Another important role of Wnt–?-catenin signaling in cancer stem cells of the mammary gland is that the frequency of CD29+CD24+ stem cells in the mammary gland is increased by 6.4 when WNT1 is activated201. When Ctnnb1 is used to delete DMBA–TPA causes a complete regression of the tumours202. These are factual demonstration that Wnt–?-catenin signaling plays an important role in the upkeep of the mammary gland and skin cancer stem cells. This could also mean that the differentiation potential of ?-catenin in cancer as oppose normal stem cells could be explored for therapy.It has also been reported that cytotoxic agents have anti-tumour activity against cancer cells, however putative cancer stem cells are resistant 203,204). It has offer a great deal of importance to determined whether the Wnt-inhibiting drugs that are presently under development show preferential targeting to cancer stem cells in the diverse tumour types. It has also been reported that in colon carcinoma and metastasis, Wnt–?-catenin pathway is activated further at late stages of tumour progression, 155,205.
Therefore, drugs that inhibit Wnt signaling might interfere with metastasis formation and maintenance. The next 25 yearsThe past 25 years, has witness rigorous components and target genes of the Wnt signaling pathway. The trend of research focus is a clear suggestive that the future holds a great deal of identification of new partners. Those research findings are expected to provide knowledge and better understanding of how components of the Wnt pathway function, and cooperate with several other proteins in cells206–209. The major signaling pathways and their interactions during development, why mutations in these major components can cause tumour progression and other diseases are also expected to be more explicit. The mechanisms that underpin the activities of some wnt pathway components such as BCL9 and PYGO in vertebrate development and disease, the activity of tyrosine phosphorylation of various Wnt components, how activating serine/threonine kinases in the Wnt pathway and the definite mechanisms by which the Wnt pathway influence gene regulation and chromatin transformation are also expected to be unraveled 210). We now know that a significant % of colon cancers and a high percentage of other cancers 211) originate from mutations of the Wnt pathway. Cell culture studies have been used to confirmed that when WIF1 or ?-catenin silencing, RNA interference targeting GSK3?, Pygopus 2 antisense oligonucleotides are used to inhibit the Wnt pathway it can inhibit proliferation and induce differentiation in cancer cells159,212–214.
Therefore, it is likely that inhibiting the Wnt pathway could make a great contribution to cures for cancer or other diseases. Attention is also expected to be given to effective therapies that interfere with protein–protein interactions at critical steps of the pathway, which is currently a challenge for present-day drug development215. Recently New protein kinases that trigger the Wnt pathway have been found96, and these serve as promising leads for the development of specific inhibitors of these enzymes. Last but not least, it is expected the combined use of inhibitors of different pathways will be examined in the next 25 years. Among such inhibitors are those of receptor tyrosine kinases and Notch, Hedgehog, or TGF-? inhibitors and a combination of Wnt inhibitors. For instance, tyrosine kinase inhibitors like imatinib mesylate as well as ?-secretase inhibitors disturb the Wnt signaling pathway 216,217 and efforts are expected to be put in the development of these drugs to enhance their ability to interfere at the target gene level.
Currently, there is evidence available that cancer stem cells, which is just a minor fraction of tumour cells, could solely be responsible for the proliferation and sustenance of tumours. These cancer stem cells appear to share many features with normal stem cells, including self-renewal and differentiation. Probably these have a similar functions in tumours. This outstanding finding provides exciting possibilities for elucidating the mechanism behind both tumour progression and likely therapeutic interference. Research is therefore expected to advance towards the demonstration of how important the Wnt–?-catenin signaling pathway as far as self-renewal of cancer stem cells are concerned.
A number of cases in which processes that regulate embryonic development have been implicated in disease over the last 20 years. The evolutionary conserved Wnt signaling pathway regulates many of the procedures required for the growth, differentiation, and regulation of animal cells, and hence, it is little wonder that mutations components of this pathway is linked to the deregulation of those processes in disease. Knowledge on how to manipulate wnt pathway through new low-molecular-weight substances or therapeutic molecules is holds great promise for therapy and research is expected to be directed to these areas in the future.