ReviewAPC: the plot thickens
Introduction
β-catenin and its Drosophila homolog Armadillo are bi-functional proteins: when associated at the plasma membrane with E-cadherin and α-catenin, they mediate cellular adhesion; as ‘free’ cytoplasmic proteins, they transduce the Wnt signal. Unstimulated cells possess a constitutive activity that keeps the free β-catenin protein unstable. Wnt signaling inhibits this activity, stabilising free β-catenin which subsequently translocates to the nucleus. Here, it binds to the transcription factor TCF (T cell factor) and functions as its co-activator to stimulate the transcription of Wnt target genes (Figure 1).
Recent work has demonstrated that the destabilisation of β-catenin in normal cells is mediated by a multi-protein complex. One component of this complex is glycogen synthase kinase 3 (GSK3) whose chief function during development is mediated by β-catenin/Armadillo, and whose kinase activity is inhibited by Wnt signaling 1, 2. GSK3 phosphorylation of β-catenin earmarks this protein for degradation by the proteasome pathway. Mutation of the GSK3-phosphorylation sites in β-catenin causes it to accumulate to high levels, which can lead to cancer in many tissues [3•].
Two further components of the β-catenin-destabilising complex are the tumour suppressor adenomatous polyposis coli (APC) and the recently discovered Axin protein. Mutational loss of APC causes β-catenin to accumulate to high levels in human colon epithelial cells [4], eventually leading to cancer. Axin, the product of the mouse fused locus, was shown to antagonise Wnt signalling [5]. In this review, I focus on recent work that has formed our current understanding of how Axin functions together with GSK3 to promote the destabilisation of β-catenin. APC’s function in this process is still poorly understood, and I shall propose a new idea regarding a possible regulatory role of this tumour suppressor.
Section snippets
β-catenin, an Armadillo repeat protein
β-catenin is a protein composed of three modules (Figure 2a): an amino-terminal domain which contains multiple GSK3 phosphorylation sites needed for its degradation, a central domain consisting of 12 so-called ‘Armadillo repeats’, and a carboxy-terminal domain required for its signalling function. The latter confers transcriptional activation in yeast and in mammalian cells 6, 7 and mediates the signalling function of dorsal β-catenin during axis formation in Xenopus embryos [8].
The
Degradation of β-catenin
Cells that are not stimulated by Wnts rapidly degrade free β-catenin. The serine/threonine kinase GSK3, or its Drosophila homolog Shaggy/Zeste-white 3, play a critical role in this process: GSK3 loss causes free β-catenin to accumulate 28, 29. Furthermore, if one of its GSK3 phosphorylation sites is mutated, β-catenin becomes refractory to degradation 4, 30, 31. GSK3 phosphorylation earmarks β-catenin for degradation by the proteasome pathway 32, 33.
Recent work has shed light on the molecular
Axin, a scaffold protein which facilitates phosphorylation of GSK3 substrates
Evidently, GSK3-phosphorylated β-catenin is earmarked for destruction. Therefore, GSK3 phosphorylation of β-catenin is a critical step which is likely to be under exquisite control. A first indication for this came from the observation that, in vitro, β-catenin is not phosphorylated efficiently by GSK3 [29]. This suggested that another protein might be required for this enzymatic reaction.
A likely candidate for such a protein was identified —namely Axin, the product of the mouse locus fused [5]
APC, a well conserved protein promoting the degradation of β-catenin
As outlined above, Axin is thought to facilitate the phosphorylation of GSK3 substrates in the quaternary complex and to be the main target for Wnt-mediated inhibition of GSK3 activity. However, the quaternary complex also contains APC, which begs the question of what the molecular function of APC might be.
Until recently, only one APC gene was known each in humans and in Drosophila; expression of the latter is largely restricted to neuronal cells [56]. During the past year, a second APC has
A regulatory role of APC
Intriguingly, overexpressed Axin is capable of downregulating β-catenin in APC-mutant colon cancer cells 12••, 13, 48•. In line with this, mutant Axin protein that lacks the APC-binding domain (RGS-less Axin) is equally active 12••, 13, if not more so [48•], than full-length Axin in destabilising β-catenin in these cells. These experiments indicated that Axin may be able to downregulate β-catenin without APC under certain circumstances. A caveat is that the cancer cells used contain truncated
A taxi function of APC?
APCs are not only associated with the plasma membrane but are also seen in the cytoplasm and in the nucleus of Drosophila and mammalian cells 17••, 69. It is therefore conceivable that APC shuttles forth and back between these subcellular compartments. I would like to suggest an idea that envisages a function of APC in concentrating β-catenin at the plasma membrane.
In this scenario (Figure 4), APC picks up β-catenin in the cytoplasm (e.g. β-catenin immobilised by a putative anchor, see [26••])
Anomalies
Two recent studies 71, 72 appear to contradict the evidence from mammalian cells and from Drosophila that APC antagonises β-catenin. First, overexpression of human or Xenopus APC in ventral cells of Xenopus embryos induces a secondary axis, mimicking overexpression of β-catenin [71]. This has been taken to mean that APC behaves in this assay like a positive Wnt component such as β-catenin itself but the scenario outlined above offers a different explanation. The levels of endogenous Axin are
Conclusions
Rapid progress has been made recently in our understanding of how Axin and GSK3 earmark β-catenin for degradation and how the doomed β-catenin is recognised by the ubiquitination machinery. The function of APC in this process has been recognised for some time, on the basis of findings in colon cancer cells, and recent loss-of-function experiments in Drosophila have provided strong evidence that APC also promotes destabilisation of β-catenin in normal cells. How APC regulates this process
Acknowledgements
I thank Matthew Freeman, Sean Munro and Rob Arkowitz for discussion and comments on the manuscript. I am particularly grateful to Matthew for earmarking various versions of my model for degradation.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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