Nuclear APC

Abstract

Mutational inactivation of the tumor suppressor gene APC (Adenomatous polyposis coli) is thought to be an initiating step in the progression of the vast majority ofcolorectal cancers. Attempts to understand APC function have revealed more than a dozen binding partners as well as several subcellular localizations including at cell-cell junctions, associated with microtubules at the leading edge of migrating cells, at the apical membrane, in the cytoplasm and in the nucleus. The present chapter focuses on APC localization and functions in the nucleus. APC contains two classical nuclear localization signals, with a third domain that can enhance nuclear import. Along with two sets of nuclear export signals, the nuclear localization signals enable the large APC protein to shuttle between the nucleus and cytoplasm. Nuclear APC can oppose beta-catenin-mediated transcription. This down-regulation of nuclear beta-catenin activity by APC most likely involves nuclear sequestration of beta-catenin from the transcription complex as well as interaction of APC with transcription corepressor CtBP. Additional nuclear binding partners for APC include transcription factor activator protein AP-2alpha, nuclear export factor Crm1, protein tyrosine phosphatase PTP-BL and perhaps DNA itself. Interaction of APC with polymerase beta and PCNA, suggests a role for APC in DNA repair. The observation that increases in the cytoplasmic distribution of APC correlate with colon cancer progression suggests that disruption of these nuclear functions of APC plays an important role in cancer progression. APC prevalence in the cytoplasm of quiescent cells points to a potential function for nuclear APC in control of cell proliferation. Clear definition of APC's nuclear function(s) will expand the possibilities for early colorectal cancer diagnostics and therapeutics targeted to APC.

Figure 1

Schematic representation of APC domains as they relate to nuclear functions. A) Human APC is 2843 amino acids, with binding sites for several proteins shown on the middle panel. The 15 aa repeats (15R) and 20 aa repeats (20R) are involved in β-catenin binding and degradation. The lower panel highlights regions of APC that facilitate nuclear-cytoplasmic shuttling. Two NESs are located near the N-terminus, with three more in the 20aa repeat domain in the center of APC. Two NLSs are located in the central domain near axin binding sites. An additional region of APC that is able to facilitate nuclear import of truncated APC is shown, NLS-Arm. B) Drosophila APC2 (dAPC2) is 1067 amino acids, with conserved domains labeled and NES-R3 and -R4 indicated (*). Figures are to scale, with amino acid increments indicated at the top. C) Nuclear localization signals and nuclear export sequences in human APC. Numbers for the initial amino acid in each sequence are shown. Boxes indicate important hydrophobic residues (leucine or isoleucine).

Figure 2

Phosphorylation of APC regulates activity. A) The activity of NLS2APC contributes to nuclear import of APC. While phosphorylation of the serine upstream of NLS2 results in a more active NLS, phosphorylation of the serine just adjacent and downstream of NLS2 inhibits NLS activity. Potential CK2 and PKA recognition sequences are shown in italics. B) p38 MAPK activates CK2 and inhibits PKA. Inhibition of p38MAPK results in reduced levels of nuclear APC. C) LEF/TCF and APC compete for β-catenin binding. β-catenin binds LEF/TCF with a higher affinity than unphosphorylated APC. In contrast, β-catenin binds CK1-phosphorylated APC with a higher affinity than LEF/TCF. CK2-phosphorylated LEF-1 shows stronger binding to β-catenin, but APC can inhibit CK2 activity.

Figure 3

Model for the roles of nuclear APC in regulation of Wnt target genes. APC modifies expression of Wnt target genes by three mechanisms. 1) APC associates with transcription corepressors, such as CtBP, potentially guiding them to the transcription complex. 2) Phosphorylated APC competes with LEF/TCF for β-catenin binding, thus displacing β-catenin from the transcription complex. 3) APC may assist in the nuclear export of β-catenin for cytoplasmic degradation.