APC2 associates with the actin cortex through a multipart mechanism to regulate cortical actin organization and dynamics in the Drosophila ovary

EB1 Directly Regulates APC-Mediated Actin Nucleation

EB1 was discovered 25 years ago as a binding partner of the tumor suppressor adenomatous polyposis coli (APC) [1]; however, the significance of EB1-APC interactions has remained poorly understood. EB1 functions at the center of a network of microtubule end-tracking proteins (+TIPs) [2-5], and APC binding to EB1 promotes EB1 association with microtubule ends and microtubule stabilization [6, 7]. Whether EB1 interactions govern functions of APC beyond microtubule regulation has not been explored. The C-terminal basic domain of APC (APC-B) directly nucleates actin assembly, and this activity is required in vivo for directed cell migration and for maintaining normal levels of F-actin [8-10]. Here, we show that EB1 binds APC-B and inhibits its actin nucleation function by blocking actin monomer recruitment. Consistent with these biochemical observations, knocking down EB1 increases F-actin levels in cells, and this can be rescued by disrupting APC-mediated actin nucleation. Conversely, overexpressing EB1 decreases F-actin levels and impairs directed cell migration without altering microtubule organization and independent of its direct binding interactions with microtubules. Overall, our results define a new function for EB1 in negatively regulating APC-mediated actin assembly. Combining these findings with other recent studies showing that APC interactions regulate EB1-dependent effects on microtubule dynamics [7], we propose that EB1-APC interactions govern bidirectional cytoskeletal crosstalk by coordinating microtubule and actin dynamics.

Low expression of adenomatous polyposis coli 2 correlates with aggressive features and poor prognosis in colorectal cancer

Currently, there are no relevant findings on the diagnostic and prognostic roles of adenomatous polyposis coli 2 (APC2) in colorectal cancer (CRC). This study investigated the clinical value of APC2 dysregulation in the prognosis of CRC. Immunohistochemical scores obtained from tissue microarrays were used to quantify the expression of APC2 protein in 201 CRC tissues and 139 adjacent normal tissues. A chi-squared test was performed to analyze the association between APC2 expression and various clinical characteristics. Differences in 5-year survival between groups were analyzed. A receiver operating characteristic (ROC) curve was generated to investigate the potential association between APC2 and CRC diagnosis. Compared with adjacent normal tissues, APC2 was downregulated in CRC tissues (P = 0.0004). Survival analyses revealed that CRC patients with high APC2 expression (96.74%) obtained better 5-year survival rates than those with low APC2 expression (88.07%). CRC patients with low APC2 expression exhibited obvious lymphovascular invasion (P = 0.010), lymph node metastasis (P = 0.007), and high tumor node metastasis (TNM) stage (P = 0.007). Furthermore, ROC curves confirmed that APC2 was associated with lymphovascular invasion (P = 0.004), lymph node metastasis (P = 0.002), and TNM staging (P = 0.002). In summary, low APC2 expression in CRC tissues was associated with poor prognosis and may be a useful biomarker for the prognosis and clinical classification of CRC.

The role of APC-mediated actin assembly in microtubule capture and focal adhesion turnover

Actin assembly by APC maintains proper organization and dynamics of F-actin at focal adhesions. This, in turn, impacts the organization of other molecular components and the responsiveness of focal adhesions to microtubule capture and autophagosome-induced disassembly.

Focal adhesion (FA) turnover depends on microtubules and actin. Microtubule ends are captured at FAs, where they induce rapid FA disassembly. However, actin’s roles are less clear. Here, we use polarization-resolved microscopy, FRAP, live cell imaging, and a mutant of Adenomatous polyposis coli (APC-m4) defective in actin nucleation to investigate the role of actin assembly in FA turnover. We show that APC-mediated actin assembly is critical for maintaining normal F-actin levels, organization, and dynamics at FAs, along with organization of FA components. In WT cells, microtubules are captured repeatedly at FAs as they mature, but once a FA reaches peak maturity, the next microtubule capture event leads to delivery of an autophagosome, triggering FA disassembly. In APC-m4 cells, microtubule capture frequency and duration are altered, and there are long delays between autophagosome delivery and FA disassembly. Thus, APC-mediated actin assembly is required for normal feedback between microtubules and FAs, and maintaining FAs in a state “primed” for microtubule-induced turnover.