Survivin-induced Aurora-B kinase activation: A mechanism by which APC mutations contribute to increased mitoses during colon cancer development

APC mutations initiate most colorectal cancers (CRCs), but cellular mechanisms linking this to CRC pathology are unclear. We reported that wild-type APC in the colon down-regulates the anti-apoptotic protein survivin, and APC mutation up-regulates it, explaining why most CRCs display survivin overexpression and apoptosis inhibition. However, it does not explain another hallmark of CRC pathology--increased mitotic figures and cell proliferation. Because survivin activates aurora-B kinase (ABK) in vitro, catalyzing mitosis, we hypothesized that in normal colonic crypts, APC controls ABK activity, while in neoplastic APC-mutant crypts, ABK activity is up-regulated, increasing mitosis. We quantitatively mapped intracryptal distributions of survivin, ABK, and markers of activated downstream signaling and mitosis (INCENP, phospho-histone-H3, phospho-centromere-protein-A). In normal crypts, gradients for these markers, ABK:survivin:INCENP complexes, and ABK activity were highest in the lower crypt (inverse to the APC gradient). In neoplastic crypts that harbor APC mutations, proliferating (Ki-67+) cells and cells expressing survivin, ABK, and phospho-histone-H3 were distributed farther up the crypt. Hence, as cells migrate up neoplastic crypts, transitions between cell phenotypes (eg, from stem to proliferating) appear delayed. In CRC cell lines, increasing wild-type APC, inhibiting TCF-4, or decreasing survivin expression down-regulated ABK activity. Thus, APC mutation-induced up-regulation of the survivin/ABK cascade can explain delayed crypt cell maturation, expansion of proliferative cell populations (including mitotic figures), and promotion of colon tumorigenesis.