Role Of PLK1 Overexpression In Colorectal Carcinogenesis

Polo-like kinase 1 (PLK1) is a member of the highly conserved serine/threonine protein kinase family. It is a key regulator of cell division and is also a central player in maintaining genome stability during DNA replication and in modulating the DNA damage response. PLK1 is frequently up-regulated in the vast majority of human tumors and the abnormal expression of PLK1 exacerbates malignant phenotype of cancer cells. Multiple preclinical trials suggest that PLK1 could be a potential target for cancer treatment. It has been reported that PLK1 is overexpressed in the intestinal adenoma, raising the possibility that PLK1 upregulation might be involved in colorectal cancer formation. The aim of the present study was to explore the role of aberrant PLK1 expression in colorectal carcinogenesis and underlying molecular mechanisms.A large number of molecular aberrations occurs during the formation and development of colorectal cancer. Mutation and inactivation of the adenomatous polyposis coli (APC) tumor suppressor gene is an early event in the formation of colorectal cancer. Inactivation of APC is not only responsible for one form of inherited colorectal cancer, familial adenomatous polyposis (FAP), but also plays the initiation and promoting roles in the majority of sporadic colorectal cancers. ApcMin/+ mice are model of FAP and are heterozygous for an APC truncation mutation, which serves as a classical genetic model for colorectal carcinogenesis. We found that Polo-like kinase 1 (PLK1) is upregulated in the intestinal adenoma formed in ApcMin/+ mice. More importantly, reduction in PLK1 levels in ApcMin/+mice through intercrossing ApcMin/+ mice with Plkl+/-mice significantly abrogated the Min (multiple intestinal neoplasia) phenotype in these mice. PLK1 downregulation not only decreased the number of intestinal adenoma, but also reduced the expression of several proliferation related markers, such as Ki67, p-ERK and c-Myc, suggesting that PLK1 overexpression promotes Apc-dependent intestinal tumorigenesis via accelerating proliferation of intestinal epithelial cells in mice.The relationship between APC mutation and PLK1 overexpression remains unkonwn. Our results showed that PLK1 mRNA and protein expression is upregulated upon APC knockdown or knockout in normal colon epithelial cell line NCM460. In contrary, PLK1 mRNA and protein expression is decreased upon enforced expression of wide type APC in human colon cancer cell line DLD-1 contain inactivating mutation of APC. These findings indicate that APC inactivation facilitates PLK1 overexpression in colorectal epithelial cells. Bioinformatics analysis results implied that several candidate transcription factor, including SP1, might be able to potentially regulate PLK1 transcription. Depletion of SP1 expression by siRNA or suppression of SP1 activity using a SP1 specific small molecule inhibitor reduced PLK1 mRNA and protein level in colorectal cancer cell lines DLD-1 and SW480, suggesting SPl positively activates PLK1 expression in/APC-mutated colorectal cancer cells. In addition, knockdown or knockout APC in normal colonic epithelial cells NCM460 increased SPl expression. Taken together, these observations suggest that PLK1 overexpression in APC-mutated colorectal epithelial cells might, at least in part, be mediated by SP1 upregulation.In summary, our present results indicate that PLK1 plays an important role in APC-dependent colorectal carcinogenesis, and suggest that SP1 is involved in the regulation of PLK1 expression in APC-mutated cells as well. These findings not only reveal a novel mechanism of APC inactivation in colorectal tumor formation, but also provide an important clue for clarifying the molecular mechanism of dysregulated PLK1 expression in colorectal cancer cells.