Deregulation of the members of the Polo-Like Kinase family in human hepatocellular carcinoma development and progression

Abstract

Members of the Polo-like kinase (PLK) protein family play critical roles during multiple stages of cell cycle progression, either favouring or inhibiting cell proliferation. Although either overexpression or downregulation of PLKs occurs frequently in various cancer types, no comprehensive analysis on the role of PLK proteins has been performed in human hepatocellular carcinoma (HCC) to date. In order to better understand the function of PLKs during human HCC development and progression, we assessed the levels and biologic role of PLK1, 2, 3, and 4 in a human HCC collection. Levels of PLK1 were progressively upregulated in HCC compared with corresponding non-tumorous livers, reaching the highest levels of expression in liver tumors characterized by a short survival length following liver partial resection. In striking contrast, PLK2, 3, and 4 were frequently downregulated in HCC, mainly in the most aggressive tumors, due to promoter hypermethylation and/or loss of heterozygosity. Subsequent experiments showed that downregulation of PLK1 by siRNA resulted in strong growth restraint of HCC cells and massive apoptosis in vitro, whereas overexpression of PLK1 led to increased proliferation and survival. Upregulation of PLK1 was driven by the transcription factor forkhead box M1 (FOXM1), since either the silencing or overexpression of the latter gene had a strong impact on PLK1 levels in HCC cells. Opposite results were instead obtained when silencing PLK2, 3, and 4 in HCC cells. Indeed, silencing of PLK2, 3 or 4 triggered a remarkable growth acceleration and resistance to apoptosis, suggesting that these three genes act as tumor suppressor genes in liver carcinogenesis. Taken together, the present data suggest that deregulation of PLK proteins is an important oncogenic event in human hepatocarcinogenesis. Therapeutic approaches aimed at suppressing PLK1 and/or reactivating PLK2–4 genes might be highly beneficial for the treatment of human HCC.