Mel-18 negatively regulates INK4a/ARF-independent cell cycle progression via Akt inactivation in breast cancer

Cancer Res. 2008 Jun 1;68(11):4201-9. doi: 10.1158/0008-5472.CAN-07-2570.

Abstract

Mel-18, a polycomb group (PcG) protein, has been suggested as a tumor suppressor in human breast cancer. Previously, we reported that Mel-18 has antiproliferative activity in breast cancer cells. However, its functional mechanism has not been fully elucidated. Here, we investigated the role of Mel-18 in human breast cancer. We saw an inverse correlation between Mel-18 and phospho-Akt, which were expressed at low and high levels, respectively, in primary breast tumor tissues from 40 breast cancer patients. The effect of Mel-18 on cell growth was examined in two breast cancer cell lines, SK-BR-3 and T-47D, which express relatively low and high levels of endogenous Mel-18, respectively. On Mel-18 overexpression in SK-BR-3 cells, cell growth was attenuated and G(1) arrest was observed. Likewise, suppression of Mel-18 by antisense expression in T-47D cells led to enhanced cell growth and accelerated G(1)-S phase transition. In these cells, cyclin-dependent kinase (Cdk)-4 and Cdk2 activities were affected by Mel-18, which were mediated by changes in cyclin D1 expression and p27(Kip1) phosphorylation at Thr(157), but not by INK4a/ARF genes. The changes were both dependent on the phosphatidylinositol 3-kinase/Akt signaling pathway. Akt phosphorylation at Ser(473) was reduced by Mel-18 overexpression in SK-BR-3 cells and enhanced by Mel-18 suppression in T-47D cells. Akt-mediated cytoplasmic localization of p27(Kip1) was inhibited by Mel-18 in SK-BR-3 cells. Moreover, Mel-18 overexpression showed reduced glycogen synthase kinase-3beta phosphorylation, beta-catenin nuclear localization, T-cell factor/lymphoid enhancer factor promoter activity, and cyclin D1 mRNA level. Taken together, we established a linear relationship between Mel-18-->Akt-->G(1) phase regulators.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Blotting, Western
  • Breast Neoplasms / enzymology
  • Breast Neoplasms / pathology*
  • Carcinoma, Ductal / enzymology
  • Carcinoma, Ductal / pathology*
  • Cell Cycle / physiology*
  • Cell Line, Tumor
  • Cyclin-Dependent Kinase Inhibitor p16 / genetics
  • Cyclin-Dependent Kinase Inhibitor p16 / physiology*
  • DNA-Binding Proteins / physiology*
  • Down-Regulation
  • Fluorescent Antibody Technique
  • Humans
  • Phosphorylation
  • Polycomb Repressive Complex 1
  • Proto-Oncogene Proteins c-akt / antagonists & inhibitors*
  • Proto-Oncogene Proteins c-akt / metabolism
  • Repressor Proteins / physiology*
  • Reverse Transcriptase Polymerase Chain Reaction
  • Tumor Suppressor Protein p14ARF / genetics
  • Tumor Suppressor Protein p14ARF / physiology*

Substances

  • Cyclin-Dependent Kinase Inhibitor p16
  • DNA-Binding Proteins
  • PCGF2 protein, human
  • Repressor Proteins
  • Tumor Suppressor Protein p14ARF
  • Polycomb Repressive Complex 1
  • Proto-Oncogene Proteins c-akt