Mel-18 negatively regulates INK4a/ARF-independent cell cycle progression via Akt inactivation in breast cancer.
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.
Lee
Jeong-Yeon
JY
Department of Pathology, College of Medicine, Hanyang University, Seoul, Republic of Korea.
Jang
Ki-Seok
KS
Shin
Dong-Hui
DH
Oh
Mi-Yun
MY
Kim
Hyun-Jun
HJ
Kim
Yongseok
Y
Kong
Gu
G
D015153:Blotting, Western; D001943:Breast Neoplasms / Q000201:enzymology / Q000473:pathology*; D044584:Carcinoma, Ductal / Q000201:enzymology / Q000473:pathology*; D002453:Cell Cycle / Q000502:physiology*; D045744:Cell Line, Tumor; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000235:genetics / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000502:physiology*; D015536:Down-Regulation; D005455:Fluorescent Antibody Technique; D006801:Humans; D010766:Phosphorylation; D063150:Polycomb Repressive Complex 1; D051057:Proto-Oncogene Proteins c-akt / Q000037:antagonists & inhibitors* / Q000378:metabolism; D012097:Repressor Proteins / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D025803:Tumor Suppressor Protein p14ARF / Q000235:genetics / Q000502:physiology*
D019941:Cyclin-Dependent Kinase Inhibitor p16; D004268:DNA-Binding Proteins; C489348:PCGF2 protein, human; D012097:Repressor Proteins; D025803:Tumor Suppressor Protein p14ARF; D063150:Polycomb Repressive Complex 1; D051057:Proto-Oncogene Proteins c-akt