Function and regulatory mechanisms of the candidate tumor suppressor receptor protein tyrosine phosphatase gamma (PTPRG) in breast cancer cells.
BACKGROUND
Protein phosphorylation is one of the essential steps in cell signaling, and aberrant phosphorylation is a common event in human cancer. The expression of receptor type protein tyrosine phosphatase gamma (PTPRG) in normal breast is found to be approximately 50-60% higher than that of breast tumor tissue. Overexpression of PTPRG inhibits anchorage-independent growth and proliferation of breast cancer cells. To understand the tumor suppression characteristics of PTPRG, we studied its tumor suppressive function in an athymic mouse model and evaluated factors that can potentially regulate its expression in breast cancer cells.


METHODS
To investigate the function of PTPRG in vivo, athymic nude mice were implanted with MCF-7 cells overexpressing PTPRG. For in vitro study, protein levels of cell cycle regulators, cell cycle re-entry, and the phosphorylation levels of extracellular signal-regulated protein kinases 1/2 (ERK1/2) were examined. In addition, methylation assays were conducted to investigate the epigenetic modification on the promoter of PTPRG.


RESULTS
Athymic nude mice bearing MCF-7 cells overexpressing PTPRG showed a reduction in tumor burden in comparison to animals implanted with MCF-7 cells transfected with vector alone. When these two cell lines were studied in an in vitro system, elevated mRNA and protein levels of cell cycle regulators, p21(cip) and p27(kip) were detected in MCF-7 cells overexpressing PTPRG compared to cells transfected with vector alone. Similarly, overexpression of PTPRG also delayed the re-entry of breast cancer cells into the cell cycle after serum starvation, and reduced the phosphorylation levels ERK1/2 in MCF-7 cells. In addition, methylation assays in PTPRG promoter in breast cancer cell lines (including SK-Br-3) revealed an aberrant methylation pattern. When SK-Br-3 and MCF-7 cells were treated with deoxy-5-azacytidine (DAC) and trichostatin A (TSA), these compounds reactivated the expression of PTPRG, suggesting an epigenetic control on its expression.


CONCLUSIONS
Our results indicated that PTPRG inhibited breast tumor formation in vivo; PTPRG may up-regulate p21(cip) and p27(kip) proteins through the ERK1/2 pathway. This study also showed methylation-mediated silencing of PTPRG in breast cancer cell lines. These data indicate that PTPRG exhibits the characteristics of a breast tumor suppressor.
Shu
Sherry T
ST
Laboratory of Reproductive and Molecular Endocrinology, Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210-1093, USA.
Sugimoto
Yasuro
Y
Liu
Suling
S
Chang
Hsiang-Lin
HL
Ye
Weiping
W
Wang
Li-Shu
LS
Huang
Yi-Wen
YW
Yan
Pearlly
P
Lin
Young C
YC
D000818:Animals; D001374:Azacitidine / Q000031:analogs & derivatives / Q000494:pharmacology; D001943:Breast Neoplasms / Q000473:pathology / Q000517:prevention & control*; D002453:Cell Cycle; D045744:Cell Line, Tumor; D018899:CpG Islands; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000032:analysis; D050760:Cyclin-Dependent Kinase Inhibitor p27; D019175:DNA Methylation; D000077209:Decitabine; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D005260:Female; D006877:Hydroxamic Acids / Q000494:pharmacology; D047908:Intracellular Signaling Peptides and Proteins / Q000032:analysis; D051379:Mice; D010766:Phosphorylation; D054633:Receptor-Like Protein Tyrosine Phosphatases, Class 5 / Q000502:physiology*
C493956:CDKN1A protein, human; C436291:CDKN1B protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D006877:Hydroxamic Acids; D047908:Intracellular Signaling Peptides and Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; C012589:trichostatin A; D000077209:Decitabine; D048049:Extracellular Signal-Regulated MAP Kinases; C516850:PTPRG protein, human; D054633:Receptor-Like Protein Tyrosine Phosphatases, Class 5; D001374:Azacitidine