ABSTRACT
The MAGE-A, MAGE-B, and MAGE-C protein families comprise the class-I MAGE/cancer testes antigens, a group of highly homologous proteins whose expression is suppressed in all normal tissues except developing sperm. Aberrant expression of class I MAGE proteins occurs in melanomas and many other malignancies, and MAGE proteins have long been recognized as tumor-specific targets; however, their functions have largely been unknown. Here, we show that suppression of class I MAGE proteins induces apoptosis in the Hs-294T, A375, and S91 MAGE-positive melanoma cell lines and that members of all three families of MAGE class I proteins form complexes with KAP1, a scaffolding protein that is known as a corepressor of p53 expression and function. In addition to inducing apoptosis, MAGE suppression decreases KAP1 complexing with p53, increases immunoreactive and acetylated p53, and activates a p53 responsive reporter gene. Suppression of class I MAGE proteins also induces apoptosis in MAGE-A-positive, p53wt/wt parental HCT 116 colon cancer cells but not in a MAGE-A-positive HCT 116 p53-/- variant, indicating that MAGE suppression of apoptosis requires p53. Finally, treatment with MAGE-specific small interfering RNA suppresses S91 melanoma growth in vivo, in syngenic DBA2 mice. Thus, class I MAGE protein expression may suppress apoptosis by suppressing p53 and may actively contribute to the development of malignancies and by promoting tumor survival. Because the expression of class I MAGE proteins is limited in normal tissues, inhibition of MAGE antigen expression or function represents a novel and specific treatment for melanoma and diverse malignancies.
Subject(s)
Antigens, Neoplasm/metabolism , Apoptosis/immunology , DNA-Binding Proteins/metabolism , Melanoma/immunology , Nuclear Proteins/metabolism , Repressor Proteins/metabolism , Transcription Factors/metabolism , Animals , Antigens, Neoplasm/biosynthesis , Antigens, Neoplasm/genetics , Antigens, Neoplasm/immunology , Cell Growth Processes/immunology , Cell Line, Tumor , DNA-Binding Proteins/immunology , Gene Expression Regulation, Neoplastic , HCT116 Cells , Humans , Melanoma/genetics , Melanoma/pathology , Melanoma, Experimental/genetics , Melanoma, Experimental/immunology , Melanoma, Experimental/pathology , Mice , Mice, Inbred DBA , Nuclear Proteins/immunology , Protein Binding , Repressor Proteins/immunology , Transcription Factors/immunology , Tripartite Motif-Containing Protein 28 , Tumor Suppressor Protein p53/immunologyABSTRACT
MAGE antigens are proteins that are normally expressed only in gametes but are often aberrantly expressed in melanomas, hematopoietic malignancies, and other "cancers". The functions of most MAGE proteins are unknown. Data have accumulated suggesting expression of MAGE proteins by malignant cells may contribute to advanced disease or resistance to chemotherapy, but direct evidence supporting this hypothesis is lacking. We show here that small interfering RNA (siRNA) suppression of MAGE-A, -B, and -C gene expression slows proliferation and induces caspase independent apoptosis in human and murine mast cell lines. Furthermore, treatment with MAGE specific siRNA suppresses growth of malignant cells in an in vivo murine model of mastocytosis. These observations demonstrate that MAGE protein expression can contribute to the development of tumors by permitting proliferation and prolonging the survival of malignant cells. We suggest a shift of the current clinical paradigm from one that envisions MAGE proteins solely as targets for immunologic attack to one in which MAGE genes and proteins are also targets for functional manipulation.
