Onco-Fetal Antigen P1A in Exocytosis and Tumorigenesis.

Abstract

P1A is the first identified tumor antigen recognized by cytotoxic T lymphocytes (CTLs). P1A represents a prototype of onco-fetal antigens as it is also expressed in embryonic stem (ES) cells and other fetus derived tissues. We observed that P1A-transgenic mice, which specifically expressed P1A in lymphoid cells, developed various thymic tumors after 7 months of age and had shorter life-span compared to control, especially in Rag2-deficient background. The P1A-transgenic bone marrow cells had higher proliferation ability and more hematopoietic progenitors compared to control. All tumors tested displayed the B-cell lineage marker B220, except for one tumor that expressed T-cell lineage markers. This latter tumor also harbored a Notch1 mutation. In order to understand the oncogenic activity of P1A, we used tandem affinity purification and mass spectrometry to identify P1A-associated proteins. Several vesicle trafficking proteins, such as RalA, AP2, IQGAP1 and Rac1, were identified and confirmed by co-immunoprecipitation. Confocal microscopy revealed co-localization of P1A and RalA in intracellular vesicles. P1A silencing resulted in alteration of RalA distribution and reduced secretion of TNFα, IL-6 and β-hexosaminidase. Ectopic overexpression of P1A increased secretion of mouse bone marrow derived mast cells. In order to determine whether P1A is essential for the function of ES cells, we deleted the gene encoding P1A in mouse ES cells. By cDNA microarray analysis, RapGEF3, Transglutaminase2, Dynein and RhoBTB1, which are related to membrane trafficking, were decreased in P1A-knockout ES cells. Therefore, our results suggest that the onco-fetal antigen P1A may contribute to tumorigenesis by promoting exocytosis of the pro-inflammatory cytokines TNFα and IL-6. Since priming of tumor-specific CTLs requires cross-presentation of tumor antigen by antigen-presenting cells (APCs), enhancement of exocytosis by the tumor cells may facilitate transfer of tumor antigens to APCs. It is intriguing that the role for P1A in exocytosis may be responsible for its recognition by the immune system. Taken together, our data suggest exocytosis as a potential link between immunobiology and cancer biology of an onco-fetal antigen. Identification of key molecules in vesicle trafficking pathways can help to prevent or minimize the biological dysfunction caused by environmental toxicants which can alter exocytosis or endocytosis.