Role of a Placenta-Specific Gene in Mammary Tumorigenesis

摘要

A solo long terminal repeat (LTR) of an endogenous retrovirus-like element (intracisternal A particle) present in the mouse MIPP gene promotes placenta-specific expression of a 1.2 kb message. We have previously shown that many mouse mammary preneoplasias and carcinomas ectopically express 2.2 and 5.6 kb MIPP-related mRNAs. We determined that the 3 ends of all MIPP transcripts, which are homologous to the kelch repeat motif, have the same sequence. PCR- based techniques were employed to clone the 5' end of the 2.2 kb transcript. Sequence analysis revealed a 1.7 kb open reading frame with an N-terminal BTB/ POZ protein-protein interaction domain and six C-terminal kelch repeats. Using recombinant proteins from bacteria, we showed that MIPP co-sediments with microfilaments in vitro. Furthermore, MIPP and actin Co- immunoprecipitate from MOD mouse mammary tumor cells. Western blotting with an antiserum we raised against recombinant MIPP protein indicates that only one MIPP protein, about 70 kDa, is translated in mouse mammary tumor cells, despite expression of two transcripts. Immunofluorescence has localized the protein to the cytoplasm, with a non-filamentous staining reminiscent of endoplasmic reticulum. We have transfected MIPP into normal mammary epithelial cells, and transformation studies are in progress to determine whether MIPP contributes to mammary tumorigenesis.