Peroxisome proliferator-activated receptor binding protein (PBP), a
nuclear receptor coactivator, interacts with
estrogen receptor alpha (
ERalpha) in the absence of
estrogen. This interaction was enhanced in the presence of
estrogen but was reduced in the presence of
antiestrogen,
tamoxifen. Transfection of PBP in
CV-1 cells resulted in enhancement of
estrogen-dependent transcription, indicating that PBP serves as a coactivator in ER signaling. To examine whether overexpression of PBP plays a role in
breast cancer because of its coactivator function in ER signaling, we determined the levels of PBP expression in
breast tumors. High levels of PBP expression were detected in approximately 50% of primary breast
cancers and
breast cancer cell lines by
ribonuclease protection analysis, in situ hybridization, and immunoperoxidase staining. Fluorescence in situ hybridization of human chromosomes revealed that the PBP gene is located on chromosome 17q12, a region that is amplified in some breast
cancers. We found PBP gene amplification in approximately 24% (6/25) of
breast tumors and approximately 30% (2/6) of
breast cancer cell lines, implying that PBP gene overexpression can occur independent of gene amplification. This gene comprises 17 exons that, together, span >37 kilobases. The 5'-flanking region of 2.5 kilobase pairs inserted into a
luciferase reporter vector revealed that the promoter activity in
CV-1 cells increased by deletion of
nucleotides from -2,500 to -273. The -273 to +1 region, which exhibited high promoter activity, contains a typical CCAT box and multiple cis-elements such as
C/EBPbeta, YY1, c-Ets-1, AP1, AP2, and NFkappaB binding sites. These observations, in particular PBP gene amplification, suggest that PBP, by its ability to function as
ERalpha coactivator, might play a role in mammary epithelial differentiation and in breast
carcinogenesis.