Y-box protein-1 involvement in cyclin A and B1 gene regulation has recently been demonstrated. A more generalized role of this protein for cell replication is hypothesized as numerous regulatory sequences of cell cycle-related genes contain putative binding sites. In the present study the DNA polymerase alpha (DPA) gene is identified as another YB-1-responsive gene with a Y-box and 3' inverted repeat sequence, designated DPA RE-1, in the serum-responsive promoter region. Overexpressed YB-1 concentration-dependently trans-activated DPA gene expression in reporter assays and Southwestern blotting as well as DNA binding analyses revealed binding of distinct endogenous proteins to the RE-1 with molecular sizes of 26, 32 and 52 kDa. Among these, YB-1 binding was confirmed using recombinant as well as endogenous proteins, with preferential single-stranded DNA binding. Early serum growth response in mesangial cells was accompanied by a nuclear YB-1 shift and nucleocomplex formation at the RE-1. Fine mapping of the DPA RE-1 sequence unraveled a dependence on co-factors for trans-regulation with gene activation in the context of a heterologous SV40 promoter but suppression in the context of the abbreviated homologous promoter sequence. A YB-1 knock down resulted in decreased DPA transcription rates and abrogated the serum-dependent induction of DPA transcription. These results link YB-1 with serum responsiveness of DPA gene expression and provide insight into the required sequence and protein binding context.

BACKGROUND

The density of breast tissue on a mammogram is a strong predictor of breast cancer risk and may reflect cumulative estrogen effect on breast tissue. Endogenous and exogenous estrogen exposure increases the risk of estrogen receptor (ER)-positive breast cancer. We determined if mammographic density is associated more strongly with ER-positive breast cancer than with ER-negative breast cancer.

METHODS

We analyzed data from 44,811 participants in the San Francisco Mammography Registry of whom 701 developed invasive breast cancer. Mammographic density was measured using the Breast Imaging Reporting and Data System (BI-RADS) classification system (1 = almost entirely fat, 2 = scattered fibroglandular, 3 = heterogeneously dense, 4 = extremely dense). We tested for associations between mammographic density and ER-positive and ER-negative breast cancer separately. Analyses were adjusted for age, body mass index, postmenopausal hormone use, family history of breast cancer, menopausal status, parity, and race/ethnicity.

RESULTS

Mammographic density was strongly associated with both ER-positive and ER-negative breast cancers. Compared with women with BI-RADS 2, women with BI-RADS 1 (lowest density) had a lower risk of ER-positive cancer [adjusted hazard ratio (HR), 0.28; 95% confidence interval (95% CI), 0.16-0.50] and ER-negative cancer (adjusted HR, 0.17; 95% CI, 0.04-0.70). Women with BI-RADS 4 (highest density) had an increased risk of ER-positive breast cancer (adjusted HR, 2.21; 95% CI, 1.64-3.04) and an increased risk of ER-negative breast cancer (adjusted HR, 2.21; 95% CI, 1.16-4.18).

CONCLUSION

Surprisingly, women with high mammographic density have an increased risk of both ER-positive and ER-negative breast cancers. The association between mammographic density and breast cancer may be due to factors besides estrogen exposure.