Involvement of microRNA in the regulation of progesterone receptor in breast cancer.

Abstract

578 Background: The progesterone receptor (PgR) is a prognostic factor in ER positive breast cancers treated by adjuvant hormonal treatment. While PgR protein is usually assessed by immunohistochemistry (IHC), RT-PCR measurement of mRNA levels is provided by Oncotype Dx and other tests. Between 20-40% of tumors are discrepant in IHC and RT-PCR PgR expression, reflecting either technical problems or biological mechanisms. MicroRNAs regulate gene expression either at the mRNA or the protein translation level. Our hypothesis is that the latter potentially explains discrepancies between IHC and RT-PCR results. Methods: ER positive tumors were divided to three groups by PgR expression according to IHC and RT-PCR: (i) positive by IHC; (ii) negative by IHC and high mRNA levels by RT-PCR; (iii) negative by IHC and low mRNA levels by RT-PCR. RNA was extracted from tumors and adjacent normal tissue and the expression of PgR and microRNAs was assessed by RT-PCR. In addition, microRNAs were transfected into MCF-7 cells. The levels of PgR mRNA and protein were analyzed. Results: The PgR gene contains potential conserved binding sites for MicroRNAs 23a, 181a, 135a and 26b. Of these, miR- 23a and miR-181a showed inverse expression relative to the PgR expression in normal and tumor tissue, as expected. MicroRNA levels were determined in tumors of all three groups. The highest expression of microRNAs 23a, 181a and 26b was seen in IHC negative tumors with low levels of mRNA. Intermediate expression was seen in IHC negative tumors with high mRNA levels, and the lowest expression was detected in IHC positive tumors. Preliminary results suggest that over expression of microRNAs 23a, 181a and 26b in MCF7 cells decreased PgR levels. Conclusions: Our results suggest that microRNAs 23a , 181a and 26b regulate the level of the progesterone receptor in breast cancer and that tumors with relatively high mRNA levels may be deficient in PgR protein due to downregulation by microRNAs. [Table: see text]