Expression of folate receptors alpha and beta in normal and cancerous gynecologic tissues: correlation of expression of the beta isoform with macrophage markers

Abstract

Background: Folate receptor alpha (FOLR1/FRA) is expressed in a number of epithelial cancers and in particular epithelial ovarian cancer (EOC), especially of the serous histotype. Recent studies have shown that EOC originates from the fallopian tube fimbriae rather than from epithelial cells lining the ovary. We have previously shown by immunohistochemistry a strong correlation between FRA expression in EOC and normal and fallopian adenocarcinoma. Folate receptor beta (FOLR2/FRB) has been described to be expressed by macrophages both in inflammatory disorders and certain epithelial cancers. Given the high sequence identity of these two folate receptor family members we sought to investigate the architectural and cell-specific expression of these two receptors in gynecologic tissues.

Methods: RNA scope, a novel chromogenic in situ hybridization assay tool, was used to examine expression of the alpha (FOLR1) and beta (FOLR2) isoforms of folate receptor relative to each other as well as to the macrophage markers CD11b and CD68, in samples of normal fallopian tube and fallopian adenocarcinoma as well as normal ovary and EOC.

Results: We demonstrated expression of both FOLR1 and FOLR2 in EOC, normal fallopian tube and fallopian adenocarcinoma tissue while very little expression of either marker was observed in normal ovary. Furthermore, FOLR2 was shown to be expressed almost exclusively in macrophages, of both the M1 and M2 lineages, as determined by co-expression of CD11b and/or CD68, with little or no expression in epithelial cells.

Conclusions: These findings further substantiate the hypothesis that the cell of origin of EOC is tubal epithelium and that the beta isoform of folate receptor is primarily restricted to macrophages. Further, macrophages expressing FOLR2 may represent tumor associated or infiltrating macrophages (TAMs) in epithelial cancers.

Fig. 1

Partial sequences for the 4 isoforms of human folate receptor are depicted to show the 16 conserved cysteine residues (red highlighted) between all isoforms. FRA and FRB have 71 % identity across their entire sequences. GPI-anchored FRA has 233 amino acids. The highlighted (blue) serine in the FRA sequence indicates the site of attachment of the GPI-anchor

Fig. 2

Expression of FOLR1 and FOLR2 were detected by dual color staining for FOLR1 (red) and FOLR2 (green) mRNA in normal fallopian tube (Fig. 2a), normal ovary (Fig. 2b), fallopian adenocarcinoma (Fig. 2c, d), and epithelial ovarian cancer (EOC) (Fig. 2e, f). Images are 40x magnification

Fig. 3

Expression of macrophage marker CD11b (red) in combination with FOLR2 (green) in normal fallopian tube (Fig. 3a) and normal ovary (Fig. 3b). Expression of macrophage markers CD11b (red) and CD68 (green) in fallopian adenocarcinoma (Fig. 3c) and EOC (Fig. 3d). Expression of macrophage marker CD68 (red) in combination with FOLR2 (green) in fallopian adenocarcinoma (Fig. 3e) and EOC (Fig. 3f). Images are 40x magnification