Cellular Localization of the Folate Receptor: Potential Role in Drug Toxicity and Folate Homeostasis

In the past year, gp38, a glycosyl-phosphatidylinositol linked membrane protein that is overexpressed in some malignant tissues, has been shown to be the folate receptor. Using immunohistochemical techniques with the monoclonal antibody MOvl9 against gp38, we evaluated the cellular localization of folate receptors in normal human tissues, which are potential target sites for drugs that utilize this uptake mechanism. The choroid plexus was intensely positive with staining limited to the epithelium, which in some foci had a distinct bilaminar pattern limited to the luminal and basal surfaces. The epithelium of the fallopian tube, uterus, and epididymis was highly immunoreactive. The acinar cells of the breast, submandibular salivary, and bronchial glands also showed intense staining as did the trophoblastic cells of the placenta. In the kidney reactivity was localized to the proximal tubules. Lung alveolar lining including type I and II pneumocytes stained intensely. Limited but focal reactivity was noted in the vas deferens, ovary, thyroid, and pancreas. This study in conjunction with previous work showing marked overexpression of folate receptor in some malignant cells suggests that the folate receptor may be an important target for diagnostic or therapeutic exploitation and indicates sites of potential drug toxicity.