Microtubule-Associated Protein 2 as a DHEA Binding Protein in Endometrial Cancer

Abstract

Dehydroepiandrosterone (DHEA) is an androgen secreted by the adrenal glands, but its binding affinity for the androgen receptor is very low. DHEA is transformed into androstenedione by 3β-hydroxysteroid dehydrogenase (HSD) and then into testosterone by 17β-HSD type 5, or into estrone by aromatase. DHEA is also converted into androstenediol by 17β-HSD type 1. Therefore, DHEA is considered to play an important role as a precursor hormone for sex steroid hormones. We performed a search for a protein having an amino acid sequence homology to the DHEA binding site of 17β-HSD type 1, and found that microtubule-associated protein 2 (MAP2) binds to DHEA (Laurine E et al., J Biol Chem. 2003). MAP2 expression is necessary for neurite extension and cessation of cell division. MAP2 is known to suppress migration and invasion and affect the assembly, stabilization, and bundling of microtubules in melanoma cells, but the function of MAP2 in endometrial cancer has not been clarified. In this study, we investigated the expression of MAP2 and its association with DHEA in order to clarify the direct non-receptor action of DHEA in endometrial cancer. We employed frozen and formalin-fixed paraffin-embedded (FFPE) tissues of 35 endometrial cancer tissues (G1, n=12; G2, n=10; G3, n=9; Serous, n=4). Hormone concentrations were measured by liquid chromatograph-tandem mass spectrometer from the frozen sample, and immunohistochemistry of MAP2 was performed using FFPE tissues. We also examined MAP2 immunoreactivity using 59 normal endometrial tissues (proliferative phase, n=33; secretory phase, n=26) of FFPE tissue microarray slides. MAP2 immunoreactivity was found in the cytoplasm of endometrial cancer cells, and the MAP2-positive rate was significantly higher in type 1 (G1 and G2) than in type 2 (G2 and G3). The cell proliferation marker Ki-67 index was significantly lower in the MAP2-positive group. MAP2 was also detected in the glandular epithelial cells of the normal endometrium. The MAP2-positive rate was lower in the proliferative phase than in the secretory phase. Furthermore, the concentration of DHEA in the cancer tissue was significantly higher in the MAP2-positive group than in the MAP2-negative group. MAP2 is known to act on the stability of microtubules and is thought to be involved in the suppression of proliferation and infiltration in cancer cells. It was suggested that DHEA is involved in the stabilization of MAP2 and suppresses the progression of cancer in a hormone receptor-independent manner.