The
LK26 antigen is a
cell surface glycoprotein (M(r)35,000 to 40,000) of normal placenta and gestational
choriocarcinomas that shows highly restricted distribution in normal tissues, being expressed primarily in a subset of simple epithelia. In this study, immunohistochemical methods were used to examine LK26 expression in 78 ovarian
tumors and > 400
tumors of other histological types. Ovarian
carcinomas derived from coelomic epithelium showed the most consistent and strongest immunostaining for LK26, with 52 of 56 cases being LK26+. Ovarian
tumors of sex cord, germ cell, and stromal origin were generally LK26-. LK26 was not found in normal fetal or adult ovary; however, it was present in the lining epithelia of some benign
ovarian cysts.
Mesotheliomas, which share a common mesothelial origin with LK26+ ovarian
tumors, expressed no or only low levels of LK26. Other epithelial
cancers expressed LK26 in subsets of cases and generally showed heterogeneous or weak immunostaining; this group of LK26+
tumors includes endometrial (10 of 11 cases tested), colorectal (six of 27), breast (11 of 53), lung (six of 18), and renal cell (nine of 18)
carcinomas. Four of five
brain metastases derived from epithelial
cancers and three of 21
neuroendocrine carcinomas showed prominent LK26 immunoreactivity. Only rare
neuroectodermal tumors (two of 70) and none of the
sarcomas (none of 58) or
lymphomas tested (none of 21) were LK26+. Tests with cultured cells showed that the LK26
proteins expressed in
choriocarcinoma and
ovarian cancer cells are biochemically similar, and transfection experiments identified LK26 as an adult-type, high-affinity
folate-
binding protein. The present study provides the first detailed specificity and sensitivity analysis for
folate-
binding protein/LK26 in human
tumors and defines a role for
folate-
binding protein/LK26 in immunobiological studies of
ovarian cancers and other LK26+
neoplasms.