Metabolic, enzymologic, and immunohistochemical techniques have been used to show that a human cell line of breast epithelial origin synthesized medium chain
fatty acids via the ubiquitous
fatty acid synthetase and a mammary-specific chain-terminating
enzyme,
thioesterase II. Previous studies in our laboratory with rodents indicated that
thioesterase II is expressed exclusively in mammary epithelial cells, an observation consistent with the physiologic role of the
enzyme in milk fat synthesis. Results of the present study suggest that the
enzyme exhibits a similar cell specificity in its expression in humans and that the specificity is maintained in normal and neoplastic tissues. Thus
thioesterase II was detected immunohistochemically in normal human breast epithelia derived from both lactating and nonlactating breast tissue, in cultured cells derived from both primary breast epithelial
tumors and from a metastatic
tumor of breast origin, and in several human breast epithelial cell lines; the
enzyme could not be detected in HeLa cells, in a colon
carcinoma, or in a mammary myoepithelial cell line. These findings raise the possibility that
thioesterase II may be of use as a diagnostic tool to identify human
tumors of breast epithelial origin.