Complementary DNA cloning of a novel epithelial cell marker protein, HME1, that may be down-regulated in neoplastic mammary cells

A full-length complementary DNA clone from a normal human mammary epithelial cell (strain 184) encoding a 25-kilodalton protein, HME1, was isolated. Expression of HME1 RNA appears to be limited to epithelial cells. The HME1 sequence has extensive sequence homology with bovine 14-3-3 protein, which is an activator of tyrosine and tryptophan hydroxylase. However, the tissue distribution, arrangement of charged amino acids, and location of potential phosphorylation sites of HME1 differ from those of 14-3-3. Compared with normal mammary epithelial cells, expression of HME1 RNA was dramatically low in two cell lines derived from human mammary carcinoma that were examined, and in a line of normal mammary epithelial cells transformed by oncogenes. HME1 therefore appears to be a cellular differentiation marker that may be down-regulated during neoplastic transformation.