The TRA-1-60 and TRA-1-81 human pluripotent stem cell markers are expressed on podocalyxin in embryonal carcinoma

We have previously identified the cell adhesion protein podocalyxin expressed in a human pluripotent stem cell, embryonal carcinoma (EC), which is a malignant germ cell. Podocalyxin is a heavily glycosylated membrane protein with amino acid sequence homology to the hematopoietic stem cell marker CD34. Since the initial discovery of podocalyxin in a cancerous stem cell, numerous new studies have identified podocalyxin in many different human cancers and in embryonic stem cells lines (ES) derived from human embryos. Embryonal carcinoma, as do all human pluripotent stem cells, expresses TRA-1-60 and TRA-1-81 antigens, and although their molecular identities are unknown, they are commonly used as markers of undifferentiated pluripotent human stem cells. We report here that purified podocalyxin from embryonal carcinoma has binding activity with the TRA-1-60 and TRA-1-81 antibodies. Embryonal carcinoma cells treated with retinoic acid undergo differentiation and lose the TRA-1-60/TRA-1-81 markers from their plasma membrane surface. We show that podocalyxin is modified in the retinoic acid-treated cells and has an apparent molecular mass of 170 kDa on protein blots as compared with the apparent 200-kDa molecular weight form of podocalyxin expressed in untreated cells. Furthermore, the modified form of podocalyxin no longer reacts with the TRA-1-60/TRA-1-81 antibodies. Thus, embryonal carcinoma expresses two distinct forms of podocalyxin, and the larger version is a molecular carrier of the human stem cell-defining antigens TRA-1-60 and TRA-1-81.