Progesterone receptor structure and protease activity in primary human endometrial carcinoma

Monoclonal antibodies were used to investigate progesterone receptor structure (isoforms) in 33 primary human endometrial tumors. The monoclonal antibodies recognized on protein blots two progesterone receptor proteins with molecular weights of 116,000 and 81,000. The Mr 116,000 protein appeared as a triplet, while a single band was found for the Mr 81,000 protein. The triplet/singlet structure was found in all progesterone receptor-positive tumors, regardless of the degree of tumor differentiation. Protease activity, which gave rise to a false-negative pattern on protein blots, was found in approximately one-half of the tumors in which it was investigated. Inclusion of a cocktail of protease inhibitors during sample preparation resulted in the maintenance of the triplet/singlet progesterone receptor structure. Mixing experiments using a progesterone receptor-rich human endometrial carcinoma (EnCa 101), which lacks protease activity, and protease-containing primary tumor homogenates indicated that the protease was leupeptin sensitive. Interestingly, while the proteolytic activity reduced or eliminated the triplet/singlet progesterone receptor structure seen on protein blot analysis, it did not affect progesterone receptor concentration measured by Scatchard analysis. Sample preparation in the presence of protease inhibitors is therefore a requisite for structural analysis of the progesterone receptor in endometrial tumors.