Estrogen-induced Leydig cell tumor in the mouse: a model system for the study of carcinogenesis and hormone dependency.

Malignant neoplasms of endocrine tissues represent almost half of the cancers diagnosed clinically in the United States, and many of these respond to hormonal therapies. Estrogen-induced testicular Leydig cell tumors in the mouse would seem to represent a realistic model for the laboratory investigation of this significant group of cancers. Data accumulated over the past few years clearly show that the Leydig cell is a target tissue for estrogens. Administering large doses of estrogen results in a reduction of enzymes converting progesterone to testosterone and induces a transient, but quantitatively very significant, synthesis of DNA in the Leydig cells of tumor-susceptible strains of mice. Neither of these actions of estrogen is mediated via the hypophysis. It has been demonstrated that the Leydig cells have specific protein receptors in their cytoplasm that bind estrogens and transport them to the nucleus where they are also bound. The genetic composition of the Leydig cells themselves is extremely important for the development of tumors. An adequately functioning pituitary gland is also essential for tumor formation. Confining the testes to the abdomen results in enzyme changes similar to those produced by estrogen administration and significantly augments the development of Leydig cell tumors. Once tumors form they frequently are dependent for their continued growth on estrogenic stimulation and/or on a functioning hypothysis. Regressed tumors may remain dormant for many months only to resume progressive growth when placed in and adequate hormone environment.