Size heterogeneity of affinity labeled estrogen receptors in the MtTF sub(4) tumor whose growth is inhibited by estradiol, in pituitary gland and uterus

Estrogen receptors (ER) of the MtTF sub(4) tumor whose growth is inhibited by estradiol (E sub(2)) were analyzed and compared to those of tissues whose growth is stimulated by E sub(2) (uterus and pituitary gland). Two classes of ER were identified. The first class is of high molecular weight. In normal tissues, it is indeed frequently made up of two subtypes as revealed by the presence of a doublet on autoradiograms. In the MtTF sub(4) tumor these subtypes were only rarely suspected and never they were as marked and distinct as in normal tissues. The second class, of low molecular weight, is also frequently made up of two subtypes in the uterus and the proportion of this class is higher in the uterus of mature than of immature rats. The MtTF sub(4) tumor contains this class of ER but, due to the presence of non-specifically labeled proteins in this region, its relative amount cannot be estimated and the doublet was exceptionally revealed. In the pituitary gland, this small receptor has not been found. Conclusions: (i) on the basis of molecular weight analyses, ER are heterogeneous, (ii) the ER pattern depends on the type of tissue and the sexual maturity of rats but all the tissues contained at least one type of the "classic" high molecular weight receptor, and (iii) no evident correlation was found between the ER pattern and the positive or negative response to estradiol.