Estrogen Regulates the Transcription of the Rat Prolactin Genein Vivothrough at Least Two Independent Mechanisms*

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Abstract
The short-acting estrogens 16.alpha.-estradiol and estriol induced transcription of the rat PRL [prolactin] gene in vivo in a biphasic manner. In these experiments, the level of PRL gene transcription was examined by measuring the amount of radiolabeled UTP incorporated into PRL-specific RNA sequences by nuclei isolated from the anterior pituitary gland at various times after hormone treatment. A single injection of 16.alpha.-estradiol stimulated PRL gene transcription within 30 min; this initial phase of stimulated transcription was observed through 2 h after treatment. A second phase of stimulated PRL gene transcription was observed by 6 h after 16.alpha.-estradiol treatment and it continued through 24 h. A biphasic stimulation of PRL gene transcription also was observed in response to a single injection of estriol. The initial phase extended into the 2nd phase, and the phases were, therefore, distinguishable only by their differing levels of stimulation. The induction of the initial phase of increased PRL gene transcription by 16.alpha.-estradiol was observed in animals in which cycloheximide or puromycin had greatly inhibited pituitary protein synthesis. Induction of the 2nd phase of stimulated transcription by 16.alpha.-estradiol was blocked by prior cycloheximide treatment. An injection of 16.alpha.-estradiol resulted in activation of the cytosol form of the pituitary estrogen receptor to its nuclear form, with maximal levels of nuclear form receptors being observed within 1 h of injection. Within 4 h of treatment, the cytosol and nuclear forms of the pituitary estrogen receptor had returned to control levels. Apparently, estrogen regulates transcription of the rat PRL gene in vivo through at least 2 independent mechanisms.