Regulation of Parathyroid Hormone‐Related Peptide Gene Expression by Estrogen in GH4C1 Rat Pituitary Cells Has the Pattern of a Primary Response Gene

The parathyroid hormone‐related peptide (PTHrP) gene has been reported to be subject to a wide variety of physiological and pharmacological controls. Two distinct patterns of PTHrP mRNA response have been recognized, one characterized by a prolonged or plateau response lasting many hours to days and the second characterized by rapid induction‐deinduction kinetics and lasting 1 to several hours. The kinetics of the second pattern are similar to those displayed by primary response genes like nuclear protooncogenes, cytokines, and growth factors. In GH4C1, rat pituitary cells, 17β‐estradiol induced a rapid and transient increase in PTHrP mRNA expression, with a peak response at 1–2 h. This response appeared to be due to a rapid and transient burst in gene transcription, which by runoff analysis was maximal at 20–40 min and declined thereafter. PTHrP mRNA half‐life was 30 min in these cells and was unaltered by estradiol. Cy‐cloheximide did not block the 17β‐estradiol‐induced response but rather prolonged it, and runoff analysis revealed that this effect was due to a prolongation or persistence of PTHrP gene transcription. These findings suggest that the transient nature of the native response reflects the effects of an estrogen‐inducible represser. All of these features are characteristic of a prototypical primary response gene.