The nuclear growth hormone receptor binding protein. Antigenic and physicochemical characterization.

The mechanisms involved in transcriptional regulation by growth hormone (GH) remain unknown. We report here that GH receptor immunoreactivity can be demonstrated in the nuclei of GH-responsive rat and rabbit tissues at both the light and electron micrograph level using monoclonal antibodies to the receptor extracellular domain. Nuclear staining is heterogeneous and associated with both chromatin and the nuclear membrane. To confirm these observations, nuclei were isolated from rabbit liver by two methods, one involving extensive nonionic detergent washes. Scatchard analysis of nuclear fractions revealed high affinity somatogenic receptor in nuclear membranes, nucleoplasm, and chromatin fractions. A panel of GH receptor monoclonal antibodies was used to further define these nuclear binding sites as being antigenically identical to microsomal receptor in all but one case. In addition, affinity cross-linking experiments showed the somatogenic binding subunit to have a reduced Mr of 67,000, similar to the Mr of the GH binding protein. We propose that the association of a GH binding protein with the nucleus may provide a means whereby GH can regulate the transcription of specific genes either directly or indirectly through nuclear kinase C activation. This speculation is congruent with the recent demonstration of a GH response element by Yoon et al. (Yoon, J. B., Berry, S. A., Seelig, S., and Towle, H. C. (1990) J. Biol. Chem. 265, 19947-19954).