Development of a site-directed polyclonal antibody against the pituitary growth hormone-releasing hormone receptor and its use to estimate GHRH receptor concentration in normal and hypothyroid rats

A site-directed polyclonal antipeptide antibody was generated in rabbits ag ainst segment 392-404 of the rat pituitary growth hormone-releasing hormone receptor (GHRH-R), using a multiple antigenic peptide system strategy of i mmunization. This C-terminal intracellular region of the rat GHRH-R exhibit s 85% sequence identity with the human GHRH-R. The purified anti-GHRH-R(392 -404) IgGs were characterized in cell lines expressing the human GHRH-R and in rat and human anterior pituitary, using immunoblotting. The polyclonal antibody recognized a 45-kD protein in human GHRH-R-transfected BHK 570 cel l membrane preparations but not in wild-type cells. A 45-kD Na-tagged human GHRH-R was immunodetected with both antitag and anti-GHRH-R antibodies in human GHRH-R-transfected HEK 293 cells. Cross-linking of [I-125-Tyr(10)]hGH RH(1-44)NH2 to GHRH-R-transfected BHK cells led to the detection of a major and specific 45-kD radioactive complex. Its probing with the anti-GHRH-R(3 92-404) IgGs led also to the detection of a 45-kD entity. In rat anterior p ituitary homogenates or membrane preparations, immunoblotting led to the de tection of 44-, 47- and 65-kD proteins. In human anterior pituitary membran e preparations, immunoblotting led to the detection of 52- and 55-kD protei ns. No immunoreactive signal was observed in the rat liver. Cross-linking o f [I-125-Tyr(10)]hGHRH(1-44)NH2 to rat anterior pituitary homogenates revea led the presence of specific 28-, 47- and 65-kD radioactive complexes. Prob ing of these radioactive complexes with the anti-GHRH-R(392-404) IgGs resul ted in the visualization of 28-, 47- and 65-kD entities and of an additiona l immunoreactive 44-kD protein. To assess the usefulness of this GHRH-R ant ibody, estimation of changes in the concentration of rat anterior pituitary GHRH-R was performed by immunoblotting and compared to binding data after a S-week antithyroid treatment. The treatment known to depress the 2.5- and 4-kb GHRH-R mRNA transcripts by at least 1.7-fold decreased the apparent m aximal concentration of high (B-max1) and low (B-max2) affinity binding sit es by 4.6- and 15.2-fold, respectively, and the 47- and 65-kD GHRH-R protei ns by 3.5- and 1.25-fold, respectively. Altogether, the characteristics of the anti-GHRH-R(392-404) polyclonal anti-body indicate that it specifically recognizes the human and rat GHRH-R. It also represents an additional valu able tool to estimate variations of GHRH-Rs in physiopathological condition s known to affect GHRH-R mRNA and/or GHRH binding site concentrations.