9198 Dimeric [Cys25]PTH(1-34) Induces Differential Transcriptional Responses Versus Monomeric PTH(1-34)

Abstract Disclosure: M. Noh: None. H.J. Kim: None. J. Kim: None. M. Kang: None. S. Lee: None. H. Lee: None. S. Lee: None. Introduction: Patients with hypoparathyroidism caused by the homozygous mutation that changes arginine to cysteine at position 25 in parathyroid hormone (PTH) exhibit high bone densities, which suggests a potential influence of the mutant hormone on bone formation. We also have found that [Cys25]PTH forms homodimers, and that the dimeric peptide exhibits altered functionality versus monomeric PTH. Dimeric [Cys25]PTH(1-34) (Dimer) thus displays selective binding to the G protein-coupled PTH1R conformation (RG) in vitro and markedly diminished cAMP-inducing capability in vivo. Despite these alterations, Dimer induced bone anabolic effects in both calvarial injection assays and after injection in ovariectomized osteoporotic mice, closely mimicking the effects of PTH(1-34) (wtPTH). Our current objective is to explore the downstream signaling pathways activated by Dimer vs wtPTH to help elucidate the bone anabolic effects of the Dimeric analog and its therapeutic potential. Methods: A phospho-protein antibody array assay developed for G protein-coupled receptor (GPCR) downstream target proteins was employed in HEK293/hPTH1R (GP-2.3) cells following treatment with chemically synthesized wtPTH or Dimer. We further conducted RNA-seq analysis of transcriptome changes in primary cells collected from calvarial bones of mice treated with wtPTH or Dimer at 6, 12, and 24-hours post-injection. Finally, we used quantitative reverse transcription PCR (qRT-PCR) to analyze changes in specific marker genes in tibiae and kidney isolated from mice after treatment with Dimer vs. wtPTH. Results: The number of phosphorylated proteins significantly regulated by PTH1R upon exposure to Dimer was remarkably enhanced, relative to the number regulated upon exposure to wtPTH. RNA-seq analysis revealed that the early effects of wtPTH and Dimer on transcriptional regulation were similar, whereas marked differences in expression levels became apparent over time. qRT-PCR analyses of RNA expression levels in kidneys versus tibiae from mice treated with wtPTH or Dimer revealed in bone that the expression of genes associated with bone catabolism, particularly RANKL, was significantly lower in response to Dimer as compared to wtPTH treatment, while in kidney there was an overall reduced response to Dimer as compared to wtPTH. Conclusions: Our study supports the hypothesis that as compared to PTH(1-34), dimeric [Cys25]PTH (1-34) induces altered signaling responses in vivo and has a lower effect on bone catabolism. Dimeric [Cys25]PTH(1-34) thus holds interest as a potential bone anabolic agent that utilizes a distinct mechanism of action. Presentation: 6/3/2024