Endogenous PTHrP C-terminus antagonises Bglap towards a non-canonical pathway by stimulating Wnt signalling independently of OSE2 element and PTH1R/PKA signalling

Abstract

PTHrP (Parathyroid Hormone-Related Protein) is a proteinaceous hormone and its signalling and functional regions are believed to be essential regulators for development for bone. Here, we report OSE2 element and PTH1R/PKA -independent physiology PTHrP C-terminus functions in bone. Using genetic models, we systematically interrogated PTHrP regions in various backgrounds in gene regulation, conducted on Ocy454 osteocytes overexpressing full-length PTHrP (Ocy454PTHrPFL), and mutant forms of PTHrP (Ocy454PTHrPΔNLSΔC, Ocy454PTHrPΔNLS, Ocy454PTHrPΔSec). PTHrP C-terminus, but not the PTHrP-NLS, remarkably limits magnitude- and numbers- dependencies of PTH/PTHrP signalling activated genes in osteocytes and osteoblastic-like osteosarcoma. C-terminus plays protective effects in bone formation signaling by down-regulation of Bglap1/2, observed in PTH /PTHrP signaling driven signalings. Unexpectedly, C-terminus gene suppression effect is cytosolic and is not dependent on canonical PTHrP-cAMP/PKA promoted signalling, nor is regulated by phosphorylated Runx2 binding to the OSE2 cis-regulatory elements. Instead, we show that PTHrP C-terminus mediated Bglap reduction results from stimulating Wnt signalling through an inhibition of Wnt inhibitor Dkk1. Collectively, our work identifies non-canonical PKA/cAMP/CREB response-independent functions of cytosolic PTHrP C-terminus to suppress Bglap, and unravel its complex endegenous C-terminus region-PTHrP interactions role in bone.