SIK Activators to Treat PTH Pathway Bone Diseases

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An overactive Parathyroid Hormone Receptor 1 (PTHR1) and its downstream signaling pathway (Gsα) are involved in several bone diseases including hyperparathyroidism, Jansen’s metaphyseal chondrodysplasia (due to a constitutively active PTHR1) or Fibrous Dysplasia-McCune-Albright Syndrome (due to a constitutively active Gsα) which, in the case of the genetic diseases, can cause major developmental and lifelong problems with their skeletons. In all three situations, the only therapies available are surgical or palliative. Protein Kinase A (PKA) is an enzyme whose activity is dependent on the activity of Gsα and cellular levels of cAMP. It is regulated by PTH binding to PTHR1 in osteoblasts. We have generated an inducible, osteoblast-specific mouse model of hyperactive PKA by collagen 1-directed deletion of the regulatory subunit, Prkar1a, and found a high bone turnover phenotype in skulls, long bones, vertebrae and caudal bones of the tail that mimics the PTHR1 diseases. We and others have shown that PTH action on the osteoblast through PKA controls gene expression, in part, through inhibition by phosphorylation of salt-inducible kinases (SIKs). These kinases tonically phosphorylate cAMP-regulated transcriptional coactivators (CRTC1, 2 and 3) and histone deacetylases 4/5 (HDAC4/5), sequestering them in the cytoplasm. Upon PTH action, PKA-mediated SIK inhibition causes CRTC2 and HDAC4/5 phosphorylation levels to decrease with subsequent nuclear translocation of CRTC2/3 and HDAC4/5 which mediates transcription of the osteoclastogenic gene, Rankl, regulation of Mmp13 and suppression of Sost transcription. SIK inhibition, deletion or knockdown mimics the effects of PTH. We have recently tested several agents for possible activation of the SIKs and resultant inhibition of PTH-induced Rankl transcription in differentiating mouse calvarial osteoblasts and found a small molecule, 9117014, as a “putative” SIK activator, to fit these goals. From these data of cells in culture and preliminary data in vivo, we have developed the hypothesis that small molecule activators of SIKs will reverse the unwanted effects of the PTH signaling pathway in bone. Consequently, the specific aims of this resubmitted R21 application are to 1) determine if activation of SIKs by a small molecule will inhibit PTH- regulated gene expression in osteoblasts, by a. performing dose responses on PTH-stimulation of Rankl, b. knocking down SIK2/3, c. testing the effects on other PTH-regulated genes, 2) assess if a small molecule activator of SIKs will inhibit a high bone turnover phenotype in mice, by a. injecting a developmental model of high bone turnover, b. injecting an adult model of high bone turnover, c. injecting an adult model of hyperparathyroidism, and conducting complete bone analyses. In so doing, the results would be of high impact and may lead to further preclinical studies that could form the basis for the first disease-modifying oral treatments for patients with hyperparathyroidism, Jansen’s metaphyseal chondrodysplasia or Fibrous Dysplasia-McCune-Albright Syndrome.