PTHrP targets salt-inducible kinases, HDAC4 and HDAC5, to repress chondrocyte hypertrophy in the growth plate

Abstract

Hypertrophy of chondrocytes is a crucial step in the endochondral bone formation process that drives bone lengthening and the transition to endochondral bone formation. Both Parathyroid hormone-related protein (PTHrP) and Histone deacetylase 4 (HDAC4) inhibit chondrocyte hypertrophy. Use of multiple mouse genetics models reveals how PTHrP and HDAC4 participate in a pathway that regulates chondrocyte hypertrophy. PTHrP/cAMP/protein kinase A (PKA) signaling pathway phosphorylates the PKA-target sites on salt-inducible kinase 3 (Sik3), which leads to inhibition of Sik3 kinase activity. Inhibition of Sik3 kinase activity decreases phosphorylation of HDAC4 by Sik3 at binding sites for 14-3-3; lower levels of HDAC4 phosphorylation then allow HDAC4 nuclear translocation. In the nucleus, the transcription factor, Myocyte Enhancer Factor 2 (Mef2), activates Runt-related transcription factor 2 (Runx2), and together these two transcription factors drive the hypertrophic process. HDAC4 binds both Mef2 and Runx2 and blocks their activities. There are genetic redundancies in this pathway. Sik1 and Sik2 also mediate PTHrP/cAMP/PKA signaling when Sik3 activity is low. HDAC5 also mediates PTHrP signaling when HDAC4 expression is low. Thus, PTHrP triggers a kinase cascade that leads to inhibition of the key transcription factors (Mef2 and Runx2) that promote chondrocyte hypertrophy.

Keywords: Chondrocyte hypertrophy; Histone deacetylase 4 (HDAC4); Myocyte enhancer factor 2 (Mef2); Parathyroid hormone-related protein (PTHrP); Protein kinase A (PKA); Salt-inducible kinase 3 (Sik3).

Figure 1.. Proposed model for regulation of chondrocyte hypertrophy.

The pathways in red letters drive chondrocyte hypertrophy. The pathways in blue letters inhibit chondrocyte hypertrophy. Chondrocyte hypertrophy is determined by the balance between the actions of Mef2 and HDAC4. Mef2 drives chondrocyte hypertrophy both by direct action and by activating Runx2 expression. Sik3 phosphorylates the 14-3-3 binding sites of HDAC4, which keeps HDAC4 in cytoplasm and subsequently inhibits HDAC4 to access Mef2 in nucleus. PTHrP inhibits the Mef2 action by releasing HDAC4 from 14-3-3. PTHrP reduces Sik3 activity through phosphorylation of PKA target sites on Sik3.