Selective HDAC4 inhibition by SP1-PTD promotes odontoblast differentiation

Abstract

Background: Vital pulp therapy is limited by incomplete dentin regeneration and dose-limiting toxicities of current histone deacetylase (HDAC) inhibitors. Previous structural studies have identified critical determinants of HDAC4-silencing mediator for retinoid and thyroid hormone receptor (SMRT) protein interactions, providing a rationale for developing selective inhibition strategies. Objective: This study evaluated SMRT peptide 1-protein transduction domain (SP1-PTD), which is a cell-penetrating peptide designed to selectively disrupt HDAC4–SMRT interaction based on structural insights, for promoting odontoblast differentiation with improved safety compared to pan-HDAC inhibitors. Methodology: SP1-PTD comprises an SMRT-derived sequence fused to a PTD, enabling targeted inhibition without affecting HDAC catalytic activity. Effects on odontoblast differentiation were assessed in murine dental papilla cell lines and primary human dental pulp cells using gene expression analysis, functional mineralization assays, and mechanistic studies including chromatin immunoprecipitation and RUNX2 acetylation analysis. Cytotoxicity was directly compared with suberoylanilide hydroxamic acid (SAHA) and trichostatin A. Results: SP1-PTD treatment significantly enhanced odontoblast differentiation with 15.9-fold increase in dentin sialophosphoprotein (Dspp) expression alongside upregulation of RUNX2, osteocalcin, and bone sialoprotein. Functional analysis revealed 1.8-fold increased mineralization capacity. Mechanistically, SP1-PTD increased RUNX2 protein acetylation and histone acetylation at the Dspp promoter, indicating derepression of RUNX2-mediated transcription. Importantly, SP1-PTD did not show cytotoxicity across a wide therapeutic range (0.1-20 μM) and promoted cell proliferation, contrasting sharply with dose-dependent toxicity of pan-HDAC inhibitors. Direct comparison revealed SP1-PTD induced 14-fold increase in Dspp expression while SAHA suppressed it despite comparable Runx2 induction. Conclusions: SP1-PTD represents a first-in-class selective HDAC4 inhibitor that achieves robust pro-differentiation effects with an exceptional safety profile. By specifically targeting HDAC4–SMRT interactions, SP1-PTD overcomes limitations of conventional HDAC inhibitors and offers translational promise for dental regenerative medicine.