Abstract
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