Inhibition of histone deacetylase 6 destabilizes ERK phosphorylation and suppresses cancer proliferation via modulation of the tubulin acetylation-GRP78 interaction.

Background: The leading cause of cancer-related mortality worldwide is lung cancer, and its clinical outcome and prognosis are still unsatisfactory. The understanding of potential molecular targets is necessary for clinical implications in precision diagnostic and/or therapeutic purposes. Histone deacetylase 6 (HDAC6), a major deacetylase enzyme, is a promising target for cancer therapy; however, the molecular mechanism regulating cancer pathogenesis is largely unknown.

Methods: The clinical relevance of HDAC6 expression levels and their correlation with the overall survival rate were analyzed based on the TCGA and GEO databases. HDAC6 expression in clinical samples obtained from lung cancer tissues and patient-derived primary lung cancer cells was evaluated using qRT-PCR and Western blot analysis. The potential regulatory mechanism of HDAC6 was identified by proteomic analysis and validated by immunoblotting, immunofluorescence, microtubule sedimentation, and immunoprecipitation-mass spectrometry (IP-MS) assays using a specific inhibitor of HDAC6, trichostatin A (TSA) and RNA interference to HDAC6 (siHDAC6). Lung cancer cell growth was assessed by an in vitro 2-dimensional (2D) cell proliferation assay and 3D tumor spheroid formation using patient-derived lung cancer cells.

Results: HDAC6 was upregulated in lung cancer specimens and significantly correlated with poor prognosis. Inhibition of HDAC6 by TSA and siHDAC6 caused downregulation of phosphorylated extracellular signal-regulated kinase (p-ERK), which was dependent on the tubulin acetylation status. Tubulin acetylation induced by TSA and siHDAC6 mediated the dissociation of p-ERK on microtubules, causing p-ERK destabilization. The proteomic analysis demonstrated that the molecular chaperone glucose-regulated protein 78 (GRP78) was an important scaffolder required for p-ERK localization on microtubules, and this phenomenon was significantly inhibited by either TSA, siHDAC6, or siGRP78. In addition, suppression of HDAC6 strongly attenuated an in vitro 2D lung cancer cell growth and an in vitro 3D patient derived-lung cancer spheroid growth.

Conclusions: HDAC6 inhibition led to upregulate tubulin acetylation, causing GRP78-p-ERK dissociation from microtubules. As a result, p-ERK levels were decreased, and lung cancer cell growth was subsequently suppressed. This study reveals the intriguing role and molecular mechanism of HDAC6 as a tumor promoter, and its inhibition represents a promising approach for anticancer therapy.