Valproic Acid Upregulates NKG2D Ligand Expression through an ERK-dependent Mechanism and Potentially Enhances NK Cell-mediated Lysis of Myeloma

Modulation of the antitumor immune response through the engagement of NKG2D receptors with their ligands (L) on targets represents a promising therapeutic approach against cancer. In this study, we tested the effect of valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, on the expression of NKG2D ligands in myeloma cells. We demonstrated that VPA was able to upregulate both protein and mRNA expression of major histocompatibility complex class I-related chain (MIC) A/B and UL16-binding protein (ULBP) 2 without any significant effect on the expression of ULBP1, ULBP3, and ULBP4 or induction of other natural killer (NK) cell ligands, such as NKp30-L, NKp44-L, and NKp46-L in myeloma cells. A ⁵¹Cr release assay and degranulation assay indicated that the induction of MICA/B and ULBP2 augmented NK cell-mediated lysis of myeloma cells, which was abolished by the addition of a blocking NKG2D antibody. Activation of constitutively phosphorylated extracellular signal-regulated kinase (ERK) by VPA is essential for the up-regulation of MICA/B and ULBP2 expressions. Inhibition of ERK using ERK inhibitor PD98059 decreased both MICA/B and ULBP2 expressions and NK cell cytotoxicity. Furthermore, overexpression of constitutively active ERK in ARK resulted in increased MICA/B and ULBP2 expressions and enhanced NK cell lysis. These data indicate that increased sensitivity of VPA-treated myeloma cells to NK cell lysis is caused by higher NKG2D ligand expression, resulting from more active ERK signaling pathway. Our results provide evidence that targeting ERK signaling pathway may be an additional mechanism supporting the antimyeloma activity of HDAC inhibitors and suggest its possible immunotherapeutic value for myeloma treatment.