Valproic acid, targets papillary thyroid cancer through inhibition of c-Met signalling pathway

Abstract

Tyrosine kinase receptors such as c-Met and its ligands are interesting therapeutic targets that have been reported to be involved in the progression of several types of cancers. Histone deacetylase inhibitor, valproic acid (VPA) is one such compound with promising anti-cancer properties. The current study was designed to evaluate the c-Met activity of VPA in thyroid carcinoma. A total 36 nu/nu mice with SW1736 cells-induced tumours were randomised into three treatment groups (5, 15, 30 mg/kg/day p.o. VPA; n = 9/group). Various cellular and enzymatic assays were performed to evaluate the dose-response relationship of VPA in c-Met inhibition. In vitro assays revealed that VPA (IC₅₀, 5-26 nmol/l) shows c-Met phosphorylation and c-Met-dependent inhibition of cellproliferation. This causes inhibition of downstream signalling pathways in human thyroid cancer cell lines (SW1736, WRO). Additionally, VPA also showed anti-angiogenetic activity in HGF-stimulated endothelial cell. VPA showed significant reduction in tumour size in xenograft model (P = 0.023) with high levels of c-Met expression. The anticancer activity was found to be dose dependent and strongly correlated with c-Met expression. Thus, this novel finding paves way for investigation of new mechanism of action and its validation in clinical settings.

Keywords: RTK inhibitor; Thyroid cancer; c-Met signalling; papillary thyroid carcinoma; valproic acid.

Figure 1

Induction of apoptosis by different concentrations (0.1, 1.0, 10 μmol) of VA (A) SW1736 and (B) WRO cells were treated with indicated concentrations of VPA for 6 and 24 hrs. Results expressed as Mean ± SD. *P<0.05.

Figure 2

VPA inhibited tubulogenesis in HMVEC endothelial cell at indicated concentrations. Using a collagen-fibrin matrix, the HMVEC cells were plated and tubulogenesis was observed within 7 days after plating.

Figure 3

A. Valproic acid shows inhibition of tumour growth in SW1736 xenograft model. B. Valproic acid induced inhibition of c-Met phosphorylation and downstream signaling pathways in SW1736 tumors.

Figure 4

A. Immunohistochemical evaluation of Ki67 expression after administration of valproic acid. B. Dose dependent inhibition of c-Met phosphorylation in xenograft model. C. Inhibition of tumour cell proliferation. *, P≤0.01, shows values are statistically significant.

Figure 5

Efficacy of valproic acid on tumour MVD (A and B) and secretion of proangiogenic factors (C and D) in SW1736 induced xenograft model. Athymic mice bearing established SW1736 xenografts were administered Valproic acid p.o. at the indicated dose levels or DMSO (control). *, P≤0.05, shows that the values are statistically significant.