An immunohistochemical study of vitamin D receptor expression in canine cutaneous mast cell tumours.

The active form of vitamin D (1alpha, 25-dihydroxycholecalciferol; calcitriol) has potent anti-neoplastic activity in the management of a number of human malignancies. Despite promising data to suggest that calcitriol is an effective adjunct to current chemotherapy modalities, the role of calcitriol in animal neoplasia is poorly understood. Vitamin D inhibits growth of canine mast cell tumours (MCTs) in vitro, presumably due to ligand-mediated activation of the vitamin D receptor (VDR). The aim of the present study was to examine immunohistochemically the expression of the VDR by reactive and neoplastic canine cutaneous mast cells. Expression was graded according to frequency, intensity and score (frequency x intensity). VDR expression was found in all samples containing reactive mast cells (n=9), and in 67 of 69 (97%) MCTs selected from each of the three Patnaik grades. The frequency and score of VDR labelling was greater in MCTs compared with reactive mast cells (P=0.0005 and 0.001, respectively). There was no difference in VDR frequency between the MCT grades, but the frequency of labelling in grade 3 MCTs was higher than for reactive mast cells (P=0.001). There was no association between tumour mitotic index and any of the three VDR variables (all P>0.16). VDR is widely expressed by reactive and neoplastic canine mast cells in vivo. VDR expression is unlikely to represent an independent prognostic factor, but its presence within biopsy specimens might be used to identify patients that are suited to high-dose vitamin D therapeutic trials.