P-glycoprotein expression in canine mammary gland tumours related with myoepithelial cells.

P-glycoprotein is influential in chemotherapy-resistance in numerous cancers and has been widely studied in human breast cancer research, but is less studied in canine mammary gland tumour (MGT). The study was to evaluate P-glycoprotein expression and its localisations related with prognostic factors with monoclonal antibody C219, by immunohistochemistry (IHC) of 68 cases of canine malignant (n=54) and benign (n=14) MGT. Additional immunofluorescence (IF) and reverse transcriptase-polymerase chain reaction (RT-PCR) were also performed. There was a novel finding that P-glycoprotein expression with C219 localised at two different cell types: epithelial and myoepithelial cells. Myoepithelial localised tumours were 5 benign (35.5%) and 21 malignant (63.6%), while epithelial localised tumours were 12 cases, all malignant (36.5%). Unlike conventional belief, semi-quantitative evaluation of IHC intensity scores of C219 expression in malignant MGT was related with favourable histopathological parameters.