Characterising the expression of the organic cation transporter OCT3 in cutaneous papillomas of dogs.

Background: The identification of the activation of the mammalian target of rapamycin (mTOR) signalling pathway as a frequent molecular event in canine cutaneous papillomas (CPs) has provided the rational foundation to explore novel molecular-targeted therapies. Recent evidence indicates that metformin reduces the size of CPs in mice by inhibiting the mTOR signalling pathway. These effects require the expression of the organic cation transporter 3 (OCT3/SLC22A3), a well-known metformin uptake transporter.

Hypothesis/objectives: The aim of the present study was to characterise the expression pattern of the metformin uptake transporter OCT3 in canine samples of CP that have shown activation of the mTOR signalling pathway in order to predict if this hyperplastic epidermal lesion is potentially sensitive to metformin.

Methods: The expression of OCT3 was evaluated by immunohistochemical investigation in sections of a previously constructed tissue microarray containing 28 samples of canine CP and compared with that previously evaluated for the mTOR activation marker pS6.

Results: OCT3 was highly expressed in the membrane and cytoplasm of the basal and suprabasal epidermal cells in all samples of canine CP. This OCT3 expression was localised at similar epidermal compartments to those observed for pS6.

Conclusions and clinical relevance: These results show that canine CPs exhibit the expression of surrogate markers that suggest sensitivity to metformin, such as upregulated OCT3 and pS6 expression. Taken together, these findings provide the rationale for the early assessment of the use of metformin as a mechanism-based therapeutic approach for treating canine patients with persistent or multiple CPs.