Characterisation of adiponectin and its receptors in the bovine mammary gland and in milk

Adiponectin is an adipocyte-derived hormone, which circulates in the form of homo-multimers. The individual oligomers have a distinct profile of activity, playing crucial roles in several biological processes, including metabolism and inflammation. Adiponectin exerts many of its effects by interacting with the receptors, AdipoR1 and AdipoR2. In the present study, mRNA expression of adiponectin, AdipoR1 and AdipoR2 was evaluated by quantitative PCR in different areas of the mammary gland in healthy lactating cows. The adiponectin isoforms in milk and blood were investigated by Western blotting and 2D-electrophoresis, and the presence of adiponectin protein was determined by immunohistochemistry. Low level expression of adiponectin mRNA was found in all areas of bovine mammary gland tissues examined. AdipoR1 and AdipoR2 mRNAs were also detected in mammary tissues and their expression was particularly prominent in the parenchyma and cistern. Western blotting revealed a heterogeneous electrophoretic pattern, indicating that different adiponectin isoforms exist in milk, compared with blood. In particular, milk shows a low molecular weight isoform of adiponectin, corresponding to the globular domain. Adiponectin in milk is characterised by a more complex 2D electrophoretic pattern, compared with blood, as illustrated by the presence of proteins of different molecular weights and isoelectric points. Adiponectin protein was detected by immunohistochemistry in epithelial cells lining the secretory alveoli, in secretum within the alveolar lumen and in small peripheral nerves. The study findings support a role for adiponectin in regulating metabolism and immunity of the bovine mammary gland and potentially the calf intestine, following ingestion of milk.