Functional relationship between cationic amino acid transporters and β-defensins: Implications for dry skin diseases and the dry eye

The ocular surface, constantly exposed to environmental pathogens, is particularly vulnerable to infection. Hence an advanced immune defence system is essential to protect the eye from microbial attack. Antimicrobial peptides, such as β-defensins, are essential components of the innate immune system and are the first line of defence against invaders of the eye. High concentrations of l-arginine and l-lysine are necessary for the expression of β-defensins. These are supplied by epithelial cells in inflammatory processes. The limiting factor for initiation of β-defensin production is the transport of l-arginine and l-lysine into the cell. This transport is performed to 80% by only one transporter system in the human, the y+-transporter. This group of proteins exclusively transports the cationic amino acids l-arginine, l-lysine and l-ornithine and is also known under the term cationic amino acid transporter proteins (CAT-proteins). Various infections associated with l-arginine deficiency (for example psoriasis, keratoconjuctivitis sicca) are also associated with an increase in β-defensin production. For the first time, preliminary work has shown the expression of human CATs in ocular surface epithelia and tissues of the lacrimal apparatus indicating their relevance for diseases of the ocular surface. In this review, we summarize current knowledge on the human CATs that appear to be integrated in causal regulation cascades of β-defensins, thereby offering novel concepts for therapeutic perspectives.