Lysinuric protein intolerance mutation is expressed in the plasma membrane of cultured skin fibroblasts

Abstract

Lysinuric protein intolerance (LPI) is an autosomal recessive phenotype consistent with impaired transport of cationic amino acids at the basolateral membrane of intestinal and renal epithelia. On the assumption that the basolateral membrane of epithelial cells and plasma membrane of parenchymal cells are functional analogues, we studied transport of cationic amino acids by cultured skin fibroblasts from LPI and control subjects matched for age, sex, and site of biopsy. We measured Na+-independent transport of radiolabeled lysine, arginine, ornithine, and homoarginine on system y+, the carrier with preference for cationic amino acids, and leucine transport on system L (as the internal control). LPI cells had increased net uptake of cationic amino acids (nmol/mg of protein) relative to leucine. LPI cells also maintained increased steady-state intracellular pools of cationic amino acids. Neither increased metabolic utilization nor increased pool size were responsible for high uptake of cationic amino acids in LPI cells. We then measured trans-stimulated efflux of homoarginine as a specific test of system y+ activity. Homoarginine efflux was significantly impaired in LPI cells (P less than 0.05), whereas leucine efflux was similar in LPI and control cells. Percent trans-stimulation of homoarginine efflux was 1.0 +/- 0.5% in homozygous LPI cells, 10 +/- 0.5% in heterozygous cells, and 22 +/- 0.5% in control cells indicating a gene-dosage effect. The LPI mutation affects system y+ asymmetrically, selectively impairing efflux in fibroblast plasma membrane. To our knowledge, this appears to be the first demonstration that the skin fibroblast can be used to study a corresponding transport defect in intestinal and renal membranes.