To avoid renal loss of large amounts of proteins, filtered proteins are reabsorbed by endocytosis along the proximal tubule. However, although protein reabsorption is a task of proximal tubular cells, it is also a threat because it may cause cell injury. This study determines whether exposure to bovine serum albumin (BSA) leads to regulatory changes in endocytosis of FITC-BSA in proximal tubule-derived opossum kidney cells. Preincubation with BSA led to a decrease of FITC-BSA endocytosis with an IC50 value of 0.58 g/L. Specific binding of FITC-BSA to the apical membrane was also reduced (IC50 = 0.69 g/L). Kinetic analyses revealed that maximal uptake rate and maximal binding capacity were decreased with no change in affinity. Similar effects were observed after preincubation with equimolar amounts of other proteins (lactalbumin, transferrin, and conalbumin), but not after preincubation with dextran. The effect of preincubation with BSA could be mimicked by preincubation with some amino acids. Preincubation with L-Ala, L-Gln, or NH4Cl, but not with L-Leu, L-Glu, or L-Asp, reduced FITC-BSA endocytosis and binding. Preincubation with BSA, but not with dextran, reduced protein degradation and increased ammonia production, vesicular pH, as well as the rate of lactate dehydrogenase release. Apical fluid-phase endocytosis and apical uptake of neutral amino acids were not reduced. It is concluded that proximal tubular cells reduce the uptake rate for proteins, but not for other substrates, in response to increased protein load. This reduction is achieved by reducing the number of apical binding sites, partially in response to increased ammoniagenesis with deranged vesicular pH and enzyme activities. Thus, increased protein filtration could result in reduced protein reabsorption, thereby enhancing proteinuria.
Transforming growth factor-beta1 reduces megalin- and cubilin-mediated endocytosis of albumin in proximal-tubule-derived opossum kidney cells