Inhibition of Na-K-ATPase activity and gene expression by a myeloma light chain in proximal tubule cells

Background: Light chain nephrotoxicity is frequently associated with Fanconi syndrome characterized by aminoaciduria, glycosuria, phosphaturia, and bicarbonaturia, The mechanisms of these transport abnormalities are unknown. To determine the role of Na-K-ATPase, we examined the effects of a lambda-light chain on both the activity and gene expression of Na-K-ATPase in primary cultures of rat proximal tubule cells. Methods: The lambda-light chain used here was isolated from urine of a patient with multiple myeloma and previously shown to inhibit sodium-dependent phosphate and glucose transport in proximal tubule cells. Na-K-ATPase was determined spectrophotometrically and the gene expression by Northern analysis in cells exposed to light chain. Results: In cells exposed to 200 mu mol/L light chain Na-K-ATPase activity was reduced significantly, up to 73%, at 2, 24, and 48 hours compared with control cells (N=12, P<0.001). Northern analysis showed that in cells exposed to light chain for 24 and 48 hours the message for the alpha-1 isoform of Na-K-ATPase was suppressed significantly compared with control cells. The messages for GAPDH, beta-actin, and 28 S RNA in light chain exposed cells were also depressed in comparison with central cells, This light chain also significantly inhibited thymidine incorporation by proximal tubule cells in a dose-dependent manner. Conclusions: These data suggest a general toxicity to cells by this light chain and indicate that inhibitory effects on both the activity and gene expression of Na-K-ATPase may be an important mechanism of light chain cytotoxicity on proximal tubule cells.