No abstract

IntroductionMultiple myeloma, a malignant plasma cell disorder, has an incidence rate of approximately 1.1% among all malignancies and constitutes 12%-13% of hematologic malignancies in the United States. 1 In a study examining newly diagnosed patients with multiple myeloma, the incidence of renal dysfunction, determined by serum creatinine elevation Ն 1.3 mg/dL, was 48%. These same investigators showed that an increase in the serum creatinine concentration to Ն 2.0 mg/dL portended a poor prognosis, with a 35% reduction in median survival compared with patients with normal serum creatinine concentrations. 2 Mortality was accentuated if patients developed end-stage kidney disease in the setting of multiple myeloma. In one study that examined outcomes in 3298 patients, the 2-year all-cause mortality was 58% compared with 31% for patients with end-stage kidney disease from other causes. 3 The immunoglobulin free light chain (FLC) is the culprit in most of these renal lesions, and the majority of patients with renal failure from monoclonal FLCs in this setting have tubulointerstitial renal disease. 4 These studies place importance on maintaining or improving renal function and emphasize the need to focus not only on the reduction of FLCs during treatment, but also on understanding the underlying renal pathophysiology.A major function of the kidney is to reclaim low-molecularweight proteins that appear in the glomerular ultrafiltrate. FLCs are low-molecular-weight proteins that readily undergo glomerular filtration and are processed by the proximal tubule epithelium. Specifically, FLCs are absorbed into the proximal tubule by a receptormediated complex that consists of megalin and cubilin. [5][6][7][8] Once endocytosed, proximal tubule epithelial cells hydrolyze the proteins and return the amino acid residues to the circulation. Under normal conditions, total serum FLC concentration is typically Ͻ 30 mg/L, and approximately 500 mg of FLCs are cleared daily by the kidney; however, in pathologic states such as multiple myeloma, serum levels exceeding 100 000 mg/L have been observed. 9,10 In addition, unlike other proteins, this renal reclamation process is complicated by intracellular oxidative stress due to the production of hydrogen peroxide, which promotes cytotoxicity and also initiates signaling cascades that produce a pro-inflammatory state with elaboration of monocyte chemoattractant protein-1 (MCP-1) and interleukin-6 (IL-6). 11,12 As is true for most progressive forms of renal disease regardless of underlying etiology, tubulointerstitial injury and fibrosis are invariant findings that contribute to a progressive loss of renal function. 13 Thus, receptor-mediated endocytosis and metabolism of monoclonal FLCs generates an intrarenal pro-inflammatory environment that exacerbates ongoing renal injury and tubulointerstitial fibrosis, promoting functional progression of the kidney disease.The intracellular signaling process is known to be mediated through oxidative activation of c-Src, the 60-kDa product of c-src, a...

As low molecular weight proteins, restriction from glomerular filtration is minimized, permitting significant amounts of Ig light chains to be endocytosed into the proximal tubule epithelium, particularly in plasma cell dyscrasias. Recent studies have shown that this effect of concentrating light chains within the proximal tubule alters cell function. This study demonstrated that light chains belonged to a class of proteins that are capable of catalyzing the formation of hydrogen peroxide. Sufficient amounts of hydrogen peroxide were produced in HK-2 cells to stimulate the production of monocyte chemoattractant protein-1 (MCP-1), a key chemokine involved in monocyte/macrophage migration and activation of the proximal tubule, and to increase lactate dehydrogenase release into the medium. The light chain-mediated effect on MCP-1 production was inhibited by co-incubation with 1,3-dimethyl-2-thiourea, which also inhibited lactate dehydrogenase release, and by pyrrolidine dithiocarbamate, an inhibitor of NF-B. The amount of light chain that stimulated an intracellular redox-signaling pathway in the proximal tubule cells was well within levels that are seen in patients who have plasma cell dyscrasias. The conclusion is that light chains possess a unique property that permits the development of intracellular oxidative stress that in turn promotes activation of the proximal tubule and elaboration of MCP-1.

The renal proximal tubule metabolizes circulating low-molecular-weight proteins such as Ig free light chains. In the setting of plasma cell dyscrasias, the burden of filtered protein can be very high. Endocytosis of certain nephrotoxic light chains induces H 2 O 2 production and monocyte chemoattractant protein-1 (MCP-1) release, leading to recruitment of inflammatory cells and interstitial fibrosis, but how these processes are linked mechanistically is not well understood. This study investigated the relationship between H 2 O 2 generated after light chain endocytosis by human proximal tubular (HK-2) cells and activation of c-Src, a redox-sensitive tyrosine kinase. HK-2 cells exposed to two different light chains upregulated c-Src activity, which increased the production of MCP-1. In parallel, we observed a time-dependent oxidation of c-Src. Inhibition of c-Src activity and silencing c-Src expression abrogated the light chain-induced MCP-1 response, but had no effect on H 2 O 2 , indicating that production of H 2 O 2 is upstream of c-Src in the signaling cascade. Silencing megalin and cubilin expression inhibited the MCP-1 response, whereas extracellular catalase did not, indicating that endocytosis is required and that intracellular generation of reactive oxygen species activates c-Src. These data show that intracellular H 2 O 2 induced by endocytosis of monoclonal free light chains oxidizes and activates c-Src, which promotes release of MCP-1.