A soluble form of human KIM-1 can be detected in the urine of patients with ATN and may serve as a useful biomarker for renal proximal tubule injury facilitating the early diagnosis of the disease and serving as a diagnostic discriminator.

We report the identification of rat and human cDNAs for a type 1 membrane protein that contains a novel six-cysteine immunoglobulin-like domain and a mucin domain; it is named kidney injury molecule-1 (KIM-1). Structurally, KIM-1 is a member of the immunoglobulin gene superfamily most reminiscent of mucosal addressin cell adhesion molecule 1 (MAdCAM-1). Human KIM-1 exhibits homology to a monkey gene, hepatitis A virus cell receptor 1 (HAVcr-1), which was identified recently as a receptor for the hepatitis A virus. KIM-1 mRNA and protein are expressed at a low level in normal kidney but are increased dramatically in postischemic kidney. In situ hybridization and immunohistochemistry revealed that KIM-1 is expressed in proliferating bromodeoxyuridine-positive and dedifferentiated vimentin-positive epithelial cells in regenerating proximal tubules. Structure and expression data suggest that KIM-1 is an epithelial cell adhesion molecule up-regulated in the cells, which are dedifferentiated and undergoing replication. KIM-1 may play an important role in the restoration of the morphological integrity and function to postischemic kidney.

KIM-1, a transmembrane tubular protein with unknown function, is undetectable in normal kidneys, but is markedly induced in experimental renal injury. The KIM-1 ectodomain is cleaved, detectable in urine, and reflects renal damage. KIM-1 expression in human renal biopsies and its correlation with urinary KIM-1 (uKIM-1) is unknown. In biopsies from various renal diseases (n = 102) and controls (n = 7), the fraction of KIM-1 positive tubules and different renal damage parameters were scored. Double labelling was performed for KIM-1 with macrophages (MØ), alpha-smooth muscle actin (alpha-SMA), proximal (aquaporin-1) and distal (E-cadherin) tubular markers and a dedifferentiation marker (vimentin). uKIM-1 at the time of biopsy (n = 53) was measured by ELISA. Renal KIM-1 was significantly increased in all diseases versus controls (p < 0.05), except minimal change. KIM-1 was primarily expressed at the luminal side of dedifferentiated proximal tubules, in areas with fibrosis (alpha-SMA) and inflammation (MØ). Independent of the disease, renal KIM-1 correlated positively with renal damage, negatively with renal function, but not with proteinuria. uKIM-1 was increased in renal patients versus controls (p < 0.001), including minimal change, and correlated positively with tissue KIM-1 and MØ, negatively with renal function, but not with proteinuria. In conclusion, KIM-1 is upregulated in renal disease and is associated with renal fibrosis and inflammation. uKIM-1 is also associated with inflammation and renal function, and reflects tissue KIM-1, indicating that it can be used as a non-invasive biomarker in renal disease.

KIM-1 (kidney injury molecule-1) is a type I transmembrane glycoprotein expressed on dedifferentiated renal proximal tubule epithelial cells undergoing regeneration after toxic or ischemic injury. The extracellular domain of KIM-1 is composed of an immunoglobulin-like domain topping a long mucin-like domain, a structure that points to a possible role in cell adhesion by homology to several known adhesion proteins. Two splice variants (a and b), of the human KIM-1 having identical extracellular domains, differ in their cytoplasmic domains and tissue distributions. In this study, we report that the KIM-1b transcript is expressed predominantly in adult human kidney. We describe the generation of 10 monoclonal antibodies against the extracellular domain of human KIM-1, the mapping of their binding sites, and their use in identifying various forms of the protein. We show that human KIM-1b is expressed in adult kidney cell lines, and we demonstrate that a soluble form of KIM-1 is shed constitutively into the culture medium of the cell lines expressing endogenous or recombinant KIM-1b by membrane-proximal cleavage. A monoclonal antibody that binds at or close to the proteolytic site can partially block the shedding of KIM-1. Release of soluble KIM-1 is enhanced by activating the cells with phorbol 12-myristate 13-acetate and can be inhibited with two metalloproteinase inhibitors, BB-94 (Batimastat) and GM6001 (Ilomastat), suggesting that the cleavage is mediated by a metalloproteinase. We propose that the shedding of KIM-1 in the kidney undergoing regeneration constitutes an active mechanism allowing dedifferentiated regenerating cells to scatter on denuded patches of the basement membrane and reconstitute a continuous epithelial layer.

The RAD6 gene of Saccharomyces cerevisiae encodes a ubiquitin-conjugating enzyme that is required for postreplication repair of UV-damaged DNA, DNA damage induced mutagenesis, sporulation, and amino-end rule protein degradation. RAD6 interacts physically with the UBR1 gene product in carrying out the multiubiquitination of amino-end rule proteolytic substrates. In mediating postreplication repair, it has remained unclear whether RAD6 acts in a pleiotropic manner distal from the site of DNA damage or is targeted to the damage site via interaction with another repair component. Here, we show that RAD6 forms a specific complex with the product of the DNA repair gene RAD18. The biological significance of this interaction is attested by the observation that overproduction of the rad6 Ala-88 mutant protein, which lacks ubiquitin-conjugating activity but retains the ability to interact with RAD18 protein, confers a high level of UV sensitivity on wild-type PAD + cells that can be corrected by the concomitant overexpression of RAD18. We demonstrate that whereas RAD6 has no affinity for DNA, RAD18 binds single-stranded DNA. Thus, association of RAD6 with RAD18 could provide a means for targeting RAD6 to damage-containing DNA regions, where the RAD6 ubiquitin-conjugating function could modulate the activity of a stalled DNA replication machinery. We also show that RAD6 forms separate complexes with RAD18 and with UBR1, and the extremely conserved amino terminus of RAD6 that is required for complex formation with UBR1 is dispensable for complex formation with RAD18.