This review paper examines the relationship between chemicals inducing ewessive accumulation of a2u-globulin (ai21g) (CIGA) in hyline droplets inmale rat kidneys and the subsequent development ofnephoit and rnal tubule nopaia in the male rt. This dose-responsive hyaline droplet accumulation distinguishes CIGA carcinogens fromclssical renal carcinogens. CIGA carcinogens also do not appear to react with DNA and are generally negative in short-term tests for genotoxicity. CIGA or their metabolites bind specifically, but reversibly, tomale rat a2,-g The resulting complex appears to be more resistant to hydrolytic degradation in the proximaltubule than native, unbound a2,-g Singlecell necrosis of the tubule epithelium, with associated granular cast formation and papillary mineralization, is followed by sustained regenerative tubule cell prolifertin, foci oftubulehyerl a in the aovoited pronal bules, and renal tubule tumor Although stnrcturally similar proteins have been detected in other species, including hu_ms, renal lesios characteristic Of a2,-g nephropathy have not been observed. Epidemiologic investigation has not specifically examined the CIGA hypothesis for humans. Based on cancer bioassays, hormone manipulation studies, investigations in an ae,-g-deficient strain of rat, and other laboratory data, an increased proliferative response caused by chemically induced cytotoxicity appears to play a role in the development ofrenal tubule tumors in male rats Thus, it is reasonable to suggest that the renal effects induced in male rats by chemicals causing a2.-g accumulation are unlikely to occur in humans.

Agency's (EPA) Risk Assessment Forum recently advised EPA risk assessors against using information on certain male rat renal tubule tumors to assess human risk under conditions specified in a new Forum report. Risk assessment approaches generally assume that chemicals producing tumors in laboratory animals are a potential cancer hazard to humans. For most chemicals, including classical rodent kidney carcinogens such as N-ethyl-N-hydroxyethylnitrosamine, this extrapolation remains appropriate. Some chemicals, however, induce accumulation of ct2,-globulin (%.-g), a low molecular weight protein, in the male rat kidney. The a2.-g accumulation initiates a sequence of events that appears to lead to renal tubule tumor formation. Female rats and other laboratory mammals administered the same chemicals do not accumulate low molecular weight protein in the kidney, and they do not develop renal tubule tumors. Because humans appear to be more like other laboratory animals than like the male rat, in this special situation, the male rat is not a good model for assessing human risk. The Forum report stresses the need for full scrutiny of a substantial set of data to determine when it is reasonable to presume that renal tumors in male rats are linked to a process involving a2ug accumulation and to select appropriate procedures for estimating human risks under such circumstances.