False-positive apoptosis signal in mouse kidney and liver detected with TUNEL assay

Apoptosis is a physiological, programmed process for the elimination of cel ls from living organisms. Currently, one of the most frequently used method s to detect apoptosis is TUNEL assay. It has provided valuable information about apoptosis in various tissues. However, the sensitivity and the specif icity of TUNEL technique have also been criticized. We detected an intense false-positive apoptotic signal in nude and Balb/c mice kidney and liver. I n kidney the signal was confined to the proximal, distal and collecting tub ular cells, and in liver to hepatocytes. Both tissues appeared normal in li ght microscopy, and no DNA ladder formation or increase in caspase-3 enzyme activity was detected. BrdU labelling and Ki-67 immunostaining did not rev eal increased cell proliferation in these tissues. On the other hand, false -positive signal was not detected in testis, spleen, pancreas or renal cell carcinoma from the same animals. Also, no false-positive signal was seen i n human liver or kidney samples. Although factors known to produce false-po sitive staining related to sample harvesting, preparation and staining prot ocols were eliminated, the cause of the false- positive apoptotic signal re mains unknown. We conclude that caution must be exercised when examining ap optosis in mouse tissues with TUNEL assay.