Increased levels of a 21-kDa protein in the circulation of tumor-bearing patients.

A novel non-ras 21-kDa protein (p21) was detected in sera of cancer patients by enzyme-linked immunosorbent assay (ELISA), using polyclonal anti-p21 antibodies. While only 4.6% of the healthy donors (n = 43) showed p21 serum levels higher than the mean +/- 2 SD of the normal group, 33 to 80% of the cancer patients (n = 94) with various tumors were positive in the ELISA test. In particular, patients with malignant lymphoma, urogenital, and gastrointestinal tumors had a 2.8- to ninefold increase in p21 serum levels. Elevated serum levels were also found in patients with benign prostatic hyperplasia (3.2-fold increase). In 17 out of 22 patients with urogenital tumors, changes in serum p21 levels correlated with the clinical course of the disease. In 15 patients, a favorable response to therapy was correlated with a decline in serum p21 level. Two patients with renal cell carcinoma showed increased or persistently high levels of p21 after surgery and were subsequently found to have distant metastases. Expression of p21 in ten renal cell carcinoma tissues was determined by Western blotting. There was a trend toward a higher content of p21 in the less-differentiated tumor tissues. In conclusion, p21 may be a useful indicator in monitoring the outcome of treatment in patients with various malignant tumors. In patients with renal cell carcinoma, p21 serum levels may be of particular importance because no other tumor marker is available.