Peanut lectin: a mitogen for normal human colonic epithelium and human HT29 colorectal cancer cells

The protein peanut agglutinin (PNA) is a galactose-binding lectin whose receptor, the Thomsen-Friedenreich (TF) blood-group antigen, shows increased expression in hyperplastic and neoplastic colonic epithelium.

Our hypothesis was that, under these conditions, increased lectin receptors could interact with dietary lectins, which would act as tumor promoters by stimulating cell proliferation. This study was designed to confirm whether active PNA is recoverable from feces after ingestion of peanuts and to assess the mitogenic effect of PNA on proliferation of epithelial cells in the colon.

Peanut lectin was extracted from feces by lactose-agarose affinity chromatography and was assayed for hemagglutinating activity. Cultured explants of histologically normal biopsy specimens of colonic mucosa from 31 patients were examined. Crypt cell production rate and incorporation of [3H]N-acetylglucosamine into mucin were assessed as indicators of proliferative and metabolic responses to PNA. In addition, we evaluated the separate and combined effects of PNA and epidermal growth factor (EGF) on cell proliferation in human HT29 colorectal cancer cells, by using tritiated thymidine incorporation and cell counts.

Peanut lectin extracted from feces showed hemagglutinating activity toward desialylated red blood cells similar to that of a lectin preparation extracted from raw peanuts. Evaluation of biopsy specimens of normal colonic mucosa demonstrated that PNA at a concentration of 25 micrograms/mL caused statistically significant increases in crypt cell production (31% [mean] +/- 5% [SD]; P = .00005) and mucus synthesis (77% +/- 12%; P less than .000001). At 7.5-100 micrograms/mL, PNA was mitogenic for the HT29 colorectal cancer cell line. At 25 micrograms/mL, PNA alone produced a statistically significant increase in thymidine incorporation (44% [mean] +/- 3.7% [SD]; P = .002). For PNA in combination with EGF at 100 pg/mL, the increase was significantly greater (222% +/- 11.2%) than that for EGF alone (57% +/- 5%; P = .003).

These results suggest that expression of the PNA receptor, TF antigen, by hyperplastic or neoplastic colonic epithelium may affect cell proliferation.

It is possible that dietary lectins such as PNA, which bind to the TF antigen, promote cell proliferation and thus cancerous growth, while galactose-containing vegetable fiber would inhibit this effect by competing for binding by these lectins.