Carotenoids: colorful cancer protection.

Carotenoids: Colorful cancer protection

Carotenoids are a class of more than 500 yellow-to-red-hued pigments, chemically related to vitamin A. Though found predominantly in green and yellow vegetables, they also color tomatoes, carrots, egg yolks, algae and even shark oil. In recent years, a few carotenoids -- most notably beta-carotene and canthaxanthin -- have gained renown for their apparent role in limiting the development of certain cancers. Now, Japanese scientists working with cultured human cancer cells report data suggesting that at least some of these nontoxic pigments fight cancers by effectively putting malignant cells to sleep and suppressing the expression of a gene that might otherwise foster tumor growth.

Cancer involves rapid and unregulated proliferation of cells. Researchers at the Kyoto Prefectural University of Medicine observed a dramatic suppression in the proliferation of human neuroblastoma cells after adding alpha-or beta-carotene. Alpha-carotene shut down cell growth at concentrations as low as 2 to 5 micromoles ([mu]M) and proved toxic at [mu]M. Beta-carotene showed similar effects at concentrations 10 times greater.

To find out what was happening, the researchers homed in on the activity of the gene N-myc, which codes for cell-growth-enhancing proteins when switched on. This so-called proto-on-cogene is present, though inactive, in healthy mature cells, but it can contribute to cancer growth if damaged or if turned on by faulty regulatory cues. Three hours after treating some of the cancer cells with a 5-[mu]M concentration of alpha-carotene, the researchers found N-myc activity 24 percent lower than in the untreated cells. Within 18 hours, activity dropped to 18 percent of that observed in untreated cancer cells.

Further examination showed cell-growth inhibition also peaked at 18 hours. In the Nov. 1 JOURNAL OF THE NATIONAL CANCER INSTITUTE, Michiaki Murakoshi and his co-workers report that the alpha-carotene apparently inhibits cancer growth by locking malignant cells into the rest phase of their growth cycle. And they remain in this sort of suspended animation until the effects of the carotenoid begin wearing off.

While these findings do not directly reveal how the pgiments inhibit cancer-cell proliferation, "they do offer the first indication -- at least in a human cancer-cell line -- the carotenoids can cause such inhibition," says Joel Schwartz, a tumor immunologist at Harvard University's School of Dental Medicine in Boston. The new data also "are consistent with what we have observed but not reported," Schwartz told SCIENCE NEWS. He says he and his colleagues, working with another cancer-cell line, have found that beta-carotene not only suppressed the expression of a proto-oncogene but also arrested cancer proliferation by preventing malignant cells from cycling through their normal stages of growth and division.