Nighttime hormone helps starve cancers.

Linoleic acid, the primary fat in corn oil, can fuel the growth of cancers in animals, a new study finds. However, a second study shows that tumors feed on this fat only in the absence of melatonin, a brain hormone produced at night. Together, these findings hold out the prospect of one day short-circuiting the growth of certain tumors with melatonin supplements.

In experiments, diets high in corn oil can leave an animal especially vulnerable to chemically induced cancers (SN: 6/24/89, p. 390). A report in the Sept. 15 CANCER RESEARCH now offers one possible explanation why.

Leonard A. Sauer and his colleagues at the Bassett Research Institute in Cooperstown, N.Y., implanted a liver cancer into the groin of rats. They encapsulated each tumor in a degradable wrap, leaving one blood vessel to feed the tissue and one to drain it. The researchers then fed the animals a diet in which the fat was mainly corn oil.

By tapping into the animals' blood as it entered and left the tumor, the researchers showed that the cancers altered the incoming linoleic acid into a chemical known as 13-HODE. In rats treated with a drug to prevent this transformation, linoleic acid exited the tumors unchanged. Moreover, these tumors grew far more slowly than those making 13-HODE did.

This fits with research by others showing that 13-HODE stimulates cell proliferation, notes David E. Blask, one of the authors.

In a second report in the same journal, Blask's group focuses on the role of melatonin, the brain hormone best known for its role in setting the body's biological clock. During the nighttime hours when an animal produced melatonin, tumor production of 13-HODE waned to negligible amounts. Cancers in these rats grew at roughly half the rate of those in rats with their melatonin-producing pineal gland surgically removed.

A third group of animals received extra melatonin. Late-afternoon supplements further slowed the growth of an implanted tumor, but early daytime melatonin had no effect. These data confirm something Blask had seen earlier--varying sensitivity to melatonin's cancer-inhibiting effect during daylight hours or nighttime illumination (SN: 10/17/98, p. 248).

Though the Bassett scientists did most of their work with liver cancer, they saw similar trends in 13-HODE's stimulation of two other cancers--and melatonin's inhibition of them. Concludes Blask, "This appears to be a generalized phenomenon."