The compound pentameric procyanidin has previously been shown to stop breast cancer progression in lab studies but the mechanism was unknown.
Writing in the April issue of Molecular Cancer Therapeutics (vol 4, issue 4, pp537-46), a team from the Lombardi Comprehensive Cancer Center at Georgetown University explain that the compound appears to work on several different proteins involved in the cancer.
Although the study was conducted in breast cancer cell cultures, the finding could potentially apply to other cancers, said lead author Robert B. Dickson.
However he added: "We need to slowly develop evidence about the selectivity of these compounds to cancer, learn how they work, and sort out any issues of toxicity."
The research, which is part of a series of studies conducted at Georgetown on the chocolate-cancer connection, is being funded by confectionery giant Mars.
Cacao beans are rich in natural antioxidants known as flavonoids. These antioxidants may protect cells from the damage caused by free radicals, which are thought to contribute to both heart disease and cancer development.
The primary family of flavonoids contributing to the antioxidant benefit in chocolate is the procyanidins, and of the various types of procyanidins, pentamer seem to be strongest, according to a number of studies.
The Georgetown researchers tested a purified preparation of pentamer on a variety of breast cancer cells, compared to treatment on normal breast cells.
They located two well known tumour suppressor genes as well as two other proteins known to be involved in regulating the 'cell cycle' - the progression of a cell from a state of being 'quiet' into division and growth.
They found that the breast cancer cells stopped dividing when treated with pentamer and that all four proteins were inactivated. Furthermore, expression of one of the genes was reduced.
Dickson notes that "the novel aspect here is that a pattern of several regulatory proteins are jointly deactivated, probably greatly enhancing the inhibitory effect compared to targeting any one of the proteins singly."
"That is also why the compound seems to work on cancer cells, irrespective of whether any of these single genes are mutated, which often happens in cancer cells."
The researchers can not yet explain why pentamer deactivates these proteins simultaneously, stopping the cell cycle.
"We don't know at a fundamental level whether a master switch that triggers cell growth is turned off, or whether the chocolate compound exerts multiple independent effects on diverse cellular processes. That will be the subject of future studies here."