Melatonin reduces metastasis in animals with breast cancer

AGÊNCIA FAPESP/DICYT When researchers at the São José do Rio Preto School of Medicine (FAMERP) in São Paulo State, Brazil, treated in vitro cultured metastatic breast cancer cells with melatonin, they observed a 55% reduction in cell migration and invasion. Experiments with mice showed that the treatment was also capable of slowing the progression of the disease in vivo.

The results of the studies, which were supported by FAPESP, have recently been described in a paper published in the Journal of Pineal Research.

“These findings reinforce the hypothesis, to which the group’s previous studies had already pointed, that therapeutic doses of melatonin, above the amount normally found in the human organism, could act as an adjunct to cancer treatment,” said Thaiz Ferraz Borin, who performed the experiments as part of her postdoctoral research at FAMERP’s Molecular Cancer Research Laboratory (LIMC) under the supervision of Professor Debora Zuccari.

Melatonin is a naturally occurring hormone in all mammals. Synthesized and released by the pineal gland at the base of the brain, it helps to regulate when we sleep and wake, as well as when most other hormones should or should not be secreted. Recent studies have shown that it also helps to regulate other important parameters and processes, including blood pressure, digestion, energy expenditure, and the synthesis and action of insulin in cells.

The group coordinated by Zuccari at FAMERP has studied the effect of melatonin on cancer for several years with FAPESP’s support. In particular, they have focused on an aggressive type of breast tumor, known as triple negative because it does not respond to anti-estrogen treatment, chemotherapy or radiotherapy, that is more likely to metastasize (read more at http://agencia.fapesp.br/17563).

“We’ve studied how melatonin affects angiogenesis, which is the formation of new vessels that will nourish the tumor, as well as the tumor microenvironment, which can facilitate or hinder the passage of malignant cells into the blood stream. We’ve also studied the formation of metastases and the expression of proteins and microRNAs that play an important role in furthering progression of the disease,” Zuccari said.

The aim of all of this research, according to Zuccari, is to understand the mechanisms of melatonin’s action because this knowledge can be leveraged to develop new approaches to cancer treatment. “A number of studies had already shown that melatonin inhibits metastasis, for example, but not exactly how,” she said.

This was the focus of Borin’s postdoctoral research. The in vivo experiments were conducted in the United States thanks to a partnership with Ali Syed Arbab, a researcher currently affiliated with Georgia Regents University.

The main thrust of the group’s research was to study the effect of melatonin on the expression of Rho-associated kinase protein. Known as ROCK1 for short, this molecule is responsible for supplying adenosine triphosphate (ATP), which stores cellular energy, and for cytoskeleton contraction, which is a necessary part of cell migration and invasion processes. Previous studies showed that expression of ROCK1 is often higher in metastatic cells.

“In earlier work, the researchers induced primary breast tumors in animals and observed whether treatment with melatonin prevented the formation of metastases,” Borin said. “In our study, we systemically injected metastatic cells into immunosuppressed mice. As a result, they all developed lung metastases, and we were able to observe the effect of treatment with melatonin.”

Human metastatic breast cancer cells were injected into mouse tail veins, and the mice were divided into three groups. The first received intraperitoneal injections of melatonin for two weeks. The second received injections of Y-27632, a pyridine compound that inhibits synthesis of ROCK1, for the same period. The third received a placebo.

After the end of the treatment, the animals were evaluated by single-photon emission computed tomography using a radiopharmaceutical that is easily absorbed by cells with high mitochondrial activity, such as tumor cells, and produces scintillation that can be imaged and measured.

Metastasis was reduced by 40% in the group treated with melatonin compared with the placebo group and by 58% in the group that received the ROCK1 inhibitor.

In another experiment, mice treated with melatonin for five weeks were found to have 25% less metastasis than the placebo group did. The ROCK1 inhibitor was not used in this case.

“The ROCK1 inhibitor Y-27632 can’t be used as medication, as it could induce the death of noncancerous cells if administered for a long period,” Borin explained. “We used it in one of the experiments merely to prove that the anti-metastatic effect of melatonin was associated with the production of ROCK1.”

A comparison of lung tissue from the groups treated with melatonin and Y-27632 showed that tumors were more localized and lung tissue better preserved in the former group, according to Borin.

The in vitro studies showed that melatonin reduced the expression of ROCK1 by 50% and tumor cell migration and invasion by 55%. It also reduced cell viability and inhibited proliferation in culture.

Borin is currently investigating how melatonin can modulate the action of certain microRNAs that are normally overexpressed in metastatic cells. MicroRNAs are small non-coding RNA molecules that regulate the expression of protein-encoding genes.

“We believe that some of these microRNAs degrade tumor-suppressor genes and that melatonin may reverse this process,” Borin said.

According to Zuccari, the group also plans to perform clinical trials with breast cancer patients who do not respond to conventional treatment.