Stress causes ovarian cancer tumors to grow and spread more quickly in mice, U.S. researchers reported in a study that provides one of the first biological links between stress and cancer.

In the mice, stress hormones attach directly to tumor cells and stimulate new blood vessel growth and other factors that lead to faster and more aggressive tumors, the researchers said.

The study published in the journal Nature Medicine also found that a blood pressure drug reverses the effect.

Studies aimed at finding out if stress causes cancer have not come up with a clear answer -- and some have clearly ruled out any link between stressful life events, such as divorce or job loss, and later cancer.

But many people believe stress can cause cancer.

Dr. Anil Sood of The University of Texas M.D. Anderson Cancer Center and colleagues noticed that ovarian cancer patients who reported high levels of stress in their lives also had higher levels of a protein called VEGF, which stimulates blood vessel growth in tumors.

The patients who had more social support in their lives had lower levels of VEGF.

So the team infected mice with ovarian cancer and then stressed some by confining alone them in a small space for two or six hours. Mice stressed for six hours had 3.6 times as many tumors.

And in half the stressed mice the tumors had already spread to the liver or spleen.

To their surprise, Sood's team found that the tumor cells have receptors, molecular doorways, that are configured for stress hormones.

When activated, they start a process known as angiogenesis -- making the little blood vessels that tumors need to nourish themselves. Not only was VEGF activated, but so were two other compounds involved in sustaining tumors called MMP2 and MMP9.

"This study provides a new understanding of how chronic stress and stress factors drive tumor growth," Sood said in a statement.

So then the researchers gave the mice a beta-blocker heart drug called propranolol, which lowers blood pressure using these same stress receptors, called beta adrenergic receptors.

"The concept of stress hormone receptors directly driving cancer growth is very new," Sood said.

"Not much had been known about how often these receptors are expressed in cancer, and more importantly, whether they had any functional significance. Our research opens a new area of investigation."