Obese women are not only at a higher risk of breast cancer than their slimmer sisters — their cancer is more aggressive. Researchers at the University of Granada wanted to understand why.

They found that the type of fat that surrounds the tumor (peritumoral fat) helps the cancer stem cells (CSCs) to grow and invade surrounding tissues, making CSCs responsible for the onset and growth of the tumor.

CSCs are few in number, but they are responsible for cancer that spreads to parts of the body far from the original tumor (called metastasis). Conventional chemotherapy and radiotherapy treatments do not kill CSCs, and they are the reason that many patients suffer a relapse after appearing to respond well to a cancer treatment.

Obesity has a sobering effect on cancer morbidity. According to cancer.org, numerous studies have shown a link between obesity, and many types of cancer, including breast, colon, and pancreas. They also estimate that obesity contributes to as many as 1 in 5 cancer-related deaths in the United States.

Obese women have a greater risk of suffering breast cancer after menopause, and the disease is more aggressive in obese women, regardless of their age, but the mechanism wasn't clear. However, scientists did know that obesity-related fat causes local inflammation and prevents the fat cells (adipocytes) from maturing.

The researchers examined the effects of culturing adipocytes with breast cancer cells to discover how aggressive they were and how capable they were of spreading.

They found that the interaction between tumor cells and immature adipocytes near the tumor during the first stages of breast cancer increased the secretion of cytokines (proinflammatory proteins).

"Said cytokines cause a greater expansion of highly metastatic CSCs," said professor Juan Antonio Marchal Corrales of the University of Granada.

The researchers also found that the cytokines activate a protein called SRC Kinase, which activates the Sox2 transcription factor that is essential for the cancer stem cells to remain healthy.

But they also discovered that drugs which inhibit SCR Kinase decrease the production of cytokines and CSCs, which may lead to new treatments for breast cancer.