Obesity and Cancer Connection

Does insulin and a related hormone play a key role in fueling tumor? The answer of this question is most probably YES. Please read the following short-article for the details. It might be interest of many people all around the world.

A growing body of research shows that insulin and a related hormone play a key role in fueling tumors. They also may be a link between obesity, diabetes, and cancer.

Growing breast cancer cells in the lab has been a revelation to Vuk Stambolic. The protocol he follows is decades old and widely used, but there’s a puzzle at its core. The recipe calls for a large dose of glucose, a growth factor called EGF, and insulin. Add these to tissue culture, and tumor cells will be fruitful and multiply. A curious thing happens if you try to wean the tumor cells off insulin, however: They “drop off and they die,” says Stambolic, a cancer researcher at the University of Toronto in Canada. “They’re addicted to [insulin].”

What makes this so “bizarre,” Stambolic says, is that this behavior is totally unlike that of the healthy breast cells from which these tumor cells are derived. Normal cells are not sensitive to insulin—or at least not nearly to the same degree. They don’t have insulin receptors, and they lack key elements of the insulin signaling pathway necessary to make insulin outside the cell immediately relevant to what goes on inside. Indeed, normal cells thrive without insulin. By contrast, the tumor cells in culture can’t live without it.

This observation, although not original, is one of the insights that drew Stambolic to investigate the tumor-promoting effects of insulin. It has led him to spend the past decade studying a signaling pathway that is activated by insulin in healthy muscle, fat, and liver cells. Named for one of its key components—the PI3 kinase pathway—it also happens to be among the most frequently mutated pathways in human cancers.

Insulin, a hormone produced in the pancreas, is more commonly known for its role in diabetes. But its reputation may be changing. Insulin and a related hormone known as insulin-like growth factor (IGF) are now at the center of a growing wave of research around the world aimed at elucidating what many scientists consider to be their critical role in fueling a wide range of cancers. Elevated levels of insulin and IGF are also the leading candidates to explain a significant correlation in epidemiology that has gained attention over the past 30 years: Obese and diabetic individuals have a far higher risk than lean healthy people of getting cancer, and when they do get it, their risk of dying from it is greater. And now that obesity and diabetes rates are skyrocketing, the need to understand this link has become far more urgent.

The correlation between obesity and cancer can be found in the medical literature going back for several decades. But it wasn’t until 2004 that two cancer epidemiologists put it all together, says Robert Weinberg, a cancer researcher at the Massachusetts Institute of Technology (MIT) in Cambridge. An article that year in Nature Reviews Cancer by Rudolf Kaaks, then of the International Agency for Research on Cancer, and the late Eugenia Calle of the American Cancer Society “laid down a challenge to the rest of us … to determine why obesity is such an important determinant of cancer risk,” Weinberg says.

The message of this research is straightforward, Kaaks says: Excess body fat seems to account for between one-quarter and one-half of the occurrence of many frequent cancer types—breast, colorectal, endometrial, renal cell, and adenocarcinoma in the esophagus, in particular. Kaaks adds, “The list is growing.”

“The magnitude of the effect is huge,” in large part because obesity and diabetes are now so common, says Michael Pollak, an oncologist at McGill University in Montreal, Canada. It seems that cancer “loves the metabolic environment of the obese person,” Pollak says. Epidemiologic studies have also found that not only is type 2 diabetes associated with increased cancer incidence and mortality but so are circulating levels of insulin and IGF.

Recent drug studies have sharpened the picture: Type 2 diabetics who get insulin therapy or drugs to stimulate insulin secretion have a significantly higher incidence of cancer than those who get metformin, a drug that works to lower insulin levels (see sidebar on metformin, p. 29). There’s a large and growing body of evidence implicating insulin and IGF in cancer, Pollak says, “and it’s causing a lot of people to stay up at night thinking about it.”

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