By Jeremy Lant and David Smith, Photo by Dave Reid
The researchers found that polar bears have an extra sticky version of Apo B grip tape, allowing them to dispose of their bad cholesterol more effectively than other animals. Future studies of polar bear Apo B might give scientists a better understanding of our own fat-slam-dunking proteins, its shortcomings, and why instances of heart disease are so high in humans.
The world’s population is becoming more and more obese. In the age of McDonald’s drive-thrus, the infamous KFC Double Down sandwich, and Krispy Kreme donuts, it is easy to overindulge. But it tastes so darn good!Of course it does: our bodies are evolutionarily hardwired to crave calorie-rich food — an adaptation from a time when food was scarce. How can we reconcile our modern diet with these ancient cravings? How can we moderate our fast-food intake, and the heart disease that accompanies it? Insights into these questions and how animals adapt to gluttony have come from a recent genetic study of one of the world’s most proficient and prodigious fat eaters: the polar bear.
An international team of researchers compared the genome sequences of polar bears to brown bears and found that the two types of bear shared a common ancestor much more recently than previously thought. Differences in their DNA sequences suggest that the polar bear and brown bear lineages diverged from one another around 350,000–500,000 years ago. That might seem like an exceptionally long time ago, but in evolutionary terms, it is surprisingly short, especially when considering that polar bears have an average generation time of 11 years. The scientists also discovered that certain polar bear genes are tailored to life in the High Arctic, and the blubbery foods that are found there.
Polar bears consume an astounding amount of fat and cholesterol. Their diet consists primarily of seal and whale blubber. What’s more, polar bear mothers produce milk that is 30 per cent fat, which is equivalent to feeding whipping cream to a newborn! But nobody is calling child services on polar bear moms. This is because all the fat and cholesterol doesn’t harm the baby. Polar bears have evolved unique cardiovascular and fatty acid metabolic systems that allow them to survive on extremely fat- and cholesterol-rich food. To understand how such a system arose, and so quickly, we need to travel back in time.
The setting is Greenland, 400,000 years ago, and the formation of a new species — Ursus maritimus (or polar bear) — is underway. This is an interglacial period, a time between ice ages when Arctic ice has retreated significantly from the land and sea. (In some ways, this reflects the contemporary Arctic landscape, although the ice retreat that the ancestors of polar bears witnessed was much slower than the human-accelerated version occurring today.) The melting of land ice in ancient Greenland permitted brown bears to move further north, into regions with copious amounts of food and unsuspecting prey — an all-youcan-eat Arctic buffet. The feast would have been easy pickings for the migrating brown bears, but would have also included new food sources. Typically used to fish and berries, bears inhabiting these northerly regions had to adapt to dining on animals with thick layers of blubber, such as seals.
This change in diet and environment would have introduced a range of “selective pressures” on the bears, including the pressures to survive a blubbery, high cholesterol diet. Bears that could flourish under such a diet would have had a greater chance of passing down their genes than those that died prematurely of heart disease, for example. Over thousands of years, fat-adapted genes became fixed in the bear population. The researchers believe that some of these adaptations involved changes to the way bears dispose of dangerous cholesterol.
There are two main types of cholesterol in animals: high-density lipoprotein, which is good, and low-density lipoprotein (LDL), which is bad. The latter exists in the blood like big slippery basketballs, which can build up over time, causing clogging and cardiovascular disease. Cells lining the blood vessels dispose of these fatty basketballs using specialized arm-like molecular machines, which grab the lipoprotein balls and slam-dunk them inside the cell where they are safely decomposed. Because LDL basketballs are slippery, cells label them with a special “grip-tape” protein, called Apo B, which helps the arm-like molecules grab hold of them.
The researchers found that polar bears have an extra sticky version of Apo B grip tape, allowing them to dispose of their bad cholesterol more effectively than other animals. Future studies of polar bear Apo B might give scientists a better understanding of our own fat-slam-dunking proteins, their short – comings, and why instances of heart disease are so high in humans. Ultimately, polar bear genes could provide a framework for scientists to engineer or modify our own genes, making heart disease a thing of the past. Ethics aside, it might allow us to gorge on Big Macs without feeling too guilty. Now wouldn’t the head honchos at McDonalds like that?
Jeremy Lant is a third-year undergraduate student studying Medical Sciences at Western University, and is a volunteer science writer in David Smith’s Lab, Biology Department. David Smith is an assistant professor in the Biology Department at Western University. You can find him online at www.arrogantgenome.com.