Intravenous Administration of Lipitor-treated Stem Cells on the Heart


Hao Zhang and colleagues from the Chinese Academy of Medical Sciences and Peking Union Medical College have published a rather unusual experiment in the American Journal of Translational Research. This experiment, however, could have implications for stem cell therapy in heart attack patients.

When heart attack patients are treated with stem cells, they are either injected directly into the heart muscle or released into the heart through the coronary arteries by means of angioplasty. Injecting stem cells directly into the heart requires special equipment and training. Releasing cells into the coronary arteries causes most cells to end up in the lungs or other organs, and the retention of the stem cells is poor. Introducing cells by means intravenous administration would be supremely simple, but in animal experiments, intravenously administered stem cells almost never get to their target organ.

When the heart undergoes a heart attack, the damaged heart cells release a molecule called SDF1 or stromal cell-derived factor 1. SDF1 summons stem cells to the damaged areas by binding to the surfaces of stem cells and drawing them to the higher concentrations of SDF1. SDF1 binds to a receptor on the surfaces of stem cells called CXCR4. Unfortunately, when stem cells are administered intravenously to animals that have just experienced a heart attack, the stem cells do not have enough CXCR4 on their surfaces to properly respond to the SDF1 being secreted by the damaged heart.

Zhang and his colleagues capitalized on an observation made several years ago. When stem cells are exposed to statin drugs that are normally used to lower serum cholesterol levels, the stem cells increase the number of CXCR4 molecules on their surfaces. Statins have also been shown to increase stem cell survival once the cells get to the heart, but Zhang and his team wanted to know if pre-treating stem cells with statins could increase their migration to the damaged heart.

The Zhang group isolated mesenchymal stem cells from rat bone marrow and treated these cells with increasing concentrations of the drug Lipitor (atorvastatin). Indeed, increasing amounts of Lipitor increased the number of CXCR4 molecules on the surfaces of the mesenchymal stem cells (MSCs), This increase in CXCR4 molecules peaked at 24 hours, after which the number of receptors declined. These Lipitor-treated MSCs also migrated much more robustly in culture when treated with SDF1.

Next, Zhang’s group pre-treated MSCs with Lipitor and labeled them with an innocuous tracking molecule. 24 hours after giving some laboratory rats heart attacks, these MSCs were administered to the rats in their tail veins. Two other groups of similarly treated rats were given either MSCs that had not been pre-treated with Lipitor, or just buffer.

The Lipitor-treated MSCs were found in significantly higher quantities in the hearts of laboratory animals, relative to the other animals. Secondly, these Lipitor pre-treated MSCs cut the size of the heart scar in half, and there was also substantially less inflammation in hearts from animals treated with Lipitor pre-treated MSCs than the other groups. Heart function was also increased in the pre-treated group.

Live MSCs were observed in the hearts of the animals given Lipitor pre-treated MSCs. This is a remarkable finding, because most experiments have shown that MSCs administered to the heart after a heart attack ad usually dead within 21 day after administration. However the Lipitor pre-treated MSCs survive and flourish in the damaged heart, which suggests that SDF1 not only attracts stem cells but also increases their rates of survival.

This is a somewhat off-beat experiment at first glance, but if MSCs could be pre-treated with a drug like Lipitor and then administered to heart patients intravenously, they would survive in the heart, convey greater benefits, and their administration would be safer, and not require special equipment or training. With a little luck, this idea will reach human clinical trials in a few years; provided that further animal and cell culture studies confirm these results, elucidate the mechanism of SDF1-mediated survival, and show that such augmentation of function is also observed in human MSCs.

A Link Between Stem Cells, Atherosclerosis, and Cholesterol


Researchers at the University of Buffalo have discovered that stem cells are involved in the inflammation that promotes atherosclerosis.

Atherosclerosis or hardening of the arteries occurs when fat, cholesterol, and other substances build up in the walls of arteries and form hard structures called plaques. With the passage of time, these plaques can grow and block the arteries, depriving tissues of oxygen and nutrition.

High serum cholesterol levels have been unequivocally linked to an increased risk of arteriosclerosis. However, the deposition of cholesterol and other molecules underneath the inner layer (intima) of arteries requires a phenomenon known as inflammation. Inflammation occurs in response to tissue damage and it involves the dilation of blood vessels, increased blood flow the damaged area, the recruitment of white blood cells to the area, and increased heart, volume, and pain at the area in question. Increased inflammation within blood vessels damages the intimal layer and allows the deposition of cholesterol and other molecules underneath it to form an atheroma or a plaque.

The stem cell link to atherosclerosis is that the bone marrow-based stem cells that make our blood cells (hematopoietic stem/progenitor cells or HSPCs) ramp up their production of white blood cells in response to increased serum cholesterol levels.

Thomas Cimato, assistant professor in the Department of Medicine in the UB School of Medicine and Biomedical Sciences, said of his publication, “Our research opens up a potential new approach to preventing heart attack and stroke, by focusing on interactions between cholesterol and the HSPCs. Cimto also suggested that these findings could lead to the development of a useful therapy in combination with statins, or a treatment in place of statins for those who cannot tolerate statins.

In Cimato’s study, high cholesterol levels were shown to cause increases in the levels of interleukin -17 (IL-17). IL-17 is a cytokine that recruits monocytes and neutrophils to the site of inflammation. IL-17 boosts levels of granulocyte colony stimulating factor (GCSF), which is a factor that induces the release of HSPCs from the bone marrow to the peripheral circulation.

Cimato also found that statin drugs reduce the number of HSPCs in circulation, but not all patients responded similarly to statins. “We’ve extrapolated to humans what other scientists previously found in mice about the interactions between LDL, cholesterol, and these HSPCs,” said Cimato.

In order to transport cholesterol through the bloodstream, cells must construct a vehicle into which the cholesterol is packaged. Cholesterol does not readily dissolve in water. Therefore, packaging cholesterol into lipoprotein particles allows for its transport around the cell. Cell use cholesterol to vary the fluidity of their membranes, and to synthesize steroid hormones. Once cholesterol is absorbed from the diet, the cells of the small intestine package cholesterol and fat into a particle known as a chylomicron.

chylomicron

Chylomicrons are released by the small intestinal cells and they travel to the liver. In the liver, chylomicrons are disassembled and the cholesterol is packaged into a particle known as a very-low density lipoprotein particle (VLDL). After its release and sojourning through the bloodstream, the VLDL looses some surface proteins and is depleted of its fat and becomes known as a low-density lipoprotein or LDL particle.  While these particles sojourn through the bloodstream, they release fat for tissues to use as an energy source.

LDL

LDL particles are gradually removed from circulation. If they build up to high concentrations, they can be taken up by a wandering white blood cell known as a macrophage. If these macrophages take up too much LDL, they can become a foam cell.  Foams cells can become lodged underneath the intimal layer of blood vessels when inflammation occurs inside blood vessels, and this is the cause of atherosclerosis.

Increased LDL levels in mice have been shown to stimulate the release of HSPCs from bone marrow and accelerate the differentiation of these cells into white blood cells (neutrophils and monocytes) that participate in inflammation.

Mice do not regulate their cholesterol levels in the same way humans do.  Cimato commented, “mice used for atherosclerosis studies have very low total cholesterol levels at baseline.  We feed then very high fat diets in order to study high cholesterol but it isn’t easy to interpret what the levels in mice will mean in humans and you don’t know if extrapolating to humans will be valid.”

Therefore, in order to properly model cholesterol regulation in their human subjects, Cimato had them take statins for a two-week period followed by one-month intervals when they were off the drugs.  “We modeled the mechanism of how LDL cholesterol affects stem cell mobilization in humans,” said Cimato.

The experiments showed that increased LDL levels tightly correlated with IL-17 levels.

IL-17 and cholesterol levels

Secondly, blood LDL levels also correlated with GCSF levels.

LDL levels and GCSF levels

Finally, increasing GCSF levels led to higher levels of circulating HSPCs.

CD34 cells and G-CSF levels

These circulating HSPCs increase the numbers of neutrophils, monocytes, and macrophages that are involved in the formation of plaque and atherosclerosis.

The next step is to determine if HSPCs, like LDL cholesterol levels are connected to stroke, cardiovascular disease and heart attacks.