The laboratory of Petri Salven at the University of Helsinki, Helsinki, Finland, has discovered a new type of stem cell that play a decisive role in the growth of new blood vessels. These stem cells are found in the walls of blood vessels and if protocols are developed to isolated these stem cells, they might very well provide news ways to treat cardiovascular diseases, cancer and many other diseases.
The growth of new blood vessels is known angiogenesis. Angiogenesis is required for the repair of damaged tissues or organs. A downside of angiogenesis is that tumors often secrete angiogenic factors that induce the circulatory system to remodel itself so that new blood vessels grow into the tumor and feed it so that it can grow faster. Thus angiogenesis research tries to promote the growth of new blood vessels when they are needed and inhibit angiogenesis when it is unwanted.
Several drugs that inhibit angiogenesis have been introduced as adjuvant cancer treatments. For example, the drug bevacizumab (Avastin) is a monoclonal antibody that specifically recognizes and binds to an angiogenic factor known as vascular endothelial growth factor or VEGF. When VEGF receptors on the surface of normal endothelial cells. When VEGF binds to receptors on the surfaces of endothelial cells, a signal is sent within those cells that initiate the growth and survival of new blood vessels. Bevacizumab binds tightly to VEGF, which prevents it from binding and activating the VEGF receptor.
Other angiogenesis inhibitors include sorafenib (Nexavar) and sunitinib (Sutent), which are small molecular inhibitors of the receptors that bind the angiogenic factors and the downstream targets of those receptors. Unfortunately, the present crop of angiogenesis inhibitors are not all that effective under certain conditions and they are also extremely expensive and have some very undesirable side effects.
Professor Salven has studied angiogenesis for some time, and his research has focused on the endothelial cells that compose blood vessels. Where do these cells come from and how can we make more or less of them as needed?
A long-standing assumption by scientists in the angiogenesis field was that new endothelial cells came from stem cells found in the bond marrow. This assumption makes sense since there are several stem cell populations in bone marrow that express blood vessel markers and can form blood vessels in culture. However, in 2008, Salven’s group published a paper that demonstrated that new endothelial cells could not come from bone marrow stem cells (see Purhonen S, et al., (2008). Proc Natl Acad Sci U S A. 105(18): 6620-5). Therefore, the mystery remained – from where do new endothelial cells come?
Salven has recently solved this conundrum in his recent paper that appeared in PLoS Biology. According to Salven, “We succeeded in isolating endothelial cells with a high rate of division in the blood vessels of mice. We found that these same cells in human blood vessels and blood vessels growing in malignant tumors in humans. These cells are known as vascular endothelial stem cells, abbreviated VESC. In a cell culture, one such cell is able to produce tends of millions of new blood vessels wall cells.”
Slaven continued: “Our study found that these important stem cells can be found as single cells among the ordinary endothelial cells in blood vessel walls. When the process of angiogenesis is launched, these cells begin to produce new blood vessel wall cells.”
Salven’s colleagues have tested the effects of these new endothelial cells in mice. A particular mouse strain that carries a mutation in the c-kit gene was examined in these experiments. The c-kit gene encodes a cell surface protein called CD117, which is a vital element in the cells that form blood vessels. IN these c-kit mutant mice, new growth of new blood vessels was very poor and the growth of malignant tumors was also quite poor. However, if new stem cells from animals that did not possess a mutation in the c-kit gene were implanted into these mutant mice, blood vessels quickly formed.
As previously mentioned, the cell surface protein CD117 does seem to mark VESCs, but other cells other than VESCs have CD117 on their surfaces. Therefore, isolating all CD177-expression cells only enriches preparations for VESCs; it does not isolate VESCs. Presently, Salven and his group are searching for better surface molecules that can be used to more effectively isolated VESCs from surrounding tissue. If this isolation succeeds, then it will be possible to isolated and propagate VESCs from patients with cardiovascular diseases and expand them in culture for therapeutic purposes.
Another potentially fertile field of research is to find a way to inhibit the activity of VESCs to prevent tumors from remodeling the circulatory system. By cutting of their blood supply, tumors will not only grow slower, but also not spread nearly as quickly.
See: Fang S, Wei J, Pentinmikko N, Leinonen H, Salven P (2012) Generation of Functional Blood Vessels from a Single c-kit+ Adult Vascular Endothelial Stem Cell. PLoS Biol 10(10): e1001407. doi:10.1371/journal.pbio.1001407