When stem cells are implanted into injured tissues, they often face a hostile environment that is inimical to their survival. A stroke, for example, can produce brain tissue without ample blood flow, low oxygen levels, and lots of cell debris and inflammation. The same can be said for the heart after a heart attack. If stem cells are going to help anyone we have to find a way for them to survive.
The first hints came in the form of genetically-engineered stem cells that expressed a host of genes that can help cells survive in low oxygen, high stress environments. However, the FDA is unlikely to approve genetically engineered cells for therapeutic purposes. Therefore, a more “user-friendly” way to precondition cells was sought, and found. Instead of loading cells up with extra genes, all you had to do was grow the cells under low oxygen, high stress conditions, and they would adapt and survive when implanted into damaged tissue. This, however, has a drawback: if you want to treat a patient, you do not always have the time it takes for extract and isolate their cells, grow them in culture over a week or two, and then implant them. Is there a better way?
The answer turns out to be yes. Treating cells with particular compounds or growth factors can induce resistance to low-oxygen, high-stress conditions, and two papers show us how it’s done.
The first paper is from the laboratory of Ling Wei at Emory University School of Medicine in Atlanta who has shown in the past that low-oxygen adaptation of mesenchymal stem cells from bone marrow made them better able to treat acute heart attacks in laboratory animals. In this paper, Wei and her colleagues exploited a biochemical pathway known to induce resistance of low-oxygen conditions known as the HIF-1 pathway. The HIF-1 pathway consists of two proteins that work as a pair; HIF1alpha and HIF1beta. HIF1beta is made all the time and HIF1alpha is oxygen sensitive. In the presence of oxygen, enzymes called prolyl hydroxylases modify HIF1alpha, marking it for destruction. In the absence of oxygen, the prolyl hydroxylases do not have enough oxygen to modify HIF1alpha and the HIF1alpha/beta complex activates the expression of a host of genes necessary for increased tolerance to low oxygen levels. Therefore, to make cells more tolerant to low oxygen levels, we need to turn on the HIF1 pathway and to do that we need to inhibit the prolyl hydroxylases.
This turns out to be pretty straight forward. A small molecule called dimethyloxalyglycine or DMOG can effectively inhibit prolyl hydroxylase and induce survival in low-oxygen, high-stress environments. Therefore Wei and her group used DMOG to treat cells and test them out.
In culture, the DMOG-treated cells made proteins known to be important for the establishment of new blood vessels and for survival. When they were compared to cultured stem cells that had not been treated with DMOG, the DMOG-treated cells expressed significantly more of VEGF, Glut-1 and HIF1alpha, all of which are important for surviving in low-oxygen environments. In a Matrigel assay, the DMOG-treated cells also made more blood vessels that were longer than their non-DMOG-treated counterparts.
When used in laboratory animals that had suffered heart attacks, the DMOG-treated cells distinguished themselves once again. They survived better than the control cells and hearts that had received the DMOG-treated cells had much smaller heart scars after heart attacks. Functional assays of heart function illustrated that the DMOG-treated cells helped their heart perform above and beyond what was shown observed in the animals implanted with stem cells that had not bee treated with DMOG.
Thus it is possible to precondition cells without long culture periods or genetic engineering. One compound can accomplish it and the cells only needed to be exposed to DMOG for 24 hours.
In a similar vein, Genshan Ma and others from Zhongda Hospital in Nanjing, China used a small peptide called bradykinin to precondition human umbilical cord endothelial progenitor cells (EPCs); the cells that form blood vessels. In this paper, Ma and colleagues used bradykinin-treated EPCs to treat heart attacks in mice. One nice aspect of this paper is the large number of controls they ran with their experimental runs.
The bradykinin-treated cells outperformed their untreated counterparts when it came to the size of the heart scar, the number of dead cells in the heart, and heart performance parameters. Cell culture experiments established that the bradykinin-treated cells expressed the Akt kinase at high levels, and expressed higher levels of VEGF, the blood vessel-inducing growth factor. Bradykinn-treated cells also were more resistant to being starved for oxygen, and survived better under unusual culture conditions. All of these benefits could be abrogated by inhibiting the activity of the Akt kinase by treating cells with LY294002, a compound that specifically inhibits the activator of Akt.
In this case, cells were treated with bradykinin for 10 minutes to 12 hours.
Two papers, two success stories. Stem cell preconditioning certainly works in laboratory animals. Since stem cell trials have been completed in human patients, it might be time to try preconditioned stem cells in human patients.