Adult stem cell transplantation has been used to treat heart attack patients in several different clinical trials. While the results have not been consistent, adult stem cells, it is clear that adult stem cells, primarily from bone marrow, and in some cases fat, help improve heart function. However, a major criticism of the use of adult stem cells is that they do not differentiate into heart muscle cells, but only improve the heart through “paracrine mechanisms,” which means that they secrete molecules that help heal the heart. This criticism is only represent part of the picture, since bone marrow stem cells transdifferentiate into heart muscle and blood vessel cells, albeit at a rather low rate, and fuse with endogenous cells to form hybrid cells that show improved function (Strauer BE, Steinhoff G. J Am Coll Cardiol. 2011 Sep 6;58(11):1095-104. doi: 10.1016/j.jacc.2011.06.016). In addition, adult stem cells activate endogenous cardiac stem cells to divide and replace lost heart muscle cells and make new blood vessels (Loffredo FS, et al., Cell Stem Cell. 2011 Apr 8;8(4):389-98. doi: 10.1016/j.stem.2011.02.002).
Embryonic stem cells, on the other hand, are thought to differentiate into heart muscle cells that integrate into the heart and directly replace the dead heart muscle cells, Animal studies do show such improvements (Caspi O, et al., J Am Coll Cardiol. 2007 50(19):1884-93). However, there is a caveat to all this: Most of the animal experiments with heart muscles derived from embryonic stem cells have only analyzed heart function for up to four weeks after transplantation. Experiments that examined heart function for longer than four weeks have not been able to show that these improvements are sustained after four weeks (van Laake LW, et al., Stem Cell Res. 2007 Oct;1(1):9-24. doi: 10.1016/j.scr.2007.06.001). Therefore, could it be possible that embryonic stem cell-derived cells also help the heart mainly through paracrine mechanisms?
A new paper from Piero Anversa’s and Richard Burt’s laboratories has shown that implantation of embryonic stem that were hit with radiation so that they cannot divide significantly improves heart function after a heart attack.
Experiments were conducted with mice and rhesus monkeys, and mouse and human embryonic stem cells (ESCs) were used. The ESCs were treated with 20 to 100 Grays of radiation, which completely abolished their ability to divide (a gray is the absorption of one joule of energy, in the form of ionizing radiation, per kilogram of matter).
The irradiated ESCs or iESCs were implanted into mice and Rhesus monkeys that had suffered a heart attack. Control animals were implanted with conditioned culture media from the ESC culture dishes.
In the mice and the Rhesus monkeys, the control animals showed little improvement and their hearts continued to deteriorate after the heart attack. However, the animals that had been implanted with the iESCs showed significant improvement of their heart function.
The authors in the discussion suggest that the iESCs might have suppressed the inflammatory response that occurs in the heart after a heart attack, but tissue sections of the hearts after the experiment showed that the iESC-implanted hearts had just as many immune cells infiltrating the tissue as the hearts of the control animals. Mesenchymal stem cells, however, do a very fine job of suppressing inflammation in the heart after a heart attack (see the recent paper by van den Akker et al., Biochimica et Biophysica Acta 1830 (2013): 2449-58). Therefore, the mechanisms by which ESCs improve heart function might be more paracrine-based than anything else. If this is the case, then why are embryonic stem cells being pursued for clinical purposes? Adult stem cells heal by means of paracrine mechanisms and can also sidestep the problem of immune rejection. Also, adult stem cells treatments do not require the dismemberment of young human beings at the embryo stage of their existence. Therefore, even though the present ESC lines are certainly appropriate for clinical and biological research, deriving more of them for clinical treatments is inappropriate, and even murderous.