HO-1 or heme oxygenase is an enzyme that degrades heme groups to biliverdin, iron, and carbon monoxide. It is induced in cells in response to oxidative stress. Overexpression of HO-1 can make cells more resistant to oxidative stress. The highest levels of HO-1 are found in the spleen, where old red blood cells are sequestrated and destroyed.
Mesenchymal stem cells (MSCs) from bone marrow have been genetically engineered to overexpress HO-1 survive much better when implanted into the hearts of animals that have recently suffered a heart attack (Zeng B, et Al, Biomed Sci. 2010 Oct 7;17:80; Yang JJ et al Tohoku J Exp Med. 2012;226(3):231-41). Such cells also increase the density of blood vessels in infarcted tissue, and HO-1 has been postulated to increase blood vessel production (Jang YB et al Chin Med J (Engl). 2011 Feb;124(3):401-7).
These previous experiments show that HO-1 can increase the survival and therapeutic abilities of MSCs. Can increasing the levels of HO-1 do the same for other types of stem cells?
Stuart Atkinson at the Stem Cell Portal web site has highlighted a new paper that was published in the journal Stem Cells that has examined increasing the levels of HO-1 in Cardiac Stem Cells (CSCs).
CSCs are a resident stem cell in the heart that can be isolated from heart patients during heart surgeries. Animal studies and clinical trials have shown that implantation of CSCs soon after a heart attack can produce significant increases in heart function (Bearzi C, et al. Proc Natl Acad Sci U S A 2007;104:14068-14073; Bolli R, et al Lancet. 2011 Nov 26;378(9806):1847-57). Unfortunately, the success of this clinical has been called into questioned by some problems with the data reported in this paper. However, animal studies suggest that the effectiveness of CSCs is compromised by their limited ability to survive in the heart after a heart attack (Hong KU, et al. PLoS One 2014;9:e96725). Therefore, increasing the survival of CSCs might increase their therapeutic efficacy.
Atkinson notes that the compound cobalt protoporphyrin (CoPP) can induce the expression of higher levels of HO-1 and thereby increase the resistance of the cells to oxidative stress and augment cell survival. Therefore, Robert Bolli from the University of Louisville, Kentucky and his colleagues, in collaboration with researchers from the Albany Medical College have treated CSCs with CoPP and these tested their ability to heal the heart after a heart attack.
Bolli and others isolated human CSCs from patients undergoing CABG (cardiac artery bypass graft) surgery, and grew them in culture to beef up the numbers of cells. After a short time in culture, the CSCs were incubated with CoPP for 12 hours. Then Bolli and his team transplanted these human CSCs that were also labeled with green fluorescent protein (GFP) into the hearts of mice that had suffered rather massive heart attacks and had undergone 35 days of reperfusion. The GFP allowed Bolli and others to detect the presence of the implanted CSCs in the rodent heart tissue.
When these hearts of these mice were examined one and five weeks after CSC transplantation, the CoPP-treated CSCs showed substantially higher levels of survival in the mouse hearts. The other two groups of mice included those transplanted with non-pretreated CSCs, and mice treated with the culture medium used to grow the CSCs, and the pretreated CSCs survival significantly better than the non-pretreated CSCs.
CoPP pretreatment seems to augment cell survival, but do the surviving cells increase heart function? Bolli and others used echocardiogram to measure heart function, and echocardiographic assessment 5 weeks after CSC transplantation showed that the CoPP-preconditioned CSCs elicited greater improvement in remodeling of the left ventricle. Additionally, the hearts of the animals that received CoPP-pretreated CSCs showed improved movement of the walls of the heart during its pumping cycle, and better overall performance of the heart in general. Both pretreated and the non-pretreated CSCs, but not CSC culture growth medium shrank the amount of scar tissue in the heart and grew new heart tissue. However, The CoPP-pretreated CSCs were obviously superior to the non-pretreated CSCs at increasing the mass of heart muscle (see here for pictures).
These experiments might very well unravel a burning controversy surrounding CSCs. Bolli’s experiment show that can definitely grow new heart muscle. However, the bulk of the experiments with CSCs strongly suggest that these cells improve heart function by secreting pro-healing molecules without directly contributing to the regrowth of heart muscle. These papers probably observed the effects of CSCs that were transplanted into the heart, but did not survive very long. Bolli and his colleagues, on the other hand, were able to implant CSCs and survived for a much longer time in the hearts. Incidentally, Bolli and his team showed that the implanted CSCs expressed heart muscle-specific genes, which corroborated that these cells were differentiating into heart muscle cells, even though the proportion of cells that formed new heart muscle was relatively small.
In summary, CoPP pretreatment of cell seems to be feasible, safe, and effective as a means to improve CSC-based therapy. Even though It is likely that paracrine mechanisms are essential for CSC-based healing, the ability of CSCs to differentiate into heart muscle cells also seems to be an essential part of the means by which CSCs heal the heart after a heart attack. Thus more work is certainly warranted, but this is a fine start to what might be a simple, but effective way to increase the effectiveness of our own CSCs.