British scientists have found that human fetal stem cells can effectively treat leg ischemic ulcers in a mouse model of Type 1 diabetes. They also discovered that the culture broth in which the stem cells had been grown mimicked the wound-healing ability of the cells, suggesting these cells may also serve as a “factory” of wound-healing substances. Alternatively, the active ingredients in the culture, once identified, could be used in place of the cells to avoid ethical concerns. The results were published online April 2 ahead of print in the journal Circulation Research.
Ischemic foot ulcers in diabetic patients have the worst outcome of all chronic skin wounds, with high amputation and mortality rates in patients with ischemic limbs. Impaired wound healing in diabetic patients results from multiple deficits, including deficient angiogenesis and impaired cell proliferation. The unique healing activity of stem cells is likely due to their ability to differentiate into the various component cells of injured tissues, as well as to produce and release growth factors that encourages the formation of new blood vessels (wound granulation). Transplantation of fetal stem cells may stimulate angiogenesis and promote tissue regeneration.
Paolo Madeddu, Professor of Experimental Cardiovascluar Medicine, and colleagues at the Bristol Heart Institute, UK, previously used fetal stem cells in a non-diabetic mouse model of limb ischemia. They showed that transplantation of a small number of CD133+ human fetal aorta-derived vascular progenitor cells promote revascularization and muscle cell regeneration, thereby supporting limb salvage. They also observed that the fetal stem cells secrete large amounts of vascular endothelial growth factor (VEGF), a potent stimulator of angiogenesis.
The current study in a diabetic model of limb ischemia confirmed the previous findings that the CD133+ cells promote blood vessel formation and salvage the diabetic limb. Three days following the graft, consisting of a piece of collagen mixed with CD133+ stem cells, very few CD133+ cells were themselves detected in the ischemic diabetic ulcer. This indicated that the transplanted cells were not persistent, but has served their role in the very first days after transplantation. The CD133+ cells also released large amounts of growth factors and cytokines with pro-angiogenic and pro-survival potential.
To confirm the importance of these released factors, Professor Madeddu and colleagues grew the CD133+ cells in vitro, and then used the “conditioned” culture to reproduce the effects on wound healing and angiogenesis. These additional experiments confirmed that wound healing and angiogenesis are equally supported either by giving stem cells or the released products from these cells. To determine which components of the healing cocktail were the most active, they eliminated likely candidates one by one using blocking antibodies. Notably, they found that the VEGF and some interleukins were the crucial factors accounting for the healing effect of transplanted stem cells.
Importantly, VEGF was recognized to be responsible for reactivation of fetal genes belonging to the Wnt gene family in wounded tissues. Withdrawal of Wnt gene products also prohibited the beneficial action of conditioned medium on wound closure and reparative angiogenesis. It is known that fetal wounds heal in a scarless fashion. It is therefore possible that, when fetal stem cells are transplanted onto diabetic ulcers, they might reactivate an intrinsic fetal program in the recipient that allows adult ulcers to undergo repair in similar ways as fetal wounds do.