Response to Letter Regarding Article, “Oxidant Stress Impairs In Vivo Reendothelialization Capacity of Endothelial Progenitor Cells From Patients With Type 2 Diabetes Mellitus: Restoration by the Peroxisome Proliferator- Activated Receptor-γ Agonist Rosiglitazone”
We thank Thum and colleagues for their letter about our article.1 We agree that it is important to gain a better understanding of mechanisms leading to impaired in vitro and in vivo function of endothelial progenitor cells (EPCs) derived from patients with diabetes. This is not only important for a better understanding of the pathophysiology of the disease, but is also a prerequisite to optimize autologous progenitor cell-based therapeutic approaches.
In both studies, an increased superoxide production of EPCs derived from diabetic patients has been observed that was suggested to contribute importantly to impaired in vitro and in vivo function.1,2 Whereas in our study, we focused on the oxidative enzyme system NAD(P)H oxidase as a major source of increased superoxide production in diabetic EPCs,1 Thum et al have demonstrated that the uncoupled endothelial nitric oxide synthase (eNOS) represents a major source of increased superoxide production in diabetic EPCs.2 These findings are not necessarily discrepant, as NAD(P)H oxidase activation has been suggested as a major mechanism leading to eNOS uncoupling, at least in part because of increased oxidation of tetrahydrobiopterin, an essential cofactor of eNOS.3 Interestingly, Thum et al have demonstrated increased oxidation of tetrahydrobiopterin in EPCs of diabetic patients, which may represent a likely mechanism leading to a reduced tetrahydrobiopterin availability in diabetic EPCs.3
Thum et al have inhibited NAD(P)H oxidase activity by the flavoenzyme inhibitor diphenyleneiodonium (DPI) that was somewhat less efficient in suppressing EPC-derived superoxide production as compared with the small interfering RNA knockdown of the NAD(P)H oxidase component p47phox used in our study. This may, at least in part, be related to a more prolonged NAD(P)H oxidase inhibition by the small interfering RNA knockdown approach as compared with DPI. A more prolonged NAD(P)H oxidase inhibition would also likely be more efficient in suppressing tetrahydrobiopterin oxidation. In support of this notion, in our previous study in p47phox-deficient mice with hypertension, we did not observe the eNOS-derived superoxide production that was observed in the wild type mice, suggesting that p47phox knockdown efficiently prevents eNOS uncoupling.3 In addition, we agree with Thum et al that their patient population had a markedly less controlled hyperglycemia, as indicated by a substantially higher glycosylated hemoglobin level, which is likely to contribute to a more severe eNOS uncoupling. In our study,1 some patients were already on another antihyperglycemic therapy (in the rosiglitazone group, 4 patients took metformin and 6 patients took sulfonylureas; in the placebo group, 3 patients took metformin, 3 patients took sulfonylureas, and 2 patients took a glinide), and therefore overall, the patients had better-controlled hyperglycemia compared with subjects in the study of Thum et al.2
After the completion of our study,1 rosiglitazone therapy in patients with diabetes became the topic of substantial debate because concerns of a possible increase of cardiovascular risk associated with this treatment.4 In our study, the effect of rosiglitazone on EPC function was mediated by peroxisome proliferator activated receptor-γ (PPAR-γ) agonism, as it was prevented by PPAR-γ small interfering RNA inhibition.1 Notably, no increase in cardiovascular risk has been observed with the PPAR-γ agonist pioglitazone,4 suggesting that the mechanism for potential adverse effects on cardiovascular risk of rosiglitazone may be independent of PPAR-γ and may involve other not yet completely understood mechanisms such as adverse effects on apolipoprotein B–containing lipoproteins.
In addition, our findings suggest that PPAR-γ activation may provide an interesting novel tool to optimize in vivo functionality of patient-derived progenitor cells for cell-based therapeutic approaches.1
Sorrentino SA, Bahlmann FH, Besler C, Muller M, Schulz S, Kirchhoff N, Doerries C, Horváth T, Limbourg A, Limbourg F, Fliser D, Haller H, Drexler H, Landmesser U. Oxidant stress impairs in vivo reendothelialization capacity of endothelial progenitor cells from patients with type 2 diabetes mellitus: restoration by the peroxisome proliferator-activated receptor-γ agonist rosiglitazone. Circulation. 2007; 116: 163–173.
Thum T, Fraccarollo D, Schultheiss M, Froese S, Galuppo P, Widder JD, Tsikas D, Ertl G, Bauersachs J. Endothelial nitric oxide synthase uncoupling impairs endothelial progenitor cell mobilization and function in diabetes. Diabetes. 2007; 56: 666–674.