**6. Potential therapeutic approaches**

Mitochondrial ROS production plays an active role in the initiation of fragmentation, since administration of a mitochondrial scavenger prevents the hyperglycemia-induced fission of mitochondria [113]. Blocking of mitochondrial fission will also restore the acetylcholinemediated eNOS (endothelial nitric oxide synthase) phosphorylation and cGMP response in hyperglycemic endothelial cells, suggesting that the vascular impairment is partly caused by

Mitochondrial ROS production results in DNA damage in the mitochondria that activates the mitochondrial DNA repair enzymes [115]. Oxidative DNA damage activates poly(ADPribose) polymerase 1 (PARP1) in the mitochondria similar to the situation in the nucleus [116]. PARP1 adds ADP-ribose polymers (PARs) to the mitochondrial base excision repair (BER) enzymes, exo/endonuclease G (EXOG) and DNA polymerase gamma (Polγ) and affects the mitochondrial DNA repair [116]. Activation of mitochondrial PARP1, as opposed to nuclear PARP1, may decrease the DNA repair and slow down the mitochondrial biogenesis. Integrity of the mitochondrial DNA (mtDNA) also relies on mitochondrial transcription factor A (TFAM), a protein that may act as a physical shield of the mitochondrial DNA, since it forms histone-like structures with mtDNA and is present in large amounts in mitochondria (~900 molecules for each mtDNA). Apart from protecting the DNA from damaging agents, it tightly binds to heavily damaged DNA parts, blocks the transcription and may promote the repair of affected sites [115]. TFAM is also implicated in mitochondrial biogenesis and the maintenance of stable mtDNA copy number. In diabetic retinas, the level of TFAM is reduced, and it decreases the mitochondrial biogenesis that can lead to fewer mitochondria and less efficient

Oxidant production will also induce several changes in the function of proteins that may be associated with cellular injury and result in altered cell metabolism, senescence and vascular dysfunction. Oxidative stress leads to oxidative DNA damage and DNA strand breaks that activates the predominantly nuclear PARP1 and may lead to ATP depletion and necrosis or apoptosis [118]. However, the level of PARP activation is mostly lower than to induce cell death; it results in higher NAD+ consumption and changes in the PARylation pattern of pro-

[67, 68, 82]. A third posttranslational modification that changes in hyperglycemia is protein

[119]. Protein S-sulfhydration is a highly prevalent modification that typically increases the activity of target proteins. The antioxidant master regulator Nrf2 (nuclear factor E2-related

ler, Kelch-like erythroid cell-derived protein with Cap 'n' collar (CNC) homology (ECH) associated protein 1 (Keap1) [120, 121]. A further target is ATP synthase in the respiratory

tein S-sulfhydration and results in lower Nrf2 activity and OXPHOS efficiency [108, 109]. All these changes contribute to the dysfunction of proteins in hyperglycemia and promote

S increases cellular bioenergetics via S-sulfhydration of Complex V [122]. Since

utilization and decreased mitochondrial output may decrease


S level in the cells and plasma, it will also decrease the pro-

concentrations and by reducing the amount of substrate

S and reactive cysteine residues

S via sulfhydration of its key control-

mitochondrial fission itself [114].

194 Endothelial Dysfunction - Old Concepts and New Challenges

OXPHOS [117].

teins [50]. The higher NAD+

for SIRT1 (another NAD+

hyperglycemia reduces the H2

cellular dysfunction.

chain: H2

the nuclear and cytoplasmic NAD+

S-sulfhydration (or persulfidation), a reaction between H<sup>2</sup>

factor 2) transcription factor is also activated by H2
