Much evidence points toward a major function for PARP-1 in tissue damage.
92 Tissue insults lead to DNA damage, which can arise from the formation of nitric-oxide derivatives such as peroxynitrite.
92 As a consequence, PARP-1 becomes overactivated and may lead to an important depletion in its substrate NAD+. In response to the NAD+ depletion, the cell’s attempt to resynthesize this substrate leads to a depletion of ATP and triggers the cell to die from energy loss. This process allows for the elimination of cells that are too damaged to progress through the many steps of the wounding process. Indeed, anterior stromal keratocytes undergo apoptosis in response to corneal epithelial injury in a proportion that may range from 0.9% to 5.1%, depending on the surgical procedure selected.
93 However, unlike stromal keratocytes, corneal epithelial cells have been reported to be resistant to apoptosis, as only a small proportion of the cells lost from the surface of the cornea through shedding enter apoptosis.
94 Growth factors, such as hepatocyte growth factor (HGF), have been shown to confer cytoprotection on corneal epithelial cells by preventing them from progressing into apoptosis through the activation of the PI3K/Akt-1/Bad- but not the ERK1/2-mediated signal transduction pathways.
95 Of note, studies by Hoyt et al.
96 97 provided evidence that engagement of β1 integrins can prevent acute DNA breakage caused by a variety of unrelated agents, such as the antitumor agent bleomycin, by dramatically reducing poly(ADPR) synthesis by PARP-1 in response to DNA damage. Integrin clustering has been proposed to alter the chromatin structure by a PARP-modulated nuclear response.
98 As FN increased PARP-1 expression in RCECs without any apparent alteration in its activation status (data not shown), it is expected that no depletion in NAD+ or ATP occurred under such culture condition. Then, what physiological advantage would such an increase in PARP-1 protein confer to RCECs during wound healing? One possible way by which PARP-1 may contribute to wound healing without the need for the cell to progress toward apoptosis is through alteration of transcription factors that regulate genes whose encoded products are necessary for cell adhesion and migration. Gene disruption or pharmacological inactivation of PARP-1 has been reported to reduce the cytokine-mediated expression of ICAM-1, P-selectin, and E-selectin, as well as mucosal addressin cell adhesion molecule (MAdCAM)-1 in human umbilical vein endothelial cells.
99 PARP-1 has been reported to modulate the expression of the integrin CD11a in the migration of microglial cells after brain injury.
100 PARP-1 may do so either by directly interacting with transcription factors, as shown for YY-1, AP-2, B-MYB, Oct-1, TEF-1, and NF-κB, or through their poly(ADP-ribosyl)ation, as evidenced for p53, fos, NF-κB, and both RNA polymerases I and II (reviewed in Ref.
101 ). Although PARP-1 has been most often reported to interfere with the positive regulatory influences mediated by these transcriptions factors, some evidence suggests that it may also act as a coactivator or enhancer factor and thereby promote gene transcription.
102 103 Target sites for some of these transcription factors (AP-1, AP-2, B-MYB, and NF-κB) were identified in many integrins genes’ promoters. Both AP-1 and -2 are of particular interest, as binding sites for these transcription factors have been identified in the promoter of the α4, α5, and α6 integrin gene subunits
26 104 105 ; and the expression of these transcription factors has been reported to be increased during corneal wound healing.
37 106 107 The transcription factor PAX-6, necessary for proper development of many eye structures including the cornea, the lens, and the retina, is also worth mentioning, as its expression has been demonstrated to be under the influence of PARP-1.
108 Pax-6 expression is increased at the migrating edge as the epithelium resurfaces the cornea after injury
109 and may contribute to corneal wound healing by modulating the expression of Pax-6 responsive genes, which comprise those encoding the integrin subunits β1, α4, and α5.
106 110 111 It is interesting that activation of the Sp1 DNA-binding activity by TNF-α or LPS requires PARP-1 activity, as Sp1 activation has been found to be lower in PARP-1
−/− glial cells relative to the level measured in PARP-1
+/+ glia
112 However, as yet no clear evidence that PARP-1 may either directly interact with Sp1 or use it as a substrate for poly(ADP-ribosyl)ation has been reported.