Carbon monoxide (CO), one of the products of heme oxygenase action on heme, prevents arteriosclerotic lesions that occur following aorta transplantation; pre-exposure to 250 parts per million of CO for 1 hour before injury suppresses stenosis after carotid balloon injury in rats as well as in mice. The protective effect of CO is associated with a profound inhibition of graft leukocyte infiltration/activation as well as with inhibition of smooth muscle cell proliferation. The anti-proliferative effect of CO in vitro requires the activation of guanylate cyclase, the generation of cGMP, the activation of p38 mitogen-activated protein kinases and the expression of the cell cycle inhibitor p21Cip1. These findings demonstrate a protective role for CO in vascular injury and support its use as a therapeutic agent.

Most evidence indicates that nitric oxide plays a role in normal wound repair; however, involvement of inducible nitric oxide synthase (iNOS) has not been established. Experiments were carried out to determine the requirement for iNOS in closing excisional wounds. Wound closure was delayed by 31% in iNOS knockout mice compared with wild-type animals. An identical delay in wound closure was observed in wild-type mice given a continuous infusion of the partially selective iNOS inhibitor N6-(iminoethyl)-L-lysine. Delayed wound healing in iNOS-deficient mice was completely reversed by a single application of an adenoviral vector containing human iNOS cDNA (AdiNOS) at the time of wounding. Reverse transcription PCR identified iNOS mRNA expression in wild-type mice peaking 4-6 d after wounding, and confirmed expression of human iNOS in the adenoviral vector containing human iNOS cDNA-treated animals. These results establish the key role of iNOS in wound closure, and suggest a gene therapy strategy to improve wound healing in iNOS-deficient states such as diabetes, and during steroid treatment.

The tumor suppressor gene product p53 plays an important role in the cellular response to DNA damage from exogenous chemical and physical mutagens. Therefore, we hypothesized that p53 performs a similar role in response to putative endogenous mutagens, such as nitric oxide (NO). We report here that exposure of human cells to NO generated from an NO donor or from overexpression of inducible nitric oxide synthase (NOS2) results in p53 protein accumulation. In addition, expression of wild-type (WT) p53 in a variety of human tumor cell lines, as well as murine fibroblasts, results in down-regulation of NOS2 expression through inhibition of the NOS2 promoter. These data are consistent with the hypothesis of a negative feedback loop in which endogenous NO-induced DNA damage results in WT p53 accumulation and provides a novel mechanism by which p53 safeguards against DNA damage through p53-mediated transrepression of NOS2 gene expression, thus reducing the potential for NO-induced DNA damage.