Cigarette smoke, which can be considered a strong chemical oxidant, has the strongest epidemiological link with Age-Related Macular Degeneration (AMD) ; it is the most important risk factor for pulmonary emphysema and fibrosis as well as is an important risk factor for stroke, mediated in part by accelerated intimal thickening and plaque development. In the Atherosclerosis Risk in Communities Study, active smoking and exposure to environmental tobacco smoke were associated with accelerated progression of intimal/medial thickness of the carotid artery.
In add exposure to secondhand smoke causes 53,000 of the 453,000 deaths caused each year by tobacco use in the United States.
Philip Morris conducted research on tobacco-specific nitrosamines (TSNA) that may explain this phenomenon. TSNAs are a group of highly carcinogenic compounds that are formed exclusively from nicotine and other tobacco alkaloids. They are found in mainstream, sidestream, and secondhand cigarette smoke. Common TSNAs include 4-(methylnitrosamino)-I-(3-pyridyl)-1-butanone (NNK), N-nitrosonornicotine (NNN), 4-(methylnitrosamino)-4-(3-pyridyl)-1-butanol (iso-NNAL), N-nitrosoanabasine (NAB), and N-nitrosoanatabine (NAT). NNK and its metabolic breakdown product NNAL are among the strongest nitrosamine carcinogens
known. NNK causes lung adenomas and adenocarcinomas, whether it is administered p.o., i.p., or via the lung. NNK can also cause cancer of the nasal mucosa and liver. The NNK metabolite NNAL is consistently found in nonsmokers exposed to secondhand smoke. Kinetic studies of TSNAs, done at Philip Morris between 1983 and 1997, show that NNK concentrations in sidestream smoke increase after the smoke is released into room air.
Carotid Artery Intimal Thickening
Increased transcription factor activation, expression of adhesion molecules, and redox gene inducible NO synthase (iNOS) have been demonstrated with exposure to cigarette smoke (CS) and may contribute to increased intimal thickening. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor mediating enzyme induction in response to CS and may have a potentiating effect on CS carcinogenicity. The mouse iNOS promoter13 contains a site that corresponds to the core xenobiotic-responsive element (XRE) to which the AhR binds. Therefore, the AhR may mediate the negative effects of CS on the arterial intima.
Anatomy of airways and transient receptor potential channels
The airways are innervated by branches of the trigeminal and vagal nerves. Among the many classes of nerve fibers are the polymodal nociceptors (PMNs). These unmyelinated neurons send signals that cause the perception of pain in response to potentially damaging thermal, mechanical, and chemical stimuli. Their activation induces protective reflexes and nocifensive behaviors (defensive behavior that is elicited by sensory stimuli that have the potential to cause injury) that include apnea, bradycardia, coughing, mucus secretion, and avoidance behavior. Some well established chemical irritants that activate PMNs include capsaicin (the pungent compound in chili pepper and Mace brand defense sprays), allyl isothiocyanate (present in mustard, wasabi, and horseradish), formaldehyde, nicotine, acid, hydrogen peroxide (H2O2), chlorine, acrolein, and, finally, smoke generated from tobacco. The latter differs from the other compounds in that it comprises at least 5,000 distinct chemicals at varying concentrations. Although there are many types of PMNs, the most common are those that are activated by capsaicin through its receptor, transient receptor potential cation channel, subfamily V, member 1 (TRPV1). TRPV1 is a member of the TRPV subfamily of ion channels that are all inhibited by the polyvalent cationic dye and ion channel blocker ruthenium red, but specific antagonists may exist for individual transient receptor potential channels. For TRPV1, one such antagonist is capsazepine, a synthetic analogue of capsaicin. When capsaicin-sensitive neurons are activated, they transmit nociceptive information to upstream relay centers within the CNS that are associated with pain perception and, importantly, these neurons also release proinflammatory mediators. With respect to cigarette smoke aqueous extract (CSE) as a stimulus, capsaicin-sensitive nociceptors appear to have an important role in physiological changes in airways and afferent control of respiration in response to CSE. Specifically, in rodent neonates, capsaicin pretreatment has been shown to induce degeneration of respiratory tract nociceptors and a longlasting desensitization of the airways to cigarette smoke. In this regard, in rat airways, it was found that capsaicin pretreatment prevented plasma extravasation (a critical component of the inflammatory response that results from the activation of sensory nerve endings and the subsequent release of proinflammatory neuropeptides) in response to exposure to cigarette smoke. Interestingly, plasma extravasation was inhibited by ruthenium red but not by capsazepine. These results indicated that TRPV1 capsaicin receptor–expressing neurons are important in airway sensitivity but that the TRPV1 capsaicin receptor is not the receptor for the majority of chemicals in cigarette smoke.
TRPA1 and cigarette smoke
The Ca2+- permeable transient receptor potential cation channel, subfamily A, member 1 (TRPA1) channel as the receptor for some of the principal components of CSE, namely crotonaldehyde and acrolein. TRPA1, like TRPV1, is expressed by trigeminal and nodose/jugular ganglia neurons and moreover, both channels are most often found in the same neuron. This means that activation of TRPA1 will likely exert effects similar to those observed following the activation of TRPV1.
Using a variety of techniques, Andrè et al. showed that the α,β-unsaturated aldehydes crotonaldehyde and acrolein, the most abundant unsaturated aldehydes in CSE, induce neurogenic inflammation by stimulating TRPA1 channels coexpressed with TRPV1 on capsaicin-sensitive nociceptors. After identifying TRPA1 neurons in guinea pig jugular ganglia, the authors cultured these neurons and used Ca2+ imaging to show that, in capsaicin-sensitive neurons, CSE and both unsaturated aldehydes activated these neurons in a dose-dependent manner. Importantly, the responses were inhibited by HC-030031, but not by capsazepine or a variety of ROS scavengers. They showed that nicotine did not activate TRPA1 and that acetaldehyde, the most abundant saturated aldehyde present in CSE, produced a small response at high concentrations but was not inhibited by HC-030031. Also, they provide evidence that HEK293 cells heterologously expressing TRPA1 were responsive to CSE as well as to the two unsaturated aldehydes. This was true also for dorsal root ganglia (DRG) neurons from Trpa1+/+ but not from Trpa1–/– mice. Using slices of guinea pig airways, they showed that acrolein or crotonaldehyde induced release of the neuropeptides substance P and calcitonin gene–related peptide. This release was reduced when the neurons were desensitized with capsaicin and/or when extracellular Ca2+ was removed, thus showing that . In physiological experiments using isolated guinea pig bronchial rings, the authors showed that CSE, as well as acrolein or crotonaldehyde, produced a contraction of the bronchial rings that was inhibited by HC-030031 but not by capsazepine or ROS scavengers. Finally, installation of CSE into the trachea of wild-type and Trpa1–/– mice revealed that plasma extravasation was only observed in wild-type mice. In summary, Andrè et al. showed that cigarette smoke–induced airway neurogenic inflammation is mediated by α,β-unsaturated aldehydes and their activation of the TRPA1 receptor.
Age-Related Macular Degeneration (AMD)
Oxidative stress has long been hypothesized to play a substantive role in the development of AMD due to the high oxidative stress environment of the fundus. The Age Related Eye Disease (ARED) study showed that high dose antioxidant vitamin therapy reduced the advancement of intermediate non-neovascular AMD, and that this benefit was associated with a reduction in plasma glutathione and cysteine oxidation. While the genetic variations of several complement factors have been associated with AMD susceptibility, different studies also have identified a susceptibility locus for AMD may be located in or near the hypothetical LOC387715 gene. Kanda et al have confirmed that this locus was the susceptibility locus for AMD, and that this gene encodes a mitochondrial protein. Interestingly, this locus may be associated with smoking, and the combination of the LOC387715 polymorphismand smoking confers a higher risk for AMD than either factor alone. This finding, along with its identification as a mitochondrial protein, raises suspicion for a role of the oxidative defense response in this disease. Further evidence for genetic susceptibility related to oxidative stress has been provided by Canter et al, who have correlated the mitochondrial DNA polymorphism A4917G with AMD and Kimura et al, who showed that a polymorphism in superoxide dismutase 2 (SOD2) is associated with AMD in a small subset of patients. Cigarette smoke, which can be considered a strong chemical oxidant, has the strongest epidemiological link with AMD.
Cigarette smoking is the most important risk factor for pulmonary emphysema and fibrosis.
In fact, cigarette smoking is thought to be the cause of recurrent epithelial injury and impaired repair. For example, previous studies have shown that CS causes death of alveolar epithelial cells, and that CS inhibits epithelial repair responses, such as chemotaxis, proliferation, and contraction of three-dimensional collagen gels.
The alveolar epithelium is often injured by a variety of inhaled toxins, such as SO2, O3, NO2, and cigarette smoke (CS), and when injured, it initiates repair responses. Appropriate repair responses by alveolar epithelial cells require their integrated ability to migrate, proliferate, and differentiate to cover defects that result from the injury. Failure of the epithelium to repair itself is assumed to be an important cause of chronic lung diseases, such as pulmonary emphysema and fibrosis. Epithelial injury and regeneration are thought to occur continually in such diseases, and the repeated cell cycles of the epithelial cells at the site of injury may shorten the length of telomeres, thereby potentially inducing replicative senescence. Inhaled toxins also generate oxidative stress and DNA damage in epithelial cells, which may cause stress-induced senescence. Once epithelial cells reach the senescence stage, they can no longer proliferate. The repair responses by alveolar epithelial cells may cease as a result, and the cessation of the repair responses may in turn result in architectural and functional disruptions in the alveolar epithelium that may allow lung diseases to progress.
Cellular senescence is a state of irreversible growth arrest induced either by telomere shortening (replicative senescence) or by telomere-independent signals, such as DNA damage and oxidative stress (stress-induced senescence). The state of cellular senescence is accompanied by various phenotypic changes, including a distinct, flat, and enlarged cell morphology, an increase in senescence-associated _-galactosidase (SA _-gal) activity, accumulation of lipofuscin, and the expression of cell
cycle inhibitors, such as p16INK4a and p21CIP1/WAF1/Sdi1. Recent evidence suggests that cellular senescence plays an important role not only in physiologic aging processes, but in pathologic disease states, such as liver cirrhosis and atherosclerosis.
Recent studies have shown that two families of cyclin-dependent kinase inhibitors (CKIs) are involved in the intracellular signaling pathways mediating cellular senescence. The first family, inhibit cyclin-dependent kinase (INK)4, includes p15INK4b, p16INK4a, p18INK4c, and p19INK4d,which bind specifically to cyclin-dependent kinase (CDK) 4 and CDK6 and prevent the formation of cyclin D–CDK complexes. p21CIP1/WAF1/Sdi1, p27KIP1, p57KIP2 form the second family, CIP/KIP, which binds to CDK4-cyclin D, CDK6-cyclinD,CDK2-cyclinE, andCDK6-cyclinDcomplexes.
CS-induced senescence of alveolar epithelial cells is associated with the accumulation of p21CIP1/WAF1/Sdi1 protein (exposure to either CSE or H2O2
increases the level of p21CIP1/WAF1/Sdi1 mRNA).