Implications of Apoptotic Cell Death of Resident and Recruited Cells in COPD

Apoptosis, or programmed cell death, is an essential physiological mechanism for the maintenance of tissue homeostasis. Pathological perturbation of this process may lead either to inappropriate deletion of tissue cells or, conversely, to the persistence of undesirable cell populations. While these processes have been heavily implicated in a number of pulmonary pathologies, their contribution to the pathogenesis of COPD in particular has only recently received attention ([1], [2], [3]). It is useful in this context to contrast structural alveolar cells, where loss of cells through excessive apoptosis is likely to correlate with loss of tissue structure and function, and recruited inflammatory cells where a prolonged life span contributes to the persistence of inflammation and exacerbation of tissue injury. In this edition of the journal, two papers, by Aldonyte et al. ([4]) and Perttunen et al. ([5]), shed light on exogenous and endogenous factors influencing cellular longevity and consider their implications for the pathogenesis and treatment of COPD.

COPD is characterised pathologically by chronic inflammation and irreversible alveolar destruction. Our understanding of the mechanisms of pathogenesis has evolved from a simple imbalance between proteases and anti-proteases to one recognising the contribution of other, equally significant pathological processes including oxidative stress, chronic inflammation and the loss of structural lung cells by apoptosis. Furthermore, the complex interplay of these mechanisms has been highlighted in a number of studies, for example, anti-proteases can have anti-apoptotic properties ([6]), and oxidative stress can influence both apoptosis and the protease/anti-protease balance ([7], [8]). While the relative contribution of each mechanism remains uncertain, many in vitro and in vivo studies have implicated dysregulation of apoptosis as a major pathogenetic mechanism in the establishment of COPD (recently reviewed by Henson et al. ([2])).

Human studies have shown levels of apoptosis of resident lung cells are higher in COPD patients when compared with those of non-smokers or smokers without COPD, although this does not establish causation ([9], [10], [11], [12]). Moreover, increased levels of apoptosis of alveolar cells persist, despite smoking cessation (13) so that characterising the relationship between pro-apoptotic stimuli and disease initiation or progression may not be straightforward. There is compelling data from animal models, in particular key studies showing emphysema can be induced through the induction of apoptosis alone, in the absence of inflammation. In a rat model, blockade of VEGF receptors resulted in apoptosis of both epithelial and endothelial cells within alveolar units, resulting in the development of emphysema ([14], [15]). Furthermore, the administration of caspase inhibitors, anti-oxidants or anti-proteases (alpha-1 anti-trypsin) could inhibit both the cellular apoptosis and the destruction of lung parenchyma ([6], [8], [14]).

The work of Aldonyte et al. complements these studies well and contributes to our understanding in this field. First, they show cigarette smoke induces apoptosis of porcine pulmonary artery endothelial cells (PAEC) when directly applied to in vitro cell cultures. Secondly, uptake of exogenous alpha-1 anti-trypsin by PAEC afforded protection against cigarette smoke induced apoptosis. This work is consistent with previous in vivo work (6), demonstrating alpha-1 anti-trypsin can mediate a protective effect via inhibition of apoptosis in a non-inflammatory animal model of emphysema. Significantly, Aldonyte et al. demonstrate a similarly protective effect for PAEC in vitro in the presence of an inflammatory and pathologically relevant stimulus, cigarette smoke. Given the dependence of the alveolar epithelium on survival signals from the vascular bed to maintain its viability, detrimental effects on pulmonary vascular cells have implications for the alveolar unit in its entirety.

While this work is revealing, in the emphysematous disease process cigarette smoke is a highly inflammatory and pleiotropic stimulus, recruiting and activating inflammatory cells. Therefore, it would be valuable in future work to explore the biological relevance of alpha-1 anti-trypsin's anti-apoptotic properties in an inflammatory in vivo model of emphysema in the presence of a multicellular and protease rich milieu. In this context, physiological levels may not be effective at inhibiting apoptotic loss of structural cells.

Collectively, the above in vitro and in vivo effects of alpha-1 anti-trypsin add to the range of anti-inflammatory effects described for this important lung serpin ([16]). The mechanism by which alpha-1 anti-trypsin inhibits endothelial apoptosis was not addressed by Aldonyte et al. in these studies but, given oxidant stress mediates apoptosis in a wide range of cell types by downstream activation of proteases such as cathepsins ([17], [18], [19]), it is plausible this is via anti-protease effects.

Pertunnen et al. consider the consequences of common pharmacological therapies used in COPD and asthma for peripheral blood neutrophil survival. Neutrophils play a key role in the pathogenesis of both COPD and asthma and airway neutrophilia can be a prominent feature ([20]). The relative contribution of pharmacological agents used to treat these conditions, as opposed to the underlying pathological process, to the observed neutrophilia is unclear. However, it has long been recognised that glucocorticoids delay neutrophil apoptosis through a glucocorticoid receptor (GR) dependent pathway ([21], [22], [23]), potentially prolonging the functional longevity and pro-inflammatory potential of these cells.

The studies of Pertunnen et al. show that a range of beta-2 agonists can enhance the anti-apoptotic effects of steroids on human neutrophils. Beta-2 agonists alone do not influence neutrophil survival, an interesting finding given neutrophils express beta-2 adrenoceptors, and ligation would be predicted to elevate intracellular cAMP levels and delay neutrophil apoptosis ([24]). Moreover, beta-2 agonists, despite augmenting the pro-survival effects of corticosteroids, attenuated the pro-survival effects of GM-CSF, although only at high concentrations (micromolar rather than nanomolar). These observations are of interest but there are relatively modest changes in levels of apoptosis, particularly on addition of beta-2 agonists to corticosteroid-treated neutrophils. Furthermore, these data are obtained from treatment of unstimulated peripheral blood neutrophils; effects on activated neutrophils within the airway may well be different. Airway neutrophils from patients with significant COPD are likely exposed to high levels of inflammatory cytokines, many of which are profoundly anti-apoptotic ([25]), as well as to high doses of inhaled steroids.

The authors propose the combined use of beta-2 agonists and corticosteroids may result in enhanced neutrophil accumulation at sites of inflammation, including the airways. This appears to contrast with many clinical studies reflecting favourable outcomes for combination therapy in both severe COPD and asthma ([26], [27]). The effects of exogenous beta-2 agonists and corticosteroids on airway neutrophils are therefore an area for future study in relevant in vivo models of neutrophilic airway inflammation.