|Home | About | Journals | Submit | Contact Us | Français|
Accumulating evidence indicates that aldosterone plays a predominant role in the pathobiology of cardiopulmonary vascular fibrosis. Mineralocorticoid receptor activation disrupts normal vessel architecture by promoting collagen and elastin degradation as well as deposition of newly synthesized collagen, which has attendant consequences for both vascular structure and function. The clinical relevance of this concept is supported by epidemiological studies that have shown that vascular stiffness occurs prior to hypertension and link increased serum aldosterone levels to incident hypertension.1, 2 Further support for this concept is found in selected patient populations, including patients with primary aldosteronism, congestive heart failure, and obesity, where evidence of mineralocorticoid receptor activation is associated with circulating markers of fibrosis and increased collagen turnover.3–5 Although these studies provide a framework to explore the association between aldosterone and fibrosis, arterial stiffness, and hypertension they do not inform on the critical molecular mechanisms or relevant signaling pathways that initiate collagen synthesis and ultimately lead to vascular fibrosis.
In this issue of Hypertension, Tarjus et al provide compelling evidence that neutrophil gelatinase associated lipocalin (NGAL), or lipocalin-2 (Lcn2), is a key mediator of mineralocorticoid-stimulated vascular fibrosis.6 They convincingly show that NGAL/Lcn2 is associated with aldosterone levels and markers of collagen metabolism in non-hypertensive asymptomatic individuals with abdominal obesity. The authors also provide mechanistic insight to support the central role of Lcn2 by conducting studies in Lcn2 conditional knockout mice using the uninephrectomy/aldosterone infusion/salt model. Under these conditions, absence of Lcn2 expression is associated with lower blood pressure, less oxidant stress, and decreased vascular fibrosis compared to controls. Thus, these studies identify NGAL/Lcn2 as a unifying regulator of aldosterone-stimulated collagen metabolism and vascular fibrosis.
The mechanism(s) by which NGAL/Lcn2 modulates vascular fibrosis is likely multifactorial. NGAL/Lcn2 binds matrix metalloproteinase (MMP)-9 via a disulfide bond, which may function as a redox switch, and is detectable in the circulation. This also suggests that one function of NGAL/Lcn2 is to deliver MMP-9 to sites of vascular remodeling. Studies performed in heart failure models found that in addition to MMP-9, NGAL/Lcn2 physically associates with MMP-2 in the heart, but not skeletal muscle, indicating that this interaction achieves greater importance at sites of tissue injury.7 While the authors were unable to study this phenomenon owing to structural differences between human and murine Lcn2 (e.g., murine Lcn2 does not form a disulfide bond with MMP-9), it does raise the possibility that NGAL/Lcn2 regulates fibrosis through an interaction with other pro- or anti-fibrotic proteins. For example, Lcn2 is known to bind and negatively regulate the actions of hepatocyte growth factor, which is a recognized anti-fibrotic peptide.8 NGAL/Lcn2 also binds to other proteins that function as receptors, such as megalin or low-density lipoprotein-related protein 2, to facilitate endocytosis of NGAL/Lcn2 and other cargo proteins.9 This is possible as megalin has been associated with fibrogenesis.10 Whether or not fibrosis resulted directly from NGAL/Lcn2 signaling remains to be determined.
The finding of decreased blood pressure in Lcn2 knockout mice despite challenge with uninephrectomy/aldosterone infusion/salt is not surprising and there is precedent for this observation. In wild-type mice, high fat feeding led to an increase in deposition of NGAL/Lcn2 in arteries and an increase in blood pressure, an effect that was not observed in high fat fed Lcn2 knockout mice. This was attributed to the state of polyamination of NGAL/Lcn2. The presence of this posttranslational modification of NGAL/Lcn2 is regulated, in part, by adipose tissue, which de-amidates NGAL/Lcn2. This results in increased time in the circulation and vascular accumulation. Moreover, studies performed with human NGAL/Lcn2 showed that mutation of Cysteine87 to alanine resulted in less polyamines, increased resident time in the circulation, and endothelial dysfunction.11 It is known that aldosterone induces adipose tissue dysfunction suggesting that aldosterone may indirectly promote NGAL/Lcn2 de-amidation leading to deposition of NGAL/Lcn2 within the vessel wall and fibrosis. Although NGAL/Lcn2 was not implicated in endothelial dysfunction in the current study, this is likely related to the differences between human and murine Lcn2.
Another interesting finding was that uninephrectomy/aldosterone infusion/salt induced oxidant stress in wild-type but not Lcn2 knockout mice suggesting that NGAL/Lcn2 regulates the redox milieu. Administration of exogenous Lcn2 to RBE4.1 rat brain endothelial cells have been shown to increase reactive oxygen species (ROS) levels as early as 3 hours following exposure due to increased intracellular iron, which is also not surprising given the well-known role of NGAL/Lcn2 in iron transport. In these studies, ROS were blocked by the addition of the NADPH oxidase inhibitor apocynin but the effects of the iron chelator deferiprone on ROS generation were not studied.12 This may be one plausible mechanism to explain an association between iron, oxidant stress, and fibrosis. In fact, in a murine high salt/uninephrectomy model placed on an iron-restricted diet, the decrease in dietary iron attenuated the development of hypertension and renal fibrosis compared to high salt/uninephrectomy-treated controls. Although the aforementioned study didn’t examine NGAL/Lcn2 it is likely that it played a role in in fibrosis.13 These observations indicate that iron homeostasis be considered a relevant factor in future studies examining aldosterone and cardiovascular injury.
Findings from this study implicate adventitial fibroblasts as the effector cell type following NGAL/Lcn2 stimulation; however, there may be a more complex interaction between vascular cells and other cell types with respect to NGAL/Lcn2 and vascular fibrosis. The adventitia is rich in T cells and exposure to deoxycorticosterone acetate/salt increases infiltration of T helper17+ cells into this space in animal models. Aldosterone activates T helper17+ cells to increase interleukin-17 (IL-17) secretion. IL-17, in turn, increases oxidant stress, expression of profibrotic mediators, and fibrosis. When deoxycorticosterone acetate/salt treated rats were administered an IL-17 antibody, these effects were abrogated.14 It is notable that IL-17 upregulates expression of NGAL/Lcn2 directly.15 These observations likely identify NGAL/Lcn2 as the mechanistic link between aldosterone, immune system activation, and fibrosis but do not tell us if these systems operate simultaneously or in sequence.
Taken together, the current study identifies NGAL/Lcn2 as an important component of mineralocorticoid-stimulated vascular fibrosis and represents another advance in our understanding of how aldosterone mediates cardiovascular fibrosis. The finding that NGAL/Lcn2, as well as posttranslationally modified forms of this protein can serve as a circulating biomarker is also relevant and may support a putative strategy for the early initiation of mineralocorticoid receptor blockade to limit fibrosis. Furthermore, more in-depth study of the binding partners of NGAL/Lcn2 may highlight other novel targets in this signaling pathway for future therapeutic intervention.
Sources of funding:
This work was supported by NIH/NHLBI R01 HL105301 and U01 HL125215.