Both experimental studies and epidemiology point to an emerging recognition of inflammatory cytokines as biomarkers of cardiovascular disease.24
However, the relative importance of specific cytokines in the regulation of arterial pressure and in the pathogenesis of hypertension has yet to be fully elucidated. Increased levels of circulating IL-6 in patients correlate with the severity of heart failure and are predictive of mortality. 6, 7
A similar relationship has also been identified between IL-6 and hypertension,8, 9
with increases in mean arterial pressure in normal healthy men being significantly associated with elevated levels of circulating IL-6.10
However, the exact contribution of IL-6 to the pathogenesis of hypertension is unclear due to the variable levels of IL-6 reported in experimental models of hypertension.1, 25-27
Nevertheless, the data presented herein clearly demonstrate the ability of IL-6 to induce a pattern of myocardial remodeling consistent with that occurring in the hypertensive heart, including concentric hypertrophy, fibrosis and diastolic dysfunction. Further, we have identified a critical role for the sIL-6R in the collagen synthesis induced by IL-6.
Our findings are the first to establish that pathologic elevations in circulating IL-6 result in extensive cardiac fibrosis. The IL-6 family of cytokines has the capacity to regulate cell function through a cell surface receptor composed of two trans-membrane proteins,28
a ligand-binding subunit designated as the IL-6 receptor and a signal transducing glycoprotein (gp130). 29, 30
The presence of the IL-6 receptor has previously been demonstrated in adult cardiac fibroblasts, where it was reported to be essential for fibroblast growth.31
. Furthermore, Siwik et al.14
found that incubation of adult rat cardiac fibroblasts with IL-6 reduced collagen content in the media by 11%. In addition, they also reported that matrix metalloproteinase activity was significantly increased after incubation of neonatal cardiac fibroblasts with IL-6. Somewhat consistent with their observation, we found no effect of IL-6 alone on collagen content in the media of isolated adult cardiac fibroblasts. However, addition of the sIL-6R, which was not evaluated in the studies by Siwik et al., elicited concentration dependent increases in collagen content. Incubation of fibroblasts with IL-6 in the presence of the sIL-6R produced a 4-fold increase in collagen content at the highest concentration of the sIL-6R. Moreover, this combination also induced a conversion in fibroblast phenotype to that of a myofibroblast, indicating the ability of the sIL-6R to regulate fibroblast function by several mechanisms. The sIL-6R is naturally occurring in the body and is the result of proteolysis of the membrane receptor or alternative mRNA splicing. 32, 33
While most soluble receptors act as antagonists in the sense that they compete with the corresponding membrane-bound receptor for the specific ligand, this is not the case with the sIL-6R, which instead acts as an agonist to activate signal transduction on cells that are not stimulated by IL-6 alone. 32
The IL-6/sIL-6 R does this by interacting with membrane-bound gp130, 33
which in turn leads to phosphorylation of downstream second messengers such as JAK and STAT and stimulation of various cellular events.
In addition to cardiac fibrosis, we also found that IL-6 induced significant concentric LV hypertrophy. This finding is consistent with the previous reports by Hirota et al.12
who described a pattern of concentric hypertrophy in the hearts of mice over-expressing both IL-6 and the IL-6 receptor. However, our study extends these findings to demonstrate that individual cardiomyocytes underwent both elongation and thickening in response to IL-6 infusion. The fact that the ratio of cell length to width was similar to normal cardiomyocytes indicates this hypertrophy consisted of proportional growth. This is in agreement with the previous report by Korecky and Rakusan,34
who determined that cell length and width increase proportionally in concentric cardiac hypertrophy. Several studies have implicated cardiotrophin-1/gp130 induced phosphorylation of STAT3 as mediating myocardial hypertrophy, 35-37
and Wollert et al.,38
reported that cardiotrophin-1 stimulation in isolated neonatal cardiomyocytes induces hypertrophy consisting of in-series addition of sarcomeric units. While this may reflect intrinsic differences between adult and neonatal cardiomyocytes, or a differential response of cells to cardiotrophin-1 and IL-6, it seems likely that differential regulation of hypertrophy is more complex, involving induction of other in vivo
pathways like the renin-angiotensin system. This is reflected in the findings of López et al.,37
who recently reported that in-series sarcomeric addition was induced by cardiotrophin-1 in adult cardiomyocytes isolated from normotensive Wistar rats, while a concentric hypertrophic response was observed in cardiomyocytes obtained from hypertensive SHR. Interestingly, cardiac fibroblasts stimulated with angiotensin II have been shown to secrete members of the IL-6 family, including IL-6 itself, which induced cardiomyocyte hypertrophy via activation of the gp130 receptor.39
In view of the possibility that increased LV stiffness could lead to higher left atrial and pulmonary pressures, it is interesting to note that RV hypertrophy was absent following IL-6 infusion. However, although LV stiffness was increased in the IL-6 infused animals, we do not know the actual LV pressures. Given that this was a one week infusion of IL-6 it may be that the pressures were not elevated long enough to result in RV hypertrophy in that time period. Furthermore, there is evidence from exercise studies in heart failure patients that pulmonary wedge pressure (a marker of LV pressure) is not coupled to right atrial pressure until a point of pericardial constraint is reached. 40
Thus, in the IL-6 infused animals, a large enough degree of LV hypertrophy and/or ventricular stiffness may not have been attained to influence the RV workload.
The IL-6 induced changes in myocardial structure were manifested functionally as a stiffer (decreased ΔV0-25
), smaller ventricle (decreased V0
). The increased stiffness was most likely the result of the marked myocardial fibrosis, since it has been shown that myocyte hypertrophy does not intrinsically alter LV stiffness.41
Together these events replicate the concentric cardiac remodeling typical of pressure overload, where there is an increase in LV mass concomitant with decreased LV chamber size and significant fibrosis. Consequently, these findings provide additional evidence suggesting IL-6 may contribute to the development of diastolic dysfunction in hypertensive patients, resulting in the eventual transition to heart failure.
The increased levels of myocardial IL-6 confirm the effectiveness of the IL-6 infusion. However, IL-6 did not induce an increase in blood pressure, nor was there an inflammatory cell response in the myocardium, indicating that: 1) IL-6 release likely occurs downstream of inflammatory cell infiltration; and 2) fibrosis and hypertrophy can be independent of blood pressure. While increases in TNF-α are known to induce subsequent increases in IL-6,42
the sustained increase in IL-6 did not induce a corresponding induction of myocardial TNF-α levels.
This is the first report demonstrating the ability of IL-6 to induce a pattern of myocardial remodeling that includes cardiac fibrosis and concentric hypertrophy. Overall, the remodeling induced by infusion of IL-6 was remarkably similar to that seen in hypertension and suggests that IL-6 may be responsible for inducing adverse remodeling in the hypertensive heart, thereby contributing to LV diastolic dysfunction. Furthermore, this study establishes the necessity of the sIL-6R for IL-6 to increase collagen content by cardiac fibroblasts. Accordingly, future investigation is warranted to evaluate IL-6 as a potential therapeutic target.