(Circulation. 1996;94:2361-2363.)
© 1996 American Heart Association, Inc.
Articles |
de Modulatione Cordis
the Cardiovascular Division, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston Mass.
Correspondence to Ralph A. Kelly, Brigham and Women's Hospital, 75 Francis St, Boston MA 02115. E-mail rakelly@bics.bwh.harvard.edu.
Key Words: Editorials • genes • molecular biology • growth substances • hypertrophy • interleukins • signal transduction
 |
Introduction |
|---|
 Top Introduction References |
|---|
When I first gave my mind to vivisection, as a means of discovering the motions and uses of the heart, and sought to discover these from actual inspection, and not from the writings of others, I found the task so truly arduous, so full of difficulties, that I was almost tempted to think, with Fracastorius, that the motion of the heart was only to be comprehended by God.—William Harveyde Motu Cordis, 1628As quoted by E.H. Starling in his Linacre Lecture on the Law of the Heart, 1918
The gradual unfolding of our awareness of the Frank-Starling, adrenergic, and cholinergic mechanisms for beat-to-beat regulation of cardiac function from the late 19th century through the 1960s has given way to an explosive growth of knowledge regarding cellular signaling starting with the elucidation of the structure and function of G-protein–coupled receptors and their downstream effectors. To this rapidly evolving picture we can now add a large, complex, and fascinating array of cytokine-activated signaling pathways that mediate the changes in gene expression that underlie long-term adaptation of cardiac form and function.
Recent developments in cell and molecular biology yield a number of important insights into the cellular and subcellular signaling events that direct cardiac morphogenesis during development and myocardial growth and adaptation in the early postnatal and adult heart. These insights have been gained through unexpected, serendipitous observations and results generated by research focusing directly on the identification of previously unidentified cardiac growth-promoting agents. For example, the targeted disruption of the gene for neuregulins (ie, a neuregulin gene "knockout"), which codes for a family of proteins known to be important in neuronal and mesenchymal cell development, or targeted disruption of either of two neuregulin receptor genes (ie, erb B2 or erb B4), leads to death of the homozygous knockout animals in utero.1 2 3 4 This was due in part to the expected abnormalities in neural development, but also to failure of the developing heart to undergo trabeculation. It is now clear that expression of neuregulins in the developing endocardium and of their cognate receptors in the subjacent developing myocardium participate in a paracrine signaling pathway that is essential for trabeculation and thus normal development to occur.
Similarly, Yoshida and colleagues5 observed that targeted disruption of the gene coding for the 130-kD glycoprotein (gp)130, the ubiquitously expressed transmembrane signaling component of the interleukin (IL)-6 family of cytokines, was found to yield an embryonic lethal phenotype due both to abnormalities of hematopoiesis and, again unexpectedly, to hypoplastic growth of the developing myocardium. These data suggested that a gp130-dependent signaling pathway was essential for normal cardiac muscle development. Moreover, previous work from this laboratory demonstrated that doubly transgenic mice constitutively overexpressing both IL-6 and its (soluble) cognate receptor IL-6R (but not singly transgenic animals expressing either IL-6 or the IL-6R transgene alone) exhibited an interesting phenotype characterized by marked hypertrophy of the ventricular myocardium.6 These authors speculated that this hypertrophic growth was likely mediated by activation of gp130 (known to be expressed in cardiac myocytes) by the constitutively high levels of both IL-6 and IL-6R in the doubly transgenic animals. However, none of the autocrine/paracrine signaling peptides associated with the IL-6 family of cytokines, including leukemia inhibitory factor (LIF), oncostatin M (OM), IL-11, and ciliary neurotropic factor (CNTF), as well as IL-6 itself, all of which utilize gp130 to elicit downstream signaling events, was known to be abundantly expressed in postnatal myocardium.
The first clue linking the IL-6 cytokine family with myocardial growth and development was discovered by another group using an approach that focused on the identification of previously undescribed cardiotrophic peptides. After the observation was made that medium conditioned by totipotent embryonic stem cells secreted one or more soluble substances that supported the growth of heart muscle cells in vitro, Pennica and colleagues7 systematically examined the biological activity of many proteins expressed by these cells. They used an "expression cloning" approach, in which hypertrophy of neonatal rat ventricular myocytes in vitro was used as a bioassay for cardiotrophic activity. They identified a 21-kD protein they termed cardiotrophin-1 (CT-1), the mRNA for which could be identified in the embryonic stem cells and in developing and adult mouse myocardium, among other tissues. CT-1 elicited hypertrophic growth in these in vitro bioassays at low nanomolar concentrations, placing it among the most potent hypertrophic agents yet identified. Moreover, as Wollert et al8 (from the same collaborating groups) have since demonstrated, CT-1 appears to elicit a form of cardiac myocyte growth that is distinct from that caused by 
-adrenergic stimulation, resulting (at least in vitro) in sarcomeric units established in series rather than in parallel (ie, an increase in cell length rather than cell width), as well as a pattern of immediate early gene expression without known precedent.
Pennica et al,7 in their original report, made another important observation: The amino acid sequence of CT-1 bore some (
20%) amino acid identity to IL-6 and other members of the IL-6 cytokine family, and the tertiary structure of CT-1 could be deduced to exhibit the four amphipathic helices characteristic of the IL-6–related proteins. These authors speculated that CT-1, acting through gp130 and one of the IL-6 receptor (IL-6R) family members (or an as yet unidentified CT-1 specific receptor), could be playing an important if not essential role in embryonic and postnatal cardiac development. Subsequent reports by Pennica et al,9 10 Sheng et al,11 and Wollert et al8 support this interpretation. Indeed, the available data indicate that CT-1 binds to the leukemia inhibitory factor receptor ß subunit (LIFR-ß), promoting heterodimerization of LIFR-ß with gp130 and the phosphorylation of tyrosine residues of both transmembrane proteins. As expected from these results, LIF (but not other IL-6 family members) could substitute for CT-1 in inducing growth of neonatal rat ventricular myocytes in vitro, while peptide antagonists of LIFR-ß inhibited CT-1 activity in this bioassay. These data do not exclude the possibility that a second high-affinity receptor for CT-1 exists and/or that there is functional redundancy with other cytokines that signal through gp130.
In this issue of Circulation, Kunisada et al,12 adding to previous reports from their laboratory, now explore downstream signaling events mediated by LIF (and presumably CT-1 as well) and by gp130 in neonatal ventricular myocytes. They demonstrate that LIF-activated gp130 in turn induces activation of two mitogen-activated protein kinases (MAPKs): p44/p42 extracellular receptor activated kinases (ERK1/ERK2). This result is not surprising, given the large number of growth stimuli that can activate these MAPKs in cardiac myocytes, as in other cell types (see review by Force et al13 ). However, these authors also demonstrate concurrent activation of a separate intracellular signaling pathway mediated by Janus kinases (JAKs) and signal transducers and activators of transcription (STATs). Janus kinases are a family of cytosolic tyrosine kinases that contain two kinase domains (hence their appellation, after the two-faced Roman god) and are constitutively associated with cytosolic domains of a number of receptors of the cytokine superfamily (see reviews by Taniguchi14 and Ihle15 and references therein). Cytokine receptor activation (ie, heterodimerization of LIFR-ß and gp130 by LIF in the report by Kunisada et al12 ) results in JAK-mediated phosphorylation of the receptor at tyrosine residues and the subsequent recruitment of one or more STAT isoforms to the activated receptor complex. STATs are a family of related proteins that, once they themselves are phosphorylated on tyrosines by a JAK at the activated cytokine receptor complex, translocate as homo- or hetero-oligomers to the nucleus where site-specific DNA binding occurs.15 16 17 Activation of JAK-STAT signaling by cytokines has been recently demonstrated in cardiac myocytes; for example, IFN-
elicits membrane recruitment, phosphorylation, and nuclear translocation of STAT1- in adult rat ventricular myocytes18 as in many other cell types. As shown by Kunisada et al,12 LIF rapidly induced phosphorylation of STAT3 in isolated neonatal ventricular myocytes in vitro and in cardiac lysates prepared from mice injected with LIF in vivo. Interestingly, these authors do not comment on the possible activation of STAT1- by LIF. This seems not altogether unlikely, since gp130, activated by IL-6 cytokine family members, is known to recruit and activate this STAT isoform in many cell types.15 16 17 Indeed, a number of additional intracellular signaling pathways are likely initiated by LIF/CT-1 and other IL-6–related cytokines. A seventh mammalian STAT family member, for example, recently identified by Tsukada et al,19 termed "LPS- and IL-1–induced factor" ("LIL factor," or LIL-STAT), was found to be activated by IL-6, presumably also a gp130-mediated signaling event.
As has been the case for the unexpected abnormal phenotypes of the neuregulin and neuregulin receptor knockouts, targeted disruption of gp130 has provided important clues that have directed subsequent research into cardiovascular development. The observation that doubly transgenic animals expressing both IL-6 and IL-6R survive to adulthood but exhibit marked cardiac hypertrophy suggests that gp130-dependent signaling may remain an important adaptive mechanism in postnatal life. The discovery of CT-1 (and its signaling through LIFR-ß/gp130) has yielded an important additional insight. While it is risky to predict the phenotype of specific knockout animals, it is of interest to speculate whether mice with targeted disruption of CT-1 will also exhibit a lethal hypoplastic cardiac developmental abnormality similar to that of the gp130 knockout. If functional redundancy among IL-6 family cytokines members "rescues" normal cardiac development in CT-1 knockout mice, these animals may exhibit abnormal cardiac growth responses to physiological stress in adulthood. Systemic infusions of CT-1 do lead to cardiac hypertrophy in adult mice, although many other organs are affected as well-not an unexpected finding, given the widespread expression of LIFR and gp130.20
While we hardly need the mastery of a new vocabulary to fill our waking hours, the likelihood of growing clinical applications of these new insights promises to keep clinicians and clinical investigators as well as basic researchers scrambling to keep up. At the least the intricacy of the molecular ballet has aesthetic appeal.
|
Acknowledgments |
|---|
This work was supported by grants HL-36141 and HL-52320 from the National Institutes of Health. The authors thank Drs Kenneth Chien and Nicholas Paoni for providing access to manuscripts before their publication and Prof Christopher McDonough for linguistic advice.
|
Footnotes |
|---|
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
|
References |
|---|
|
TopIntroductionReferences |
|---|
1. Marchionni MA. neu tack on neuregulin. Nature. 1995;378:334-335.[Medline] [Order article via Infotrieve]
2. Meyer D, Birchmeier C. Multiple essential functions of neuregulin in development. Nature. 1995;378:386-390.[Medline] [Order article via Infotrieve]
3. Lee K-F, Simon H, Chen H, Bates B, Hung M-C, Hauser C. Requirement for neuregulin receptor erb B2 in neural and cardiac development. Nature. 1995;378:394-398.[Medline] [Order article via Infotrieve]
4. Gassmann M, Casagranda F, Orioli D, Simon H, Lal C, Klein R, Lemke G. Aberrant neural and cardiac development in mice lacking the erb B4 neuregulin receptor. Nature. 1995;378:390-394.[Medline] [Order article via Infotrieve]
5.
Yoshida K, Taga T, Saito M, Suematsu S, Kumanogoh A, Tanaka T, Fujiwara H, Hirata M, Yamagami T, Nakahata T, Hirabayashi T, Yoneda Y, Tanaka K, Wang W-Z, Mori C, Shiota K, Yoshida N, Kishimoto T. Target disruption of gp130, a common signal transducer for the interleukin 6 family of cytokines, leads to myocardial and hematological disorders. Proc Natl Acad Sci U S A. 1996;93:407-411.
6.
Hirota H, Yoshida K, Kishimoto T, Taga T. Continuous activation of gp130, a signal-transducing receptor component for interleukin 6-related cytokines, causes myocardial hypertrophy in mice. Proc Natl Acad Sci U S A. 1995;92:4862-4866.
7.
Pennica D, King KL, Shaw KJ, Luis E, Rullamas J, Luoh S-M, Darbonne WC, Knutzon DS, Yen R, Chien KR, Baker JB, Wood WI. Expression cloning of cardiotrophin 1, a cytokine that induces cardiac myocyte hypertrophy. Proc Natl Acad Sci U S A. 1995;92:1142-1146.
8.
Wollert KC, Taga T, Saito M, Narazaki M, Kishimoto T, Glembotski CC, Vernallis AB, Heath JK, Pennica D, Wood WI, Chien KR. Cardiotrophin-1 activates a distinct form of cardiac muscle cell hypertrophy: assembly of sarcomeric units in series VIA gp130/leukemia inhibitory factor receptor-dependent pathways. J Biol Chem. 1996;271:9535-9545.
9.
Pennica D, Shaw KJ, Swanson TA, Moore MW, Shelton DL, Zioncheck KA, Rosenthal A, Taga T, Paoni NF, Wood WI. Cardiotrophin-1: biological activities and binding to the leukemia inhibitory factor receptor/gp130 signaling complex. J Biol Chem. 1995;270:10915-10922.
10. Pennica D, Wood WI, Chien KR. Cardiotrophin-1: a multifunctional cytokine that signals via LIF receptor-gp130 dependent pathways. Cytokine Growth Factor Rev. In press.
11. Sheng Z, Pennica D, Wood WI, Chien KR. Cardiotrophin-1 displays early expression in the murine heart tube and promotes cardiac myocyte survival. Development. 1996;122:419-428.[Abstract]
12.
Kunisada K, Hirota H, Fujio Y, Matsui H, Tani Y, Yamauchi-Takihara K, Kishimoto T. Activation of JAK-STAT and MAP kinases by leukemia inhibitory factor through gp130 in cardiac myocytes. Circulation. 1996;94:2626-2632.
13.
Force T, Pombo CM, Avruch JA, Bonventre JV, Kyriakis JM. Stress-activated protein kinases in cardiovascular disease. Circ Res. 1996;78:947-953.
14.
Taniguchi T. Cytokine signaling through nonreceptor protein tyrosine kinases. Science. 1995;268:251-255.
15. Ihle JN. STATs: signal transducers and activators of transcription. Cell. 1996;84:331-334.[Medline] [Order article via Infotrieve]
16.
Gerhartz C, Heesel B, Sasse J, Hemmann U, Landgraf C, Schneider-Mergener J, Horn F, Heinrich PC, Graeve L. Differential activation of acute phase response factor/STAT3 and STAT1 via the cytoplasmic domain of the interleukin 6 signal transducer gp130, I: definition of a novel phosphotyrosine motif mediating STAT1 activation. J Biol Chem. 1996;271:12991-12998.
17.
Hemmann U, Gerhartz C, Heesel B, Sasse J, Kurapkat G, Grotzinger J, Wollmer A, Zhong Z, Darnell JE Jr, Graeve L, Heinrich PC, Horn F. Differential activation of acute phase response factor/STAT3 and STAT1 via the cytoplasmic domain of the interleukin 6 signal transducer gp130, II: Src homology SH2 domains define the specificity of STAT factor activation. J Biol Chem. 1996;271:12999-13007.
18.
Singh K, Balligand J-L, Fischer TA, Smith TW, Kelly RA. Regulation of cytokine-inducible nitric oxide synthase (NOS2) in cardiac myocytes and microvascular endothelial cells: role of ERK1/ERK2 (p44/p42) mitogen-activated protein kinases and Stat 1. J Biol Chem. 1996;271:1111-1117.
19. Tsukada J, Waterman WR, Koyama Y, Webb A, Auron PE. A novel STAT-like factor mediates lipopolysaccharide, interleukin 1 (IL-1), and IL-6 signaling and recognizes a gamma interferon activation site-like element in the IL1B gene. Mol Cell Biol. 1996;16:2183-2194.[Abstract]
20. Jin H, Yang R, Keller GA, Ryan A, Ko A, Finkle D, Swanson TA, Li W, Pennica D, Wood WI, Paoni NF. In vivo effects of cardiotrophin-1. Cytokines. In press.
This article has been cited by other articles:
 |
|
 |
|
  Q. Yang, W. Huang, C. Jozwik, Y. Lin, M. Glasman, H. Caohuy, M. Srivastava, D. Esposito, W. Gillette, J. Hartley, et al. Cardiac glycosides inhibit TNF-{alpha}/NF-{kappa}B signaling by blocking recruitment of TNF receptor-associated death domain to the TNF receptor PNAS, July 5, 2005; 102(27): 9631 - 9636. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. A. Fischer, S. Ludwig, E. Flory, S. Gambaryan, K. Singh, P. Finn, M. A. Pfeffer, R. A. Kelly, and J. M. Pfeffer Activation of Cardiac c-Jun NH2-Terminal Kinases and p38-Mitogen-Activated Protein Kinases With Abrupt Changes in Hemodynamic Load Hypertension, May 1, 2001; 37(5): 1222 - 1228. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. R. Baliga, D. R. Pimental, Y.-Y. Zhao, W. W. Simmons, M. A. Marchionni, D. B. Sawyer, and R. A. Kelly NRG-1-induced cardiomyocyte hypertrophy. Role of PI-3-kinase, p70S6K, and MEK-MAPK-RSK Am J Physiol Heart Circ Physiol, November 1, 1999; 277(5): H2026 - H2037. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||