Файл: Extracellular and Intracellular Signaling (книга).pdf

84.W. Parks, D. Frank, C. Hu , C. Renfrew Haft, J. Martin and X. Meng, J. Biol. Chem., 2001, 276, 19332.

85.M. Topham and S. Prescott, J. Biol. Chem., 1999, 274, 11447.

86.Z. Liu, G. Chang and S. Leibowitz, J. Biol. Chem., 2001, 276, 5900.

87.I. Trougakos and E. Gonos, Int. J. Biochem. Cell. Biol., 2002, 34, 1430.

88.T. Bajari, V. Strasser, J. Nimpf and W. Schneider, FASEB J., 2003, 17, 1505.

4.2 Leptin: A Brief Introduction

As noted above, among the adipokines primary attention has been directed towards leptin, a 16-kDa protein secreted primarily by adipocytes but also produced by many tissues including the heart.2–4 The production of cardiomyocyte-derived leptin is increased by both endothelin-1 and angiotensin II suggesting a paracrine or autocrine role of leptin in the regulation of cardiac functions, particularly under pathological conditions.3 Indeed, leptin mediates the prohypertrophic e ect of both endothelin-1 and angiotensin II in cultured neonatal rat ventricular myocytes.3 The primary cardiac response to leptin in terms of physiological function appears to be a negative inotropic response, which has been shown primarily in cardiomyocytes and which is mediated by endogenously produced nitric oxide.4

Circulating total leptin levels are generally positively related with body mass index and the degree of adiposity with plasma levels ranging from 5 to 15 ng/ml in non-obese individuals and greater than 100 ng/ml in very obese subjects.5 Interestingly, circulating leptin exists primarily in the free form in obesity whereas in lean individuals leptin circulates primarily bound to plasma proteins.5 This di erence may be of biological importance since it suggests that, in obesity, substantially greater amounts of leptin are available to exert biological e ects. The e ects of leptin occur through leptin binding to its receptors, termed OBR, LEPR or LR, although the OBR designation will be used in this review for consistency. OBRs are expressed as splice variants classified as short (OBRa, c, d and f), secreted (OBRe) and long (OBRb) forms with OBRb generally considered as the primary functional isoform linked to full cell signaling processes.6 These receptors are expressed abundantly in many di erent cells including cardiomyocytes and intact myocardium.2,3 The intracellular domain of OBRb belongs to the Janus kinase signal transduction and translation system (Jak2/STAT3). As will be discussed below, it has been reported that leptin leads to the activation of various kinases in cardiomyocytes including RhoA/ROCK, ERK1/2, p38 MAPK, phosphoinositide 3-kinase (PI 3-kinase), Akt and protein kinase C.

4.3Expression of Leptin Receptors in Cardiovascular Tissues

The first demonstration of the presence of OBR gene expression in cardiac tissue was reported in 1996 upon the discovery of the gene encoding the db/db mutation.7 Further characterization of OBR isoforms indicated that cardiac tissue expressed OBRa, OBRb and OBRe.8,9 Recent work from the authors’ laboratory suggest that OBR gene expression in the heart di ers in terms of regional distribution and is also a ected by gender.2 Semi-quantitative real-time polymerase chain reaction revealed that in both males and females all three isoforms investigated were expressed in both atria, left and right ventricular walls as the interventricular septum, although the greatest gene abundance was found in the atria. In terms of

gender di erences, OBR expression was generally higher in tissues from female rats, especially in the right atria.2

The functions of each of the OBR isoforms in the heart, the relevance of regional distribution expression patterns or the influence of gender are currently unclear, although some potential functions for leptin signaling in the heart will be discussed later in this review. The identification of OBRe in cardiac tissue was of particular interest since this soluble receptor represents the primary binding protein for leptin in plasma and may thus dictate leptin availability to tissues. It is possible that the presence of OBRe in cardiac tissues

is a consequence of proteolytic cleavage of the extracellular domains of one of the other isoforms.10,11 Although the function of OBRe in the heart is currently

unknown, it is interesting to speculate that its local tissue production serves to ‘‘fine tune’’ leptin concentrations in that specific tissue, which would be in keeping with its role as a clearance receptor, although evidence for this hypothesis needs to be obtained with further studies.

In addition to cardiac tissue, leptin receptors have also been identified in both cerebral and coronary vessels.12,13 With respect to the latter it was proposed that OBR-mediated leptin-induced vasodilatation occurs through a nitric- oxide-dependent process and which was abolished by hyperleptinemia. This finding emphasizes the potential dual role of leptin on vascular tissue, a direct NO-dependent vasodilatation and vasoconstriction occurring secondarily to central stimulation of the sympathetic nervous system.

4.4 E ect of Leptin on Cardiomyocyte Function

Under in vivo conditions, the cardiovascular actions of leptin can be predicted based on the central sympathetic stimulatory e ect of the polypeptide resulting in sympathetic nervous system-dependent e ects such as elevations in blood pressure and positive inotropic and chronotropic e ects. However, leptin can exert direct e ects on both the heart and blood vessels through OBR-dependent cell signaling mechanisms. In isolated ventricular myocytes leptin produces a negative inotropic e ect via a NO-dependent pathway as the e ect was abrogated by NO synthase inhibition with L-NAME and associated with increased

NO synthase activity.4 The negative inotropism is also associated with both JAK-STAT and MAP kinase p38 activation.8,14 Leptin has also been shown to

stimulate fatty acid oxidation in working perfused rat hearts in the absence of any e ect on glucose oxidation while lowering cardiac triglyceride content.15

4.5 Cardiomyocyte Hypertrophic E ects of Leptin

Evidence for leptin as a hypertrophic and pro-growth factor stems primarily from studies examining the direct e ect of the polypeptide on myocyte preparations. For example, our laboratory reported that leptin produces marked hypertrophy in cultured neonatal rat ventricular myocytes as manifested by increased cells size, elevated protein synthesis and upregulation of a number of

hypertrophic e ects of either endothelin-1 or angiotensin II were associated with increased OBR expression and release of leptin into the culture medium. Moreover, anti-OBR antibodies completely abrogated the hypertrophic responses to both endothelin-1 and angiotensin II.3 These results need to be confirmed in other models but if validated they suggest that leptin plays a critical paracrine or autocrine obligatory role in mediating the hypertrophic to both endothelin-1 and angiotensin II and possibly other pro-hypertrophic factors.

Leptin has been shown to increase hyperplasia of the murine atrial HL-1 cell line as well as pediatric cardiomyocytes.19 Activation of ERK and phosphatidylinositol 3-kinase was demonstrated and implicated in the increase in cell number. It should be noted that leptin-induced hypertrophy has also been shown in human pediatric ventricular myocytes, which was associated with increased ERK, p38 and JAK phosphorylation.20

As will be discussed later in this chapter, activation of the RhoA/ROCK pathway likely plays a unique and critical role in mediating the cardiomyocyte hypertrophic e ect of leptin.

4.6 Post Receptor Leptin Signaling

In general, the complexity and diversity of leptin’s e ects are exemplified by its ability to activate several signal transduction pathways. It is beyond the scope of this review to comprehensively discuss leptin-mediated signaling in all tissues or organ systems. Instead, the discussion below focuses on signaling pathways that have been elucidated in cardiovascular tissue or that appear to be particularly relevant for understanding leptin-mediated cardiac signaling and its possible relationship to pathology, particularly in view of harnessing these pathways for cardiac therapeutics. For a more general treatise of this subject interested readers can consult various recent publications.21–24 In terms of cardiac pathology it is important to consider that the status of the leptin cell signaling system is likely substantially altered in disease states. For example, the leptin system in the myocardium including leptin expression and expression of OBR is upregulated in heart failure.25 Accordingly, the cell-signaling processes described below are most likely in a state of enhanced activity under pathological conditions.

4.6.1JAK-STAT Pathway Activation

It is generally accepted that OBRb is the fully competent signal transduction isoform of the receptor, and that the short-form OBRs (a,c,d,f), while capable of signal transduction, do so to a lesser extent.26,27 The major signaling pathway activated by leptin binding to OBRb is the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway,26 although this pathway may also be stimulated secondary to activation of the short form of the OBR.28 Upon binding of leptin to its receptor JAK1 and JAK2 are both capable of associating with the cytoplasmic domain of OBRb; however,

recently it has been demonstrated that JAK2 activation likely represents the physiologically relevant activated JAK during OBR signaling.29 Activation of JAKs results in transphosphorylation of other JAK as well as phosphorylation of tyrosine residues of OBRb.30 Recently, protein tyrosine phosphatase 1B

(PTP1B) has been shown to be a negative regulator of JAK-STAT signaling.31,32 PTP1B dephosphorylates the consensus recognition motif on JAK2,

resulting in inactivation of downstream STAT proteins.33 PTP1B knockout mice exhibit increased leptin sensitivity, STAT3 activation and decreased leptin to body-weight ratios.33 Phosphorylation of the cytoplasmic domain of the receptor

results in a docking site for STAT protein binding. STAT1, STAT3, STAT5 and STAT 6 have all been associated with leptin signaling in vitro.34–36 Upon binding

the receptor complex, STAT is phosphorylated by JAK where it dissociates

from the receptor, forms a homoor heterodimer and then translocates to the nucleus to act as a transcription factor.34,35,37 STAT3 can be inhibited by PIAS3,

an endogenous protein inhibitor of this transcriptional factor.38

Evidence for JAK-STAT-dependent signaling in cardiac tissue in terms of physiological e ects is at present limited but recent evidence suggests that it may be involved in the negative inotropic e ect of leptin in cardiomyocytes based on the ability of the JAK2 inhibitor AG-490 to abrogate these e ects.8 Interestingly, however, the e ect of AG-490 was mimicked by the MAPkinase inhibitor SB203580 suggesting that leptin exerts its e ects via multiple, and likely independent, cell signaling pathways.8

4.6.2Mitogen Activated Protein Kinase Stimulation

Mitogen-activated protein kinase (MAPK) represents an additional target for leptin-mediated e ects. In fact, OBRa has signal transduction capabilities through

MAPK pathways both dependently and independently of JAK phosphorylation.12,26,30 JAK2 phosphorylation of OBR tyrosine residue-985 (Tyr985) results

in docking of an SH2-domain containing protein tyrosine phosphatase (SHP-2), which associates with an adaptor molecule, Grb-2, to activate extracellular regulated kinase (ERK) signaling.30 Although ERK activation is possible in the absence of Tyr985, it still requires SHP-2 phosphatase activity.30 SHP-2 activation by leptin-OBR interaction leads to ERK activation, possibly through MEK1, but this has not yet been confirmed.22 Activation of ERK results in alterations in gene expression patterns for several genes including c-fos.12 Another MAPK, p38, has not been studied as extensively as ERK, but has been shown to be activated by leptin in mononuclear cells.39 In contrast, leptin was shown to reduce insulininduced p38 activation, while having no e ect on p38 activation on its own.23 The role of leptin signaling through c-jun NH2-terminal protein kinase (JNK) has not been well characterized. However, there are two reports of leptin activating JNK in endothelial cells,40 and in prostate cancer cells.41

In the cardiovascular system, leptin has been demonstrated to activate components of the MAPK pathways. In cultured neonatal myocytes, ERK1/2 and p38, but not JNK, were activated by leptin; inhibiting ERK had no e ect, while inhibition of p38 completely inhibited leptin-induced cardiomyocyte