A cytokine axis regulates elastin formation and degradation

1. Introduction

Elastogenesis is the process by which elastin-containing fibers and lamellae are formed. The process is subject to dynamic regulation during the development of tissues such as the lung and aorta where elastin biosynthesis is active in late stages of development and in the early postnatal period but is subsequently attenuated (Pierce et al., 1995; Bruce and Honaker, 1998; Kozel et al., 2011). Steady state, basal levels of elastin expression and breakdown remain low in adult tissues as well as in cells isolated from adult tissues (Starcher, 1986; Parks et al., 1988). Elastin expression is reactivated in diseases such as pulmonary hypertension, dermal elastosis, pseudoxanthoma elasticum, Buschke–Ollendorff syndrome, Moyamoya disease, cigarette smoke-induced emphysema, severe chronic obstructive pulmonary disease (COPD) and forms of progeria (Holbrook and Byers, 1982; Sephel et al., 1988; Botney et al., 1992; Bernstein et al., 1994; Stenmark et al., 1994; Yamamoto et al., 1997; Hoff et al., 1999; Deslee et al., 2009; Rangasamy et al., 2009), although in many of these disorders the elastin produced is disorganized and dysfunctional (Kuhn et al., 1976; Fukuda et al., 1989; Hoff et al., 1999).

Much is known about the molecular mechanisms underlying elastin formation in development and disease. This understanding has been advanced through studies of the many human genetic disorders and pathologies of elastin-rich tissues e.g., Costello syndrome (Vila Torres et al., 1994), cutis laxa syndrome (Morava et al., 2009) and Ehlers–Danlos syndrome (Zweers et al., 2004). Studies of mouse models have contributed greatly to our understanding of the genes involved in developmental elastin formation (Dietz and Mecham, 2000; Wagenseil and Mecham, 2007; Yanagisawa and Davis, 2010). In addition, numerous positive and negative effectors of tropoelastin expression and elastin formation have been identified through in vitro experimentation (Tables 1 and ​and2).2). Emerging from studies of elastin biology/pathobiology is a cytokine regulatory axis comprised of pro- and anti-elastogenic cytokines. In the present review we focus on highlighting salient components of this regulatory axis and their mechanisms of action on elastin biosynthesis and degradation.

2. Cytokines that promote elastin formation

2.1. Transforming growth factor β-1 (TGFβ1)

TGFβ1 is a member of the TGFβ superfamily of cytokines. TGFβ activation is normally kept under tight negative control through the process of extracellular matrix sequestration/latency (Annes et al., 2003). Increased TGFβ1 signaling drives pathogenesis of multiple diseases (e.g., fibrotic diseases of the liver, kidney, lung and skin), most of which impact elastin formation and breakdown. For example, TGF-β1 overexpression results in severe interstitial and pleural fibrosis associated with increased deposition of elastin (Sime et al., 1997). Overexpression of active TGFβ1 in experimental abdominal aortic aneurysms is associated with preservation of medial elastin (Dai et al., 2005). Indeed, numerous studies have established that TGFβ1 augments both tropoelastin mRNA abundance (Liu and Davidson, 1988; McGowan and McNamer, 1990; Kahari et al., 1992b; Zhang et al., 1995) and elastin formation (McGowan and McNamer, 1990; Katsuta et al., 2008). Mechanistically, TGFβ1 mediates these effects by modulating tropoelastin promoter activity, mRNA stability and elastin degradation.

A number of studies demonstrate that TGFβ1 exerts positive effects on tropoelastin transcription. In human embryonic lung fibroblasts, TGFβ1 activates tropoelastin transcription via the phosphatidylinositol 3-kinase/Akt/p38 signaling pathway (Kuang et al., 2007). In transgenic mice expressing a chloramphenicol acetyl-transferase (CAT) reporter gene under control of the human elastin promoter, CAT activity is greatly elevated in the skin of TGFβ1 treated animals (Katchman et al., 1994). Furthermore, in chick embryo aorta cells transfected with an elastin promoter-CAT construct, CAT activity is increased by addition of TGFβ1 (Marigo et al., 1993). By contrast, elastin promoter/CAT reporter studies performed in skin fibroblasts showed that TGFβ1 did not change the promoter activity (Kahari et al., 1992b). These and other findings indicate that the effects of TGFβ1 on elastin transcription are cell-type specific. Consistent with this conclusion are findings of McGowan (McGowan, 1992) showing that TGFβ1 stimulates elastin formation in neonatal lung fibroblasts but not in adult lung fibroblasts or adult lung smooth muscle cells. Furthermore, embryonic aorta cells and tendon fibroblasts also display differential responsiveness to TGFβ1 (Marigo et al., 1993). These differential effects have been attributed to cell type-specific nuclear transcription factors that bind to a TGFβ1-responsive element located in the −196 to −12 region of the elastin promoter (Marigo et al., 1993, 1994).

TGFβ1 also acts post-transcriptionally to stabilize tropoelastin mRNA transcripts (Kahari et al., 1992b; Kucich et al., 1997). Indeed, TGFβ1 can relieve tropoelastin mRNA instability in cutis laxa fibroblasts in which tropoelastin mRNA is highly unstable (Zhang et al., 1995). Mechanistically, this involves a regulatory GA-rich sequence located in the 3′ UTR of the tropoelastin transcript referred to as the G3A site (Zhang et al., 1999; Hew et al., 2000). Individuals of several families with inherited cutis laxa have mutations in the tropoelastin gene located in the vicinity of the coding sequencing for this regulatory element (i.e., located near the 5′ end of exon 30) (Zhang et al., 1999). Elements related to the elastin GA-rich sequence are enriched in stable RNAs of other genes and mediate binding of mRNA stability factors, including CUGBP1 (Lee et al., 2010). Whether CUGBP1 or another such protein binds to the G3A site and influences decay of the tropoelastin mRNA and whether TGFβ1 influences binding to the site remain to be established.

The tropoelastin mRNA stabilization effects of TGFβ1 are mediated through several TGFβ signaling pathways including the Smad signaling pathway, the phosphatidylcholine (PC)-specific phospholipase C (PLC)-protein kinase C (PKC)-delta pathway and the TGFβ-activated kinase (TAK1)-stress-activated protein kinase p38 pathway (Kucich et al., 1997, 2002). This is supported by evidence showing that TGFβ-stimulated tropoelastin mRNA accumulation can be blocked by inhibitors of PLC, PKC and p38 as well as by transgenic expression of the inhibitory Smad, Smad7 (Kucich et al., 2002).

TGFβ1 may also stabilize elastin mRNA by reducing the expression of microRNAs. Recent studies show that expression of the microRNA, miR-29, is reduced by TGFβ1 (van Rooij et al., 2008) and that the 3′ UTR of elastin mRNA is a target of miR-29 (Boon et al., 2011; Ott et al., 2011). Furthermore, miR-29 mimics decrease elastin mRNA levels in dermal fibroblasts and vascular smooth muscle cells (Zhang et al., 2012b). Moreover, in the developing mouse aorta, an up regulation in the expression of miR-29 as well as several other microRNAs that have targets in the elastin mRNA (i.e., the miR-15 family members miR-195 and miR-497) accompanies the down regulation of elastin mRNA in the period between the newborn and adult (Boon et al., 2011; Ott et al., 2011).

The ability of TGFβ to augment elastin expression also relates to hyaluronan signaling. Studies have shown that TGFβ1 and hyaluronan oligomers (consisting of 3 to 9 glucuronate and N-acetylglucosamine disaccharides) act synergistically to enhance elastin levels in the extracellular matrix of cultured vascular smooth muscle cells (Joddar and Ramamurthi, 2006). Intriguingly, TGFβ treatment of vascular smooth muscle cells from experimentally induced aortic aneurysms does not elicit a change in elastin synthesis, but treatment with a combination of TGFβ and hyaluronan oligomers enhances elastin protein levels in the extracellular matrix (Kothapalli et al., 2009). Hyaluronan oligomers interact with the major receptor for hyaluronan, CD44, and it is thought that hyaluronan oligomers disrupt endogenous hyaluronan interactions with CD44 (Toole, 2009). Since CD44 has direct and indirect interactions with other signaling receptors, including TGFβ receptor type I and epidermal growth factor receptor (EGFR), distruption of hyaluronan-CD44 binding can influence the activity of a variety of downstream signaling pathways. The CD44-EGFR interaction may be of particular relevance to the pro-elastin effects of hyaluronan oligomers since EGFR signaling appears to be anti-elastogenic (DiCamillo et al., 2006). Thus, hyaluronan oligomers may interfere with suppressive effects of EGFR signaling on elastin expression. Studies describing EGFR signaling effects on elastin are discussed below.

TGFβ1 also limits elastin degradation by decreasing levels and activity of elastolytic proteases including matrix metalloproteinase (MMP)-2 and -9 (Dai et al., 2005). Indeed, TGFβ blockade exacerbates elastin degradation and decreases levels of elastin in medial layers of blood vessels (Alvira et al., 2011). By contrast, overexpression of a mutated, active form of TGFβ1 in animals having experimentally induced aortic aneurysms leads to reduction in the expression of elastolytic MMPs and preservation of elastic fibers in the medial layers of injured aortas (Dai et al., 2005). TGFβ1 also elicits effects on MMP activity by augmenting expression of tissue inhibitors of MMPs (TIMPs) (Akool el et al., 2005). Similarly, TGFβ1 sustains levels of the plasmin inhibitor, PAI-1, which prevents activation of the elastolytic protease MMP-9 (Kutz et al., 2006; Alvira et al., 2011).

2.2. Insulin-like growth factor-I (IGF-I)

IGF-1 has been implicated as a positive regulator of elastogenesis occurring in the developing aorta (Foster et al., 1989; Rich et al., 1993) and in pathologies including atherosclerosis (Conn et al., 1996). Like TGFβ, IGF-I elicits positive effects on elastin expression (Badesch et al., 1989; Rich et al., 1992; Wolfe et al., 1993). Also similar to TGFβ, the effects of IGF-I on elastogenesis appear to be cell type specific. For example, IGF-I increases steady state levels of tropoelastin mRNA and soluble elastin production in neonatal aortic smooth muscle cells, but it elicits no change in tropoelastin expression in pulmonary fibroblasts, which also express the type I IGF-I receptor (Rich et al., 1992).

IGF-I regulation of elastin expression involves the interaction of two members of the Sp-family of transcription factors, Sp1 and Sp3, with elements of the elastin promoter (Wolfe et al., 1993; Jensen et al., 1995; Conn et al., 1996; Sen et al., 2011). These transcription factors have opposing effects on elastin transcription such that Sp1 promotes elastin transcription whereas Sp3 represses Sp1-mediated enhancement of elastin transcription. IGF-I acts to both promote Sp1 binding to the elastin promoter and abrogate Sp3 interaction with the elastin promoter (Conn et al., 1996).

Key to the process by which IGF-I mediates Sp1 interaction with the elastin promoter is the formation of a complex between Sp1 and phosphorylated retinoblastoma protein (Rb) (Wolfe et al., 1993; Jensen et al., 1995; Conn et al., 1996; Sen et al., 2011). IGF-I acts to promote the formation of a cyclinE–cyclin-dependent kinase 2 (cdk2) complex, which mediates cdk2-dependent phosphorylation of Rb on threonine-821 (Thr821) (Sen et al., 2011). Phosphorylation of Rb(Thr821) recruits Sp1 to Rb. The resulting complex binds to a retinoblastoma control element within the elastin gene promoter and stimulates tropoelastin transcription (Sen et al., 2011).

Intriguingly, the IGF-I-induced effects on Rb phosphorylation and elastin expression are counter balanced by pro-proliferative stimuli transmitted through the mitogenic Ras/MEK/ERK pathway. For example, dermal fibroblasts treated with the mitogen, PDGF-BB, display decreased levels of phosphorylated Rb(Thr821) and reduced elastin deposition (Sen et al., 2011). PDGF-BB also increases levels of phosphorylated Rb(Ser780), which is elevated in Costello syndrome fibroblasts that display H-Ras driven proliferation and decreased elastogenesis. Quenching of the increased proliferation of Costello syndrome fibroblasts by treatment with inhibitors of Ras, MEK or cdk4 induced up-regulation of Rb(Thr821) phosphorylation, decreased Rb(Ser780) phosphorylation and recovery of their elastogenesis deficiency (Sen et al., 2011). These findings fit with observations indicating that an inverse relationship exists between elastin deposition and cell proliferation (Urban et al., 2002).

3. Cytokines that inhibit elastin formation

3.1. Basic fibroblast growth factor (bFGF)

bFGF (also known as FGF2) is a major negative regulator of elastogenesis. bFGF decreases elastin gene transcription in aortic smooth muscle cells (Carreras et al., 2002) and pulmonary fibroblasts (Rich et al., 1996). Furthermore, in mice bearing compound deficiency of the FGF receptors, 3 and 4, elastin deposition is not downregulated postnatally in the lungs (Weinstein et al., 1998). While these findings highlight that FGF signaling plays a role in attenuating developmental elastogenesis in the lung, it is unclear whether this involves bFGF or other FGFs. bFGF is the candidate elastin suppressor in the periodontal ligament (PDL), in which elastin expression is normally negligible. bFGF is expressed in the PDL (Gao et al., 1996) and has been shown to suppress transcription of tropoelastin in cultured PDL cells (Palmon et al., 2001). Interestingly, PDL cells grown in culture express tropoelastin mRNA (Palmon et al., 2001), suggesting that the suppressive mechanism operative in the PDL in vivo is released when the cells are placed in culture.

bFGF represses elastin gene transcription by augmenting expression of the transcription factor Fra-1 (also known as fos-like antigen 1), that subsequently binds to a sequence located at −564 to −558-bp in the elastin promoter, heterodimerizing with c-Jun to form an inhibitory complex (Rich et al., 1999). bFGF-dependent repression of tropoelastin mRNA expression can be blocked by inhibition of MEK1, indicating that the MEK/ERK pathway mediates the response of the growth factor (Carreras et al., 2001).

The potent suppressive effects of bFGF may underlie attenuated/defective elastogenesis observed in wound healing processes in which bFGF levels are increased, such as in dermal, cerebral, hepatic, renal and tendon wound healing (Takamiya et al., 2003). bFGF expressed in these in vivo settings or even in cultured cells may oppose the action of positive effectors of elastogenesis. Indeed, bFGF has been shown to inhibit the ability of TGFβ1 to induce elastin expression by vascular smooth muscle cells (Davidson et al., 1993). The suppressive effects of bFGF on elastogenesis would appear to be an unfavorable consequence to therapeutic use of bFGF in wound healing, if elastin synthesis is a desired endpoint.

bFGF has also been shown to decrease the expression of mRNA encoding lysyl oxidase (Hong and Trackman, 2002), the enzyme that catalyzes tropoelastin crosslinking. Interestingly, lysyl oxidase inactivates bFGF through promoting covalent crosslinking of bFGF monomers to form multimers (Li et al., 2003). It is not known whether lysyl oxidase might cross link bFGF to elastin, but bFGF is released by elastase treatment of cultures of pulmonary fibroblasts and pulmonary artery smooth muscle cell (Rich et al., 1996; Thompson and Rabinovitch, 1996), suggesting that the growth factor may be bound to elastin. Release of extracellular matrix-bound bFGF has also been reported to occur in response to burn injury of skin, another tissue rich in elastin (Gibran et al., 1994). Elastase release of bFGF could contribute to impaired elastin synthesis in pulmonary obstructive diseases in which elastase degradation of elastin is operative.

3.2. Heparin-binding epidermal growth factor-like factor (HB-EGF)

HB-EGF is an EGF receptor (EGFR/ErbB1) ligand that reduces tropoelastin mRNA expression and matrix accumulation of elastin protein (Liu et al., 2003; Bertram and Hass, 2009). HB-EGF mediates these effects through a mechanism that, like bFGF, involves stimulation of ERK1/2 phosphorylation and nuclear accumulation of Fra-1 (Liu et al., 2003). This is supported by findings showing that siRNA suppression of HB-EGF expression leads to increased levels of elastin protein and decreased levels of Fra-1 protein (Bertram and Hass, 2009). In addition, other findings show that inhibition of ERK1/2 activation using MEK1/2 inhibitors reduces both HB-EGF-induced Fra-1 accumulation and HB-EGF-downregulation of tropoelastin mRNA expression (Liu et al., 2003). HB-EGF also induces the expression of bFGF, which could amplify the suppressive effects of HB-EGF on elastin expression (Liu et al., 2003).

3.3. Epidermal growth factor-like growth factor (EGF)

EGF isa cytokine produced in response to injury (Moulin, 1995; Van Winkle et al., 1997). EGF and TGFα-like cytokines are released by neonatal rat lung fibroblasts treated with neutrophil elastase, a protease associated with excessive elastolysis in emphysema (DiCamillo et al., 2002). Similar to the action of HB-EGF, EGF causes decreases in tropoelastin mRNA stability and insoluble elastin deposition (Ichiro et al., 1990; DiCamillo et al., 2006). Tropoelastin mRNA destabilization by EGF can be blocked by the EGFR inhibitor, AG1478, and the MEK/ERK inhibitor, U0126 (DiCamillo et al., 2006). Dicamillo et al. (2006) concluded that the EGFR/MEK/ERK signaling cascade acts in opposition to TGFβ1 signaling to override the positive effects of TGFβ1 on tropoelastin mRNA stability.

3.4. Transforming growth factor-α (TGFα)

TGFα is an EGFR ligand that is expressed by airway epithelial and interstitial cells following lung injury (Madtes et al., 1994; Strandjord et al., 1995; Van Winkle et al., 1997), including the pathogenesis of cystic fibrosis lung disease (Hardie et al., 1999). TGFα is thus implicated in the elastolysis and defective elastin repair associated with pathological remodeling of the lung. Indeed, overexpression of TGFα in the neonatal lung leads to disorganized and fragmented elastin fibers in the alveolar septae (LeCras et al., 2004). Like EGF, TGFα is released by lung fibroblasts in response to neutrophil elastase treatment, causing tropoelastin mRNA destabilization via activation of the EGFR/MEK/ERK signaling cascade (Kohri et al., 2002; DiCamillo et al., 2006).

3.5. Tumor necrosis factor-alpha (TNF-α)

TNF-α, a proinflammatory cytokine, suppresses tropoelastin mRNA levels in skin fibroblasts, aortic smooth muscle cells and lung fibroblasts (Kahari et al., 1992a; DiCamillo et al., 2002). Conversely, TNF-α knockout mice display increased vascular elastin expression (Carneiro et al., 2009), which may account for findings that TNF-α-deficient mice are resistant to aneurysm formation (Xiong et al., 2009).

In cultured aortic smooth muscle cells, the inhibitory effects of TNF-α on elastin gene expression involve the transcription factor AP-1 binding to a region in the elastin promoter located between −290 and −198 bases pairs (Kahari et al., 1992a). AP-1 complexes are heterodimers of Jun and Fos family members, each activated via the MAP kinase signaling pathway. As mentioned above, bFGF exerts its repressive effects on elastin transcription via the Fos family member Fra-1 (Rich et al., 1999), downstream of MEK1/ERK1/2 pathway signaling (Carreras et al., 2001). It is therefore possible that TNF-α also downregulates tropoelastin transcription via MAP kinase pathway effects on AP-1.

Another dimension to the relationship between TNF-α and elastin is that TNF-α promotes elastin breakdown through enhanced release of elastolytic enzymes (She et al., 1993; Fujita et al., 2001; Lau et al., 2008). For example, TNF-α induces production of the elastolytic MMPs, MMP-2 and MMP-9, by vascular smooth muscle cells (Kothapalli and Ramamurthi, 2010).

Like EGF, TNF-α is released by lung fibroblasts in response to neutrophil elastase treatment and has been shown to abrogate TGFβ-induced enhancement of tropoelastin mRNA expression (Kahari et al., 1992b; DiCamillo et al., 2002). Enhanced TNF-α expression also occurs in response to cigarette smoke exposure and its increase correlates with pulmonary elastin breakdown, a precursor to emphysema (Churg et al., 2002). TNF-α induced elastin degradation is also linked to vasculitis stemming from Kawasaki disease as well as vascular aneurysms (Hui-Yuen et al., 2006). In fact, in Lactobacillus casei cell wall extract-induced coronary arteritis, abrogation of TNF-α activity by TNF-α-blocking agents or deletion of the TNF receptor-1 protects against elastin degradation and aneurysm formation (Hui-Yuen et al., 2006). Furthermore, doxycycline, which inhibits MMP-9 enzymatic activity, can mitigate TNF-α-induced coronary elastin breakdown (Lau et al., 2009).

3.6. Interleukin (IL)-1β

IL-1β is a pro-inflammatory cytokine released in pathologies involving elastin-rich tissues (e.g., COPD and asthma) (Chung, 2006). Similar to TNF-α, IL-1β enhances the release of elastases in skin and uterine fibroblasts (Ito et al., 1990; Croute et al., 1991). Induction of elastase expression may underlie the disruption of elastin fibers observed in alveolar septa of mouse lungs induced to express IL-1β (Lappalainen et al., 2005; Bry et al., 2007). Moreover, mice deficient in a negative regulator of IL-1β signaling, interleukin-1 receptor antagonist, develop femoral artery aneurysms displaying degradation of both the internal and external elastic lamina (Isoda et al., 2012).

In addition to its effects on elastase expression, IL-1β causes changes in elastin transcription. In neonatal lung fibroblasts IL-1β reduces elastin gene transcription (Berk et al., 1991; Kuang et al., 2002). The mechanism by which elastin transcription is negatively regulated in response to IL-1β is most thoroughly defined in neonatal lung fibroblasts. In these cells, IL-1β promotes nuclear localization of the p65 subunit of NF-κβ that subsequently interacts with Sp1 to downregulate elastin transcription (Kuang et al., 2002). This is supported by findings showing that: 1) overexpression of p65 downregulates tropoelastin mRNA levels (Kuang et al., 2002), and 2) that inhibition of NF-κβ blocks the effects of IL-1β on elastin transcription in neonatal rat lung fibroblasts (Kuang and Goldstein, 2003). Similarly, other studies show that p65 binds Sp1 proteins and interferes with transcription of at least one other extracellular matrix protein, type α1(I) collagen (Rippe et al., 1999).

The effect of IL-1β on NF-κβ also increases the expression of the transcriptional repressor CCAAT/enhancer-binding protein (C/EBP)-β (LIP), which binds to a GCAAT-containing sequence located at −56 to −62 bp in the elastin promoter (Kuang and Goldstein, 2003). Like IL-1β, C/EBPβ protein levels are increased in lung fibroblasts exposed to cigarette smoke (Miglino et al., 2012). Importantly, levels ofC/EBPβ proteins are regulated in lung fibroblasts by the ubiquitin-proteasome pathway, and proteasome inhibitor treatments downregulate tropoelastin mRNA expression (Kuang and Goldstein, 2005). C/EBP family members also bind to Rb, and the complexes bind to C-EBP binding sites on DNA (Charles et al., 2001). Whether Rb-C/EBP proteins interfere with the pro-elastin transcriptional activity of Rb-Sp1 remains to be determined.

4. Cytokines that have dual effects on elastin formation

4.1. IL-1β

By contrast to the anti-elastogenic effects exerted by IL-1β on neonatal lung fibroblasts, several studies show that IL-1β promotes tropoelastin mRNA expression in adult dermal fibroblasts (Mauviel et al., 1993; Song et al., 1999). For example, in dermal fibroblasts isolated from transgenic mice expressing the human elastin promoter linked to a CAT reporter, IL-1β increases CAT activity (Mauviel et al., 1993). Furthermore, subcutaneous IL-1β injection into these transgenic mice increases CAT activity in the skin, indicative of a robust positive effect on elastin transcription (Uitto et al., 1995). The molecular basis for the effects of IL-1β on elastin expression in skin fibroblasts is not clear.

4.2. TGFβ

Despite the preponderance of evidence indicating that TGFβ is a pro-elastogenic factor, increased TGFβ activity as seen in Marfan syndrome (MFS) (Neptune et al., 2003) is associated with elastic fiber degeneration and decreased tropoelastin mRNA levels (Yao et al., 2007). This, together with the fact that noncanonical, Smad-independent, TGFβ-mediated Erk1/2 activation drives aortic aneurysm progression in MFS (Holm et al., 2011), appears to indicate that TGFβ signaling through the noncanonical, Smad-independent, MAP kinase pathway has suppressive effects on elastin RNA levels.

5. Conclusions

The cytokine-governed elastin regulatory axis is comprised of pro- and anti-elastogenic cytokines that control the biosynthesis and breakdown of elastin through transcriptional and post-transcriptional mechanisms. How cytokines exert opposing effects on elastin transcription (Fig. 1) hinges on the way in which they influence cell cycle regulatory constituents, particularly cyclin–cyclin dependent kinase complexes and their substrates. For example, cytokines such as IGF-I induce tropoelastin transcription by promoting cyclin E/cdk2-dependent phosphorylation of Rb(Thr821) which recruits Sp1 to bind the elastin gene promoter and stimulate transcription (Sen et al., 2011). Counterbalancing this process is mitogenic cytokine signaling via the Ras/MEK/ERK cascade. Cytokines such as EGF, HB-EGF and TGFα, which stimulate this cascade through activation of EGFR, exert suppressive effects on elastin biosynthesis and/or catabolism. Likewise, Ras/MEK/ERK pathway signaling mediated by bFGF, PDGF-BB and TNF-α also elicit suppressive effects on elastin expression, as does noncanonical, Smad-independent Ras/MEK/ERK signaling stimulated by TGFβ1. Importantly, cytokine stimulated Ras/MEK/ERK signaling acts in opposition to the pro-elastin effects of canonical TGFβ1 signaling, possibly through attenuation of nuclear accumulation of activated SMADs (Kretzschmar et al., 1999). Similarly, Ras/MEK/ERK-mediated initiation of the cell-cycle progression acts to attenuate the elastin transcription-promoting effects of IGF-I.

Fig. 1

Cytokine signaling cascades that influence tropoelastin transcription. Cytokines and signaling pathway intermediates highlighted in red act to suppress tropoelastin transcription. Cytokines and signaling pathway intermediates highlighted in green act to promote tropoelastin transcription. RTK, receptor tyrosine kinases.

Acknowledgments