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Fibrotic disorders of the renal, pulmonary, cardiac, and hepatic systems are associated with significant morbidity and mortality. Effective therapies to prevent or curtail the advancement to organ failure, however, remain a major clinical challenge. Chronic kidney disease, in particular, constitutes an increasing medical burden affecting >15% of the US population. Regardless of etiology (diabetes, hypertension, ischemia, acute injury, urologic obstruction), persistently elevated TGF-β1 levels are causatively linked to the activation of profibrotic signaling networks and disease progression. TGF-β1 is the principal driver of renal fibrogenesis, a dynamic pathophysiologic process that involves tubular cell injury/apoptosis, infiltration of inflammatory cells, interstitial fibroblast activation and excess extracellular matrix synthesis/deposition leading to impaired kidney function and, eventually, to chronic and endstage disease. TGF-β1 activates the ALK5 type I receptor (which phosphorylates SMAD2/3) as well as non-canonical (e.g., src kinase, EGFR, JAK/STAT, p53) pathways that collectively drive the fibrotic genomic program. Such multiplexed signal integration has pathophysiological consequences. Indeed, TGF-β1 stimulates the activation and assembly of p53-SMAD3 complexes required for transcription of the renal fibrotic genes plasminogen activator inhibitor-1, connective tissue growth factor and TGF-β1. Tubular-specific ablation of p53 in mice or pifithrin-α-mediated inactivation of p53 prevents epithelial G(2)/M arrest, reduces the secretion of fibrotic effectors and attenuates the transition from acute to chronic renal injury, further supporting the involvement of p53 in disease progression. This review focuses on the pathophysiology of TGF- β1-initiated renal fibrogenesis and the role of p53 as a regulator of profibrotic gene expression.

Plasminogen activator inhibitor-1 (PAI-1), the primary inhibitor of urokinase-and tissue-type plasminogen activators (uPA and tPA), is an injury-response gene implicated in the development of tissue fibrosis and cardiovascular disease. PAI-1 mRNA and protein levels were elevated in the balloon catheter-injured carotid and in the vascular smooth muscle cell (VSMC)-enriched neointima of ligated arteries. PAI-1/uPA complex formation and PAI-1 antiproteolytic activity can be inhibited, via proteolytic cleavage, by the small molecule antagonist tiplaxtinin which effectively increased the VSMC apoptotic index in vitro and attenuated carotid artery neointimal formation in vivo. In contrast to the active full-length serine protease inhibitor (SERPIN), elastase-cleaved PAI-1 (similar to tiplaxtinin) also promoted VSMC apoptosis in vitro and similarly reduced neointimal formation in vivo. The mechanism through which cleaved PAI-1 (CL-PAI-1) stimulates apoptosis appears to involve the TNF-α family member TWEAK (TNF-α weak inducer of apoptosis) and it’s cognate receptor, fibroblast growth factor (FGF)-inducible 14 (FN14). CL-PAI-1 sensitizes cells to TWEAK-stimulated apoptosis while full-length PAI-1 did not, presumably due to its ability to down-regulate FN14 in a low density lipoprotein receptor-related protein 1 (LRP1)-dependent mechanism. It appears that prolonged exposure of VSMCs to CL-PAI-1 induces apoptosis by augmenting TWEAK/FN14 pro-apoptotic signaling. This work identifies a critical, anti-stenotic, role for a functionally-inactive (at least with regard to its protease inhibitory function) cleaved SERPIN. Therapies that promote the conversion of full-length to cleaved PAI-1 may have translational implications.

Colchicine and nocodazole, both established microtubule disruptors, are useful tools to investigate cytoskeletal-dependent signaling cascades and the associated downstream transcriptional targets. Since cytoskeletal events impact pathophysiologic consequences in the vascular system, the signaling requirements underlying colchicine-stimulated expression of PAI-1 and CTGF, two prominent cell deformation-sensitive fibrosis-initiating proteins, were evaluated in vascular smooth muscle cells. Microtubule disruption rapidly induced EGFR transactivation (at the src kinase-sensitive EGFR(Y845) site) in a ROS-dependent manner. Genetic deficiency of EGFR, inhibition of EGFR signaling with AG1478 or introduction of a kinase-deficient EGFR construct effectively blocked colchicine-stimulated PAI-1 and CTGF expression. MEK/ERK involvement downstream of ROS generation was critical for PAI-1, but not CTGF, expression following cytoskeletal perturbation suggesting bifurcation of signaling pathways downstream of EGFR activation. Colchicine also stimulated SMAD2/3 phosphorylation by a Rho/ROCK-dependent mechanism independent of TGF-beta1 release or receptor activity. Rho/ROCK signaling initiated by tubulin network collapse was required for both CTGF and PAI-1 induction. Colchicine-initiated SMAD3 phosphorylation, however, was essential for PAI-1, but not CTGF, expression further highlighting divergence of signaling events downstream of Rho/ROCK that mediate microtubule deformation-associated changes in profibrotic gene transcription.

TGF-beta1 and its target gene encoding plasminogen activator inhibitor-1 (PAI-1) are major causative factors in the pathology of tissue fibrosis and vascular disease. The increasing complexity of TGF-beta1 action in the cardiovascular system requires analysis of specific TGF-beta1-initiated signaling events that impact PAI-1 transcriptional regulation in a physiologically-relevant cell system. TGF-beta1-induced PAI-1 expression in both primary cultures and in an established line (R22) of vascular smooth muscle cells (VSMC) was completely blocked by inhibition of epidermal growth factor receptor (EGFR) activity or adenoviral delivery of a kinase-dead EGFR(K721A) construct. TGF-beta1-stimulated PAI-1 expression, moreover, was preceded by EGFR phosphorylation on Y845 (a src kinase target residue) and required pp60(c-src) activity. Infection of VSMC with an adenovirus encoding the EGFR(Y845F) mutant or transfection with a dominant-negative pp60(c-src) (DN-Src) expression vector effectively decreased TGF-beta1-stimulated, but not PDGF-induced, PAI-1 expression implicating the pp60(c-src) phosphorylation site EGFR(Y845) in the inductive response. Consistent with these findings, TGF-beta1 failed to induce PAI-1 synthesis in src kinase-deficient (SYF(-/-/-)) fibroblasts and reexpression of a wild-type pp60(c-src) construct in SYF(-/-/-) cells rescued the PAI-1 response to TGF-beta1. TGF-beta1-induced EGFR activation, but not SMAD2 activation, moreover, was virtually undetectable in SYK(-/-/-) fibroblasts in comparison to wild type (SYK(+/+/+)) counterparts, confirming an upstream signaling role of src family kinases in EGFR(Y845) phosphorylation. Genetic EGFR deficiency or infection of VSMCs with EGFR(K721A) virtually ablated TGF-beta1-stimulated ERK1/2 activation as well as PAI-1 expression but not SMAD2 phosphorylation. Transient transfection of a dominant-negative RhoA (DN-RhoA) expression construct or pretreatment of VSMC with C3 transferase (a Rho inhibitor) or Y-27632 (an inhibitor of p160ROCK, a downstream effector of Rho) also dramatically attenuated the TGF-beta1-initiated PAI-1 inductive response. In contrast to EGFR pathway blockade, interference with Rho/ROCK signaling effectively inhibited TGF-betaR-mediated SMAD2 phosphorylation and nuclear accumulation. TGF-beta1-stimulated SMAD2 activation, moreover, was not sufficient to induce PAI-1 expression in the absence of EGFR signaling both in VSMC and mouse embryonic fibroblasts. Thus, two distinct pathways involving the EGFR/pp60(c-src)/MEK-ERK pathway and Rho/ROCK-dependent SMAD2 activation are required for TGF-beta1-induced PAI-1 expression in VSMC. The identification of such novel interactions between two TGF-beta1-activated signaling networks that specifically impact PAI-1 transcription in VSMC may provide therapeutically-relevant targets to manage the pathophysiology of PAI-1-associated cardiovascular/fibrotic diseases.

TO THE EDITOR Serum-stimulation of quiescent (G0) keratinocytes initiates a temporally regulated program of transcriptional activity required for G0/G1 transit and subsequent entry into the proliferative cycle (Qi and Higgins, 2003). Expression profiling of such “activated” keratinocytes identified physiologically relevant subsets of cell cycle/growth state-regulated genes (Gromov et al., 2002; Gazel et al., 2003). Indeed, non-cycling human (HaCaT) keratinocytes express a differentiated (i.e., super-basal) genetic signature, whereas the serum-stimulated transcriptome approximates that of transient amplifying cells (Pivarcsi et al., 2001; Lemaitre et al., 2004). Clearly, the associated transcriptional responses dictate epidermal cell lineage commitments by impacting the expression of pathway-relevant genes (Banno et al., 2004; Lemaitre et al., 2004). This report provides early evidence regarding the comprehensive inventory of genes expressed by human HaCaT-II4 keratinocytes during the initial stage of cell-cycle re-entry. Re-introduction of serum to quiescent HaCaT-II4 cells stimulates G0 exit and residence in a short-lived “activated G0 substate” (i.e., the kinetically defined G0→G1 transition state) (Qi et al., 2006). Microarray analysis of quiescent and 2 hours fetal bovine serum (FBS)-“ activated” HaCaT-II4 cells defined the transcriptional signature of this early G0→G1 window. A total of 54,675 expressed sequence-tagged genes were analyzed with 41,083 directly compared for groups A (quiescent) and B (2 hours FBS-stimulated) and a total of 35,991 reproducibly assessed for all three experimental conditions (i.e., 66% of the total sequences available; this includes group C [FBS for 2 hours in the presence of puromycin included as a first approximation of the immediate-early response cluster]). Genes exhibiting statistically significant (analysis of variance) changes (two-fold increase or decrease) distributed as follows: 1151 for A versus B, 1241 for A versus C, and 1319 for B versus C. Among the most prominently upregulated mRNA transcripts were those encoding proteins involved in the initial growth response (EGR1-4), extracellular matrix remodeling and tissue invasion (uPA, uPAR, tPA, SERPINE1 (PAI-1), PAI-2, MMP-2, MMP- 12, CYR61), transcription (Myc, Fos, Jun, KLF4, AT3, p300/pTAF), signal transduction (DUSP1, -4, -8, -10, MAPK3, TGF-α), proliferation (GADD45a, GADD45b, CDK7, cyclins), and apoptosis (CASP9, MCL2) (e.g., Figure 1a). Reverse transcription-PCR and northern blotting validated the expression data for selected genes. When more stringent criteria were applied to data filtering (i.e., set threshold of ≥ 10-fold increase), 79 genes were identified (Table 1) of which 75 also partitioned to the puromycin-resistant subset. Rank order analysis indicated that PAI-1 (SERPINE1) and the uPA receptor (PLAUR) were the most significantly elevated transcripts. uPA increased (by 12-fold) as well (by microarray and northern analyses), although maximal uPA expression occurred several hours later than PAI-1. Additional genes that comprise the “tissue repair” subset and that were upregulated > 10-fold within the first 2 hours included DTR, IL8, EREG, HBEGF, IL11, CTGF, LIF, IL6, IL1A, HAS3, SERPINB1, and TGFA. Northern blotting confirmed that PAI-1 transcripts were low to undetectable in quiescent HaCaT-II4 cells, peaked in puromycin-resistant manner 2 hours after serum addition (during residence in the initial activated G0 substate; Qi et al., 2006), and then rapidly decreased (Figure 1b and c). Expression required EGFR/MEK/rho-ROCK signaling during the G0→G1 transition (Figure 1d and e). Actinomycin chase/mRNA decay and temporal assessments of mRNA abundance indicated, moreover, that PAI-1 transcripts were substantially reduced (from a maximum at 2 hours) as early as 4 hours post-stimulation decreasing further by 6–8 hours post-stimulation (i.e., approximately mid-G1) likely due to E2F1-mediated suppression (Koziczak et al., 2001), consistent with a narrow window of serum-initiated transcription and short mRNA half-life (1.5–2 hours) (Mu et al., 1998; White et al., 2000). Figure 1 A significant fraction of FBS-induced transcripts encode proteins involved in cell proliferation, transcriptional reprogramming, and control of pericellular proteolysis Table 1 Genes exhibiting a ≥ 10-fold increase in expression 2 hours after serum stimulation of quiescent HaCaT-II4 cells Activation of a wound repair transcript profile appears to be a general response to serum addition (Iyer et al., 1999; this study). The present findings indicate, furthermore, that PAI-1 is the most prominent member of the keratinocyte “serum response transcriptome”. Several SERPINS (i.e., PAI-1, protease nexin-1), in fact, modulate the complex process of injury resolution through control of focalized plasminmediated matrix remodeling, cell migration, and apoptosis (e.g., Bajou et al., 2001; Li et al., 2000; Deng et al., 2001; Degryse et al., 2004; Rossignol et al., 2004; Wang et al., 2005). Targeted PAI-1 knockdown/overexpression and protein add-back approaches, moreover, support the contention that PAI-1 participates within the global program of injury to coordinate cycles of cell-to-substrate adhesion/detachment and/or maintain a stromal “scaffold” to satisfy the prerequisites for both G1/S transition and effective cellular migration (Planus et al., 1997; Chazaud et al., 2002; Palmeri et al., 2002; Providence et al., 2002; Czekay et al., 2003; Providence and Higgins, 2004). PAI-1 is also expressed at high levels in senescent cells where it likely interferes with uPA-dependent growth factor activation (Mu et al., 1998; Kortlever et al., 2006). Certain “senescence-associated” genes (i.e., p16INK4a, PAI-1) may actually function in the wound repair program by inhibiting proliferation while promoting migration (Chan et al., 2001; Ploplis et al., 2004; Darbro et al., 2005; Kortlever et al., 2006; Natarajan et al., 2006). Indeed, keratinocytes at the leading edge during wound re-epithelialization are less mitotically active than cells more displaced from the motile front and express relatively high levels of PAI-1 (Garlick and Taichman, 1994; Jensen and Lavker, 1996; Providence and Higgins, 2004). Collectively, these data suggest that PAI-1 may regulate the temporal cadence of cell-cycle progression in replicatively competent cells as part of the injury repair program.

Transforming growth factor-beta1 (TGF-beta1) transcriptionally regulates the expression of genes that encode specific proteins (e.g., plasminogen activator inhibitor-1; PAI-1) important in stromal remodeling and cellular invasion. Definition of molecular events underlying TGF-beta1-initiated PAI-1 transcription, therefore, may lead to the identification of new therapeutic targets for diseases associated with elevated PAI-1 synthesis (e.g., tissue fibrosis, vascular disorders, tumor progression). An intact upstream stimulatory factor (USF)-binding E box motif (5'-(-165)CACGTG(-160)-3') at the HRE-2 site in the rat PAI-1 gene was required for PAI-1 transcription in TGF-beta1-treated cells. Mutation of the CA dinucleotide to TC at position -165/-164 in a reporter construct driven by 764 bp of PAI-1 promoter sequence decreased TGF-beta1-dependent CAT activity by >80% indicating the necessity for a consensus hexanucleotide E box motif in induced expression. The same CA --> TC substitution eliminated USF binding to an 18-bp HRE-2 DNA target highlighting the importance of site occupancy to transcriptional activation. Transfection of a dominant-negative USF construct, moreover, completely inhibited formation of USF/HRE-2 probe complexes, attenuated PAI-1 promoter-driven luciferase activity and reduced the response of the endogenous PAI-1 gene to TGF-beta1 (to that approximating quiescent controls). Maximal immediate-early PAI-1 induction upon exposure to TGF-beta1 required EGFR, p21ras, MEK and pp60(c-src) signaling as pharmacologic or dominant-negative inhibition of any of the four intermediates (EGFR, p21ras, MEK, pp60(c-src)) virtually eliminated TGF-beta1-augmented PAI-1 levels. U0126 titering experiments, furthermore, revealed that the same MEK inhibitor concentration that blocked the TGF-beta1 increase in ERK1/2 phosphorylation (20 microM) also effectively attenuated the PAI-1 inductive response suggesting a requirement for stimulated ERK signaling in TGF-beta1-mediated PAI-1 expression. These data suggest a model whereby TGF-beta1 activates a complex signaling cascade to affect PAI-1 gene control and involves USF occupancy of a critical E box motif at the HRE-2 site in the PAI-1 gene.