Regulation of the expression of a secreted acidic protein rich in cysteine (SPARC) in human fibroblasts by transforming growth factor β

Abstract

Transforming growth factor β (TGF‐β) and secreted protein acidic rich cysteine (SPARC) have been associated with the rapid remodeling of connective tissues that occurs in wound healing and developmental processes. To study the temporal and mechanistic aspects of TGF‐β‐regulated extracellular‐protein gene expression in human fibroblasts, confluent cells were pulse labeled for 30 min with [35S]methionine at various times following the single addition of 1.0 ng/ml TGF‐β. After a 4‐h chase period, specific radiolabeled media proteins were isolated by either immunoprecipitation or affinity chromatography and quantitated. Stimulation of SPARC synthesis was first apparent 5 h after addition of TGF‐β, reached a maximum (3.5‐fold increase) at 24 h and persisted for at least 96 h. A similar temporal response to TGF‐β was observed for the extracellular matrix proteins collagen and fibronectin. In contrast, TGF‐β induced a strong (> sixfold increase at 9 h after addition of TGF‐β), but transient stimulation of the synthesis of endothelial‐type plasminogen activator inhibitor. Northern blot analysis showed that SPARC mRNA levels were increased by TGF‐β in parallel with increase in SPARC synthesis; a maximum 3.9‐fold increase in SPARC mRNA being reached at 24 h. Similarly, the levels of both collagen and fibronectin mRNA were increased by TGF‐β treatment. In each case the stimulation of mRNA was blocked by the presence of the translation inhibitor, cycloheximide. Stability of SPARC mRNA (half‐life of approximately 50 h) was not significantly altered by TGF‐β. In contrast, the stability of collagen and fibronectin mRNA were both increased in the presence of TGF‐β; the increased stability being pronounced in less dense cells. In addition to effects on stability, transcription of the collagen and fibronectin genes was increased 7 h after TGF‐β addition, but returned to control levels by 24 h. However, transcription of the SPARC gene was unaffected by TGF‐β at both time points and, together with the stability data, indicates that TGF‐β regulates SPARC expression via a nuclear post‐transcriptional mechanism. Differential regulation of gene expression by TGF‐β in a precise temporal pattern via transcriptional and post‐transcriptional pathways may be an important aspect of the response of fibroblast cells in a wound environment.

Transcriptional control of SPARC by v-Jun and other members of the AP1 family of transcription factors

“…As shown in Figure 2 there was no detectable di erence in the steady-state level of the mRNA between the two extracts. This experiment also indicated that the SPARC message was extremely stable with a half-life of 38+2 h, a value which is in agreement with an independent study conducted in mammalian cells (Wrana et al, 1991). These data led us to pursue the analysis of the repression by v-Jun at the level of an isolated fragment of the SPARC promoter.…”

TP53INP1 decreases pancreatic cancer cell migration by regulating SPARC expression

“…TGFb is a master regulator of wound-healing and fibrosis by inducing the synthesis of several ECM proteins including collagen and FN1 (Verrecchia and Mauviel, 2007). Ample data demonstrate that TGFb induces SPARC expression (Wrana et al, 1991;Ford et al, 1993;Reed et al, 1994). However, there is also evidence that SPARC regulates the expression and activity of TGFb, suggesting that there is a reciprocal regulatory feedback loop between SPARC and TGFb.…”

SPARC is expressed by macroglia and microglia in the developing and mature nervous system

“…SPARC negatively regulates signaling by various growth factors known to be involved in neural stem cell proliferation and differentiation, including fibroblast growth factor 2 (FGF2) , epidermal growth factor (EGF) (Said et al, 2007a), insulin-like growth factor-1 (IGF-1) (Francki et al, 2003), and transforming growth factor ␤ (TGF␤) (Schiemann et al, 2003;Francki et al, 2004). However, the expression of SPARC is in turn regulated by each of these factors; decreased by FGF2 and EGF, increased by IGF-1, and reciprocally regulated by TGF␤ (Wrana et al, 1991;Chandrasekhar et al, 1994;Delany and Canalis, 1998;Shiba et al, 1998Shiba et al, , 2001. Furthermore, SPARC can positively and negatively modulate growth factor-and adhesion-dependent cell survival signaling (Shi et al, 2004(Shi et al, , 2007Said et al, 2007a), with the potential to maintain neural stem cell equilibrium.…”