Presence of elastin-related 45-kDa fragment in culture medium: specific cleavage product of tropoelastin in vascular smooth muscle cell culture

In vitro degradation of human tropoelastin by MMP-12 and the generation of matrikines from domain 24

Degradation of elastic fibers in tissues can result in the development of disorders that include aneurysms, atherosclerosis, and loss of skin elasticity. Tropoelastin is the precursor of the cross-linked elastin and its expression is triggered by elastin-degrading factors as a response to damage. Factors like UV radiation not only increase the expression of tropoelastin but also potent metalloelastases such as macrophage elastase (MMP-12). The development of elastin-degrading diseases, moreover, is a chronic process during which elastin and tropoelastin are repeatedly exposed to attacks by MMP-12. Hence, in this work we report the in vitro susceptibility of tropoelastin and the potential of MMP-12 to generate matrikines. This work provides evidence that tropoelastin is substantially and rapidly degraded by MMP-12 even at very dilute enzyme concentrations. MMP-12 cleaves at least 86 sites in tropoelastin. Analysis of the generated peptides revealed that some small peptides contained the motif GXXPG that may enable them to bind with the elastin binding protein (EBP). Furthermore, using synthesized peptides it was confirmed that several sites in the sequence encoded by exon 24 which contains repetitive units of biologically active VGVAPG domains are susceptible to attack by MMP-12, provided that the active subsites in MMP-12 (S(4) to S(4)') are occupied. Such cleavage events have lead to the generation of ligands that may bind to EBP.

Tropoelastin expression is up-regulated during activation of hepatic stellate cells and in the livers of CCl(4)-cirrhotic rats

BACKGROUND/AIMS

Activated hepatic stellate cells (HSC) are regarded as the principal cells synthesizing extracellular matrix components in fibrotic liver. Elastin content is increased in cirrhotic livers, but the cellular source is not known. Contribution of HSC to the production of elastin was investigated.

METHODS

Expression of elastin in CCl(4)-cirrhotic rat liver was studied by immunohistochemistry and in situ hybridization, liver myofibroblasts were identified in histological sections by alpha-smooth muscle actin (alpha-SMA) staining. LightCycler PCR and Northern blotting were used to detect tropoelastin mRNA in isolated HSC; tropoelastin protein was detected in the cells and in cell-conditioned medium by Western blotting.

RESULTS

HSC, isolated from normal rat liver, displayed increasing tropoelastin mRNA expression during transdifferentiation in culture. Expression of tropoelastin mRNA was accompanied by the production of tropoelastin protein in vitro. Increased levels of tropoelastin transcripts were found in the connective tissue septa of CCl(4)-cirrhotic rats and co-localized with alpha-SMA positive cells. Immunohistochemistry demonstrated elastin presence in the septa.

CONCLUSION

HSC express tropoelastin and its expression increases during transdifferentiation to myofibroblast-like cells.

Cultured human keratinocytes express tropoelastin

We detected elastin mRNA in cultured normal human keratinocytes by RNase protection assay. The content of elastin mRNA was estimated at approximately one-twentieth of that of cultured skin fibroblasts. Tropoelastin polypeptide with a molecular weight of 68 kDa was detected in the preparation of culture medium of normal human keratinocytes by western blot assays using anti-tropoelastin antibody. Immunohistochemical studies also demonstrated positive staining in cultured normal human keratinocytes as well as in skin fibroblasts. The expression of elastin by normal human keratinocytes was found to reach a maximum level at the quiescent phase of keratinocyte growth. When normal human keratinocytes were cultured on tropoelastin-coated dishes, their growth potential was greatly suppressed compared with other matrix protein-coated dishes. These results suggest that cultured normal human keratinocytes can actively synthesize elastin and that keratinocyte elastin may act as a growth-regulator for keratinocytes.