No abstract

In normal adult tissue tenascin-C (TN-C) is usually expressed at low levels. However, it is strongly induced in many tumors as well as in other pathological conditions often associated with inflammation. To evaluate the diagnostic significance of TN-C, we established a sensitive sandwich ELISA to determine TN-C levels in serum. Furthermore, we investigated the distribution of TN-C variants in serum and found the large TN-C isoforms to be predominant. We measured TN-C in sera from 15 healthy persons, 75 tumor patients and 84 patients selected due to their elevated levels of the acute-phase protein C-reactive protein (CRP), which is a very specific marker for infection and inflammation. It was found that sera from cancer patients can have elevated TN-C levels; however, the increase was more pronounced in persons with high levels of CRP. There appeared to be a correlation of TN-C levels with the levels of CRP. In view of these facts, the diagnostic value of TN-C levels in serum as a potential tumor marker seems to be questionable, since our data show that TN-C levels can be elevated as a consequence of infection and inflammation.

The tenascins are a growing family of extracellular matrix proteins of typical multidomain structure. The prototype to be discovered was tenascin-C. It shows a highly regulated expression pattern during embryonic development and is often transiently associated with morphogenetic tissue interactions during organogenesis. In the adult organism reexpression of tenascin-C occurs in tumors and many other pathological conditions. Tenascin-C expression can be regulated by many different growth factors and hormones. Furthermore, mechanical strain exerted by fibroblasts seems to induce the expression of tenascin-C. This could represent a mechanism of translating mechanical forces into protein patterns, a step of potential relevance in the organization of embryogenesis. Tenascin-C as well as tenascin-R are believed to counteract the cell adhesion and spreading activity of fibronectin, thereby facilitating cell movement.

Cellular transformation into myofibroblasts is a central physiological process enabling tissue repair. Its deregulation promotes fibrosis and carcinogenesis. TGF-b is the main inducer of the contractile gene program that drives myofibroblast differentiation from various precursor cell types. Crucial regulators of this transcriptional program are serum response factor (SRF) and its cofactor MKL1 (also known as MRTF-A). However, the exact mechanism of the crosstalk between TGF-b signaling and MKL1 remains unclear. Here, we report the discovery of a novel MKL1 variant/isoform, MKL1_S, transcribed from an alternative promoter and uncover a novel translation start for the published human isoform, MKL1_L. Using a human adipose-derived mesenchymal stem cell differentiation model, we show that TGF-b specifically upregulates MKL1_S during the initial phase of myofibroblast differentiation. We identified a functional N-terminal motif in MKL1_S that allows specific induction of a group of genes including the extracellular matrix (ECM) modifiers MMP16 and SPOCK3/testican-3. We propose that TGF-b-mediated induction of MKL1_S initiates progression to later stages of differentiation towards a stationary myofibroblast.

Induction of tenascin-C mRNA by cyclic strain in fibroblasts depends on RhoA and Rho dependent kinase (ROCK). Here we show that integrin-linked kinase (ILK) is required upstream of this pathway. In ILK-deficient fibroblasts, RhoA was not activated and tenascin-C mRNA remained low after cyclic strain; tenascin-C expression was unaffected by ROCK inhibition. In ILK wild-type but not ILK-/- fibroblasts, cyclic strain-induced reorganization of actin stress fibers and focal adhesions, as well as nuclear translocation of MAL, a transcriptional co-activator that links actin assembly to gene expression. These findings support a role for RhoA in ILK-mediated mechanotransduction. Rescue of ILK -/- fibroblasts by expression of wild-type ILK restored these responses to cyclic strain. Mechanosensation is not entirely abolished in ILK -/- fibroblasts, since cyclic strain activated Erk-1/2 and PKB/Akt, and induced c-fos mRNA in these cells. Conversely, lysophosphatidic acid stimulated RhoA and induced both c-fos and tenascin-C mRNA in ILK -/- cells. Thus, the signaling pathways controlling tenascin-C expression are functional in the absence of ILK, but are not triggered by cyclic strain. Our results indicate that ILK is selectively required for the induction of specific genes by mechanical stimulation via RhoA-mediated pathways.