Differential effects of cytotactin/tenascin fusion proteins on intracellular pH and cell morphology

Expression of tenascin-C and its isoforms in the breast

Tenascin-C (TNC) is an extracellular matrix glycoprotein which is frequently up-regulated in a variety of pathological conditions including chronic inflammation and cancer. TNC has been implicated in the modulation of cell migration, proliferation, invasion and angiogenesis. Multiple isoforms of TNC can be generated through the alternative splicing of nine exons located in the fibronectin type III region of the molecule. The profile of isoforms expressed differs between cancers and normal breast, with the fully truncated TNC isoform being predominant in normal and benign tissues and higher molecular weight isoforms induced predominantly in cancer. The addition of extra domains within the fibronectin type III repeat domain greatly affects TNC function with multiple exon combinations available for splicing. Exons 14 and 16 are considered to be tumour-associated and have been shown to affect breast cell line invasion and growth in vitro to a greater extent than the full-length TNC isoform. This mini review will provide a summary of the literature to date regarding the expression of TNC isoforms in the breast and also discuss more recent developments in the field regarding exon AD1.

Alkaline induces metallothionein gene expression and potentiates cell proliferation in Chinese hamster ovary cells

Metallothionein (MT) gene expression is increased in cadmium resistant Chinese hamster ovary cells (CHO Cd(R)) upon medium (regular or serum-free) change during culturing. Among the major components of the medium, NaHCO3 was found to be able to induce MT gene expression in a dose- and time-dependent manner. The same effect was observed with other alkaline solutions, such as HEPES and NaOH. Using MT promoter-luciferase reporter gene constructs, we found that the presence of metal response elements (MREs) in the promoter region is necessary for NaHCO3-induced MT gene transcription. This finding is further supported by the observation that the binding activity between the metal-responsive transcription factor 1 (MTF-1) and the MRE were increased after NaHCO3 treatment. Following NaHCO3 treatment, an increase in cell proliferation was observed in CdR cells but not in the parental CHO K1 cells that do not express MT transcripts due to MT gene methylation. Using synchronized cells, an increase in cell proliferation was observed 9 h after NaHCO3 addition. Notably, proliferation of CHO K1 cells was increased when transfected with an MT gene. The effect of MT on cell growth was affirmed by treating CHO K1 cells with 5-azacytidine (Aza) to demethylate the MT gene. Proliferation increased in Aza-treated CHO K1 cells after NaHCO3 treatment. These results demonstrate that NaHCO3 stimulates MT gene expression and causes an enhancement of cell proliferation in CHO cells.

Tenascin-C in development and disease: gene regulation and cell function

Tenascin-C (TN-C) is a modular and multifunctional extracellular matrix (ECM) glycoprotein that is exquisitely regulated during embryonic development and in adult tissue remodeling. TN-C gene transcription is controlled by intracellular signals that are generated by multiple soluble factors, integrins and mechanical forces. These external cues are interpreted by particular DNA control elements that interact with different classes of transcription factors to activate or repress TN-C expression in a cell type- and differentiation-dependent fashion. Among the transcriptional regulators of the TN-C gene that have been identified, the homeobox family of proteins has emerged as a major player. Downstream from TN-C, intracellular signals that are relayed via specific cell surface receptors often impart contrary cellular functions, even within the same cell type. A key to understanding this behavior may lie in the dual ability of TN-C-enriched extracellular matrices to generate intracellular signals, and to define unique cellular morphologies that modulate these signal transduction pathways. Thus, despite the contention that TN-C null mice appear to develop and act normally, TN-C biology continues to provide a wealth of information regarding the complex nature of the ECM in development and disease.

The tenascin family of ECM glycoproteins: structure, function, and regulation during embryonic development and tissue remodeling

The determination of animal form depends on the coordination of events that lead to the morphological patterning of cells. This epigenetic view of development suggests that embryonic structures arise as a consequence of environmental influences acting on the properties of cells, rather than an unfolding of a completely genetically specified and preexisting invisible pattern. Specialized cells of developing multicellular organisms are surrounded by a complex extracellular matrix (ECM), comprised largely of different collagens, proteoglycans, and glycoproteins. This ECM is a substrate for tissue morphogenesis, lends support and flexibility to mature tissues, and acts as an epigenetic informational entity in the sense that it transduces and integrates intracellular signals via distinct cell surface receptors. Consequently, ECM-receptor interactions have a profound influence on major cellular programs including growth, differentiation, migration, and survival. In contrast to many other ECM proteins, the tenascin (TN) family of glycoproteins (TN-C, TN-R, TN-W, TN-X, and TN-Y) display highly restricted and dynamic patterns of expression in the embryo, particularly during neural development, skeletogenesis, and vasculogenesis. These molecules are reexpressed in the adult during normal processes such as wound healing, nerve regeneration, and tissue involution, and in pathological states including vascular disease, tumorigenesis, and metastasis. In concert with a multitude of associated ECM proteins and cell surface receptors that include members of the integrin family, TN proteins impart contrary cellular functions, depending on their mode of presentation (i.e., soluble or substrate-bound) and the cell types and differentiation states of the target tissues. Expression of tenascins is regulated by a variety of growth factors, cytokines, vasoactive peptides, ECM proteins, and biomechanical factors. The signals generated by these factors converge on particular combinations of cis-regulatory elements within the recently identified TN gene promoters via specific transcriptional activators or repressors. Additional complexity in regulating TN gene expression is achieved through alternative splicing, resulting in variants of TN polypeptides that exhibit different combinations of functional protein domains. In this review, we discuss some of the recent advances in TN biology that provide insights into the complex way in which the ECM is regulated and how it functions to regulate tissue morphogenesis and gene expression.

Glial cell interactions with tenascin-C: adhesion and repulsion to different tenascin-C domains is cell type related

The multimodular glycoprotein tenascin-C is transiently expressed, predominantly by glial cells, during the development of the central and peripheral nervous systems. This extracellular matrix glycoprotein is involved in the control of cell adhesion, neuron migration and neurite outgrowth. Distinct functional properties for neuronal cell types have been attributed to separate tenascin-C domains using antibody perturbation studies and in vitro experiments on tenascin-C fragments. In order to study potential roles of tenascin-C for glial cell biology, a library of recombinant tenascin-C domains was used in a bioassay in vitro. Embryonic day 14 astrocytes, various astroglial-derived cell lines (C6, A7 and Neu7) and oligodendroglial-derived cell types (Oli-neu and G26-20) were examined in an adhesion assay and compared to the neuroblastoma cell line N2A. A binding site for most cell types, except for A7 and N2A, could be assigned to the first three fibronectin type III domains. Repulsive properties could be mapped to three different sites the epidermal growth factor-like repeats, fibronectin type III repeats 4 and 5 and to the alternatively spliced region of the molecule. The responses to these repulsive sites varied according to the cell type. These data are consistent with the interpretation that different cell types express distinct sets of tenascin-C receptors which might regulate cellular responses via distinct second messenger pathways.

Separate cell binding sites within cytotactin/tenascin differentially promote neurite outgrowth

Cytotactin/tenascin (CT/TN) is an extracellular matrix protein that binds to a variety of cell types and that influences neurite outgrowth. It has a multidomain structure with regions homologous to epidermal growth factor (EGF)-like repeats, fibronectin (FN) type II repeats, and the beta and gamma chains of fibrinogen (fg). The current study demonstrates that a fusion protein corresponding to the sixth fibronectin type III repeat in CT/TN (CTfn6) supported cell attachment and promoted an increase in the number of cells with neurites in both central and peripheral neurons in tissue culture. The third fibronectin type III repeat, CTfn3, like intact CT/TN, supported attachment of peripheral neurons but not of central neurons and, while it caused an increase in neurite length, it did not increase the number of cells that sprouted neurites. When CTfn3 and CTfn6 were combined, an increase in both the number of cells sprouting neurites and in neurite length was observed for peripheral neurons that resembled their response to intact CT/TN. Cell attachment to CTfn6 was inhibited in the presence of function-blocking antibodies against beta 1 integrins. In contrast, the interaction with CTfn3 was not inhibited by antibodies to beta 1 integrins, but was inhibited by RGD-containing peptides. The results suggest that cell binding to CT/TN involves two different sites within the molecule and occurs via different receptors which may be differentially expressed on different neuronal cell types. The location of these sites within the whole molecule in the context of other adhesive and counteradhesive domains may modulate their influence on cellular responses such as cell attachment and neurite outgrowth.