Periostin family of proteins: Therapeutic targets for heart disease (2005)

This review is prompted by several recent discoveries expanding our knowledge of periostin, periostin-like factor, and βig-h3. These proteins are assigned to one family based on their homology to fasciclin I identified in insects. Proteins that share homology with fasciclin I include βig-h3, stablin I and II, MBP-70, Algal-CAM, periostin (osf2), and, most recently, periostin-like-factor (PLF) (Zinn et al.,1988; Terasaka et al.,1989; Skonier et al.,1992; Takeshita et al.,1993; Huber and Sumper,1994; Horiuchi et al.,1999; Litvin et al.,2004). Of these, periostin, PLF, and βig-h3 are rapidly becoming popular as putative therapeutic targets because of their role in embryonic heart development and in adult heart disease, in osteogenesis, and in tumor suppression (Stanton et al.,2000; Kruzynska-Frejtag et al.,2001; Sasaki et al.,2001; Gillan et al.,2002; Yoshioka et al.,2002; Katsuragi et al.,2004; Kern et al.,2005). The characteristic that is used to assign these proteins to the same family is that they all share homology in multiple 150 amino acid (aa) domains referred to as repeat or fasciclin domains. The number of fasciclin domains varies among these proteins but the significance of this difference is not known. Within each fasciclin domain is a smaller conserved region referred to as the fas domain (Figs. 2 and 3, Table 1). In addition, several of the members contain YH motifs (non-RGD-binding domains), which in the case of βig-h3 has been shown to interact with integrin receptors (Kim et al.,2000; Bae et al.,2002; Kim et al.,2002; Nam et al.,2003; Jeong and Kim,2004; Park et al.,2004). This review will summarize the essentials on select members in the family and will speculate on the usefulness of periostin and PLF as therapeutic targets based on known facts about their function(s) and expression in the heart. The fasciclins are proteins identified in insects and include four members called fasciclin I, II, III, and neuroglian. The fasciclins are differentially expressed on subsets of nerve fascicles and are surface recognition molecules involved in growth cone guidance and nerve terminal arborization (Bastiani et al.,1987; Snow et al.,1988; Zinn et al.,1988; Hortsch and Goodman,1990; McAllister et al.,1992a; Hu et al.,1998). Fasciclin II and neuroglian have structural motifs similar to those of cell adhesion molecules such as NCAM and L1 (Hortsch et al.,1998; Forni et al.,2004) and will not be discussed in this review. Fasciclin I and III have been found to mediate homophilic cell adhesion (Patel et al.,1987; Snow et al.,1989; Chiba et al.,1995). Fasciclin I is structurally most closely related to proteins discussed in this review. It is a glycoprotein of approximately 72 kD and contains three 150 amino acid fasciclin domains; through alternative splicing, multiple forms are generated, each with differences in their binding specificity (McAllister et al.,1992b). One form of fasciclin I is soluble and the other is anchored to the plasma membrane by a phosphotidylinositol lipid moiety (Hortsch and Goodman,1990). The amount of each isoform is temporally regulated during embryonic development and thereby may differentially regulate adhesion. In both Drosophila and grasshoppers, fasciclin I is expressed on the surface of a subset of commissural axon pathways in the embryonic central nervous system (CNS) and on sensory axonal pathways in the peripheral nervous system (PNS) (McAllister et al.,1992a). The null mutation resulted in viable offspring without defects in nervous system morphogenesis. However, a double mutation for fasciclin I and abl resulted in a defect in growth cone guidance (abl is a proto-oncogene homolog that encodes a cytoplasmic tyrosine kinase expressed during embryogenesis in CNS and the null mutation in abl resulted in no overt phenotypic change in the CNS) (McAllister et al.,1992a). While the details on the molecular mechanisms involved are unclear, fasciclin I mediates interactions between cell surfaces in the nervous system. βig-h3 was first identified in adenocarcinoma cells treated by TGF-β. It is composed of 683 amino acids and has a molecular weight of 68 kD. It has a signal sequence at the N-terminus, an Arg-Gly-Asp (RGD) sequence at the C-terminus, and four internal Fas domains each containing 140 amino acids (Skonier et al.,1992). The gene coding for βig-h3 is located on chromosome 5q31. βIG-H3 can polymerize to form a fibrillar structure and strongly interacts with other extracellular proteins, such as type I collagen, laminin, and fibronectin. It has been detected in conditioned medium of keratinocytes, endothelial cells, renal proximal tubular epithelial cells, fibroblasts, astrocytes, and smooth muscle cells confirming that it is secreted from cells (LeBaron et al.,1995; Billings et al.,2000; Bae et al.,2002; Kim JE, et al.,2000,2002; Ferguson et al.,2003a,2003b; Kim MO et al.,2003; Nam et al.,2003; Jeong and Kim,2004; Park et al.,2004). It has also been detected in nuclei of human bladder smooth muscle cells and fibroblasts (Ferguson et al.,2003b). The major function of βIG-H3 is to mediate cell spreading, adhesion, proliferation, and migration (LeBaron et al.,1995; Billings et al.,2000; Kim JE et al.,2000,2002; Bae et al.,2002; Ferguson et al.,2003a,2003b; Kim MO et al.,2003; Nam et al.,2003; Jeong and Kim,2004; Park et al.,2004). These functions are mediated through interactions between the Fas domains and integrin receptors, such as α3β1, αvβ5, αvβ3, and α6β4 (Bae et al.,2002; Kim JE et al.,2002; Kim MO et al.,2003; Nam et al.,2003; Jeong and Kim,2004; Park et al.,2004). Although βIG-H3 contains RGD domains, it is clear that they are not motifs used for binding integrins. Two motifs within the Fas domain responsible for this interaction are the YH motif and the Asp-Ile motif. Each of the four Fas domains contains YH motifs consisting of tyrosine and histidine that interact with integrin receptors (Kim et al.,2002; Nam et al.,2003). Only the second and the fourth Fas domains contain Asp-Ile motifs consisting of aspartic acid and isoleucine that interact with integrin receptors (Bae et al.,2002; Jeong and Kim,2004; Park et al.,2004). The peptides NKDIL and EPDIM within the second and the fourth Fas domains, respectively, and that represent the Asp-Ile motif were identified as those that bind integrins (Park et al.,2004). In conformation, they resemble RGD peptides that are commonly known to bind integrins. βIG-H3 also functions as a tumor suppressor in immortalized human bronchial epithelial cells (BEP2D) by regulating the expression of α5 β1 (Zhao et al.,2003). Other functions of βIG-H3 include a role in modulating collagen VI microfibril function (Hanssen et al.,2003) and facilitating TGF-β-induced apoptosis by releasing its RGD peptides, which occurs upon its secretion from the cell (Park et al.,2004). Mutation of the βIG-H3 gene causes corneal dystrophy (Fujiki et al.,2000; Schmitt-Bernard et al.,2000). Overexpression of βIG-H3, which is induced by TGF-β, is implicated in atherosclerotic and restenotic human vascular lesions (O'Brien et al.,1996), and when upregulated by high glucose in vascular smooth muscle cells, βIG-H3 may play an important role in diabetic angiopathy (Ha et al.,2004). The human hepatic clearance receptors (HACRs), stabilin 1 and 2, were identified based on homology with the MS-1 antigen (Politz et al.,2002). Human stabilin 1 (275 kD) is located on chromosome 3p and stabilin 2 (273 kD) on chromosome 12q. Human stabilin 2 has been identified as either full length or a truncated form. Mouse and rat stabilins have been identified based on their homology to the human clones (Politz et al.,2002). Structurally, these proteins are similar to others in this family in that they contain fasciclin domains (seven). They differ from the other members in that they contain 15–17 EGF-like domains, 2–4 laminin EGF-like domains, an X-link domain, a B-X-B hepatic hyaluron (HA)-binding motif, and a transmembrane domain at the C-terminus. Predicted amino acid sequence analysis of stabilin 2 suggested membrane localization, whereas that of stabilin 1 may be involved in adenosine 5′-diphosphate-ribosylation factor-binding sorting processes and may function in endocytosis and secretory processes (Kzhyshkowska et al.,2004; McCourt et al.,2004; Hansen et al.,2005). Hstabilin 1 expression is highest in endothelial cells, especially organs with discontinuous capillaries (sinuses) such as the liver and spleen. Hstabilin 2 was expressed in the same organs as stabilin 1 but was not detected in endothelial cells. Mouse stablin expression was similar to that of hstabilin except that mstabilin 1 was not detected in the spleen. An assessment of the temporal and spatial pattern of expression in organ systems showed expression of stabilin 2 in hepatic and lymph node sinusoidal, splenic and bone marrow venous endothelial cells, heart valve mesenchymal cells, brain ependymal cells, and epithelial cells of the renal papillae (Falkowski et al.,2003). It was not detected in pathological conditions in newly formed vasculature. It has been suggested that stabilin 2 may be involved in the clearing of hepatic hyaluronan from the blood stream and thereby regulating blood viscosity (Falkowski et al.,2003). Reports on the function of stabilin 1 differ. One suggests that it may serve as a recycling scavenger receptor (Prevo et al.,2004); another that it mediates cellular locomotion or adhesion (Salmi et al.,2004). The physiological role(s) of the stabilins has not been established and therefore their relationship with the other members of the family of proteins containing fasciclin domains at this time remains structural. Periostin first identified in bone was implicated in regulating adhesion and differentiation of osteoblasts (Horiuchi et al.,1999; Litvin et al.,2004) and recently as being antiosteogenic (Kern et al.,2005). We now know that periostin is also highly expressed in developing and mature heart valves (Kruzynska-Frejtag et al.,2001), under pressure or volume overload in the adult heart (Stanton et al.,2000; Katsuragi et al.,2004), in developing teeth (Kruzynska-Frejtag et al.,2004), and it regulates adhesion and migration of ovarian epithelial cells via its binding to the αvβ3 and αvβ5 integrins (Gillan et al.,2002; Yoshioka et al.,2002). Its involvement in oncogenesis was suspected when Yoshioka et al. (2002) found that periostin mRNA levels were significantly reduced in many human cancer cell lines and tissues, and that these cancer cell lines had reduced anchorage-independent growth when periostin levels were increased by infection with recombinant retrovirus. Also, it was determined that the C-terminal region and not the signal peptide containing N-terminal of periostin was sufficient to impart the growth-suppressive action of the protein. This finding was surprising, because periostin, a secreted molecule, was previously believed to mediate its effects exclusively in the extracellular environment. These findings make evident the possible importance of the differences between the isoforms PLF and periostin, which reside within the C-terminal region of the protein, and suggest that periostin and its isoforms may function within the cell to mediate its effects. Mouse periostin is located on chromosome 3 and human periostin on chromosome 13q. A pronounced increase in periostin was observed by Northern and Western blot analyses in rat carotid arteries after balloon injury (Lindner et al.,2005). mRNA transcripts were detected in smooth muscle cells in the neointima and adventitia (Lindner et al.,2005) and the protein was associated with smooth muscle cell (SMC) differentiation and migration in vitro. Similar findings on PLF in our laboratory (data not shown) suggest a role for periostin in vascular proliferative diseases such as restenosis by regulating vascular smooth muscle cell (VSMC) proliferation and migration. In response to the stress of hypoxia, periostin was increased in pulmonary arterial smooth muscle cells and the response was mediated through the P13K/p70Sk6, Ras/MEK1/2, and Ras/p38MAPK signaling pathways (Li et al.,2004). In general, it appears that periostin is upregulated in adult tissues under adverse conditions such as damage, overload, and/or stress. Its function under these conditions is not known. Periostin and βig-h3 have also been identified in the developing heart, specifically in the endocardial cushions (Kruzynska-Frejtag et al.,2001; Norris et al.,2004). Cushions are mesenchymal swellings that form in the walls of the ventricular inlets and outlets, which subsequently differentiate into valve leaflets and their supporting apparati (Mjaatvedt et al.,1999). Cushion mesenchymal cells have the potential to form bone, cartilage, and cardiac muscle (van den Hoff et al.,2001) but normally differentiate into fibrous connective tissue. Screening by DNA microarray revealed several gene candidates that might regulate the differentiation of cushion cells (Kruzynska-Frejtag et al.,2001). One of these was periostin. Periostin message was expressed primarily in cushions and their valvular derivatives during embryonic, fetal, and postnatal development (Kruzynska-Frejtag et al.,2001; Norris et al.,2004). As the known variants of periostin occur in the C-terminal domain (Takeshita et al.,1993), a polyclonal antibody prepared against a conserved N-terminal region (AA residues 123–142) enabled confirmation of RNA expression for all isoforms at the protein level (Kruzynska-Frejtag et al.,2001). Western blotting revealed that periostin can form disulfide-linked multimers, which may correlate to the fibrillar pattern of expression seen in developing valve leaflets (Kern et al.,2005). Based on both message and protein expression, periostin appears to be developmentally regulated. Periostin protein is associated initially with the surfaces of early-formed cushion cells but progressively becomes more organized into fibers as the cushions elongate and differentiate into valve leaflets (Kern et al.,2005). While fibrillar staining for periostin is visible in cushion cells cultured in a collagen matrix after 24 hr, it is more pronounced after 48 hr (Fig. 1). The mechanism for this increase in fibrillogenesis is unknown. Fibrillar staining is most intense in the tendinous cords of the valve supporting the atrioventricular (AV) apparatus, where bundles of periostin fibers appear to extend and ramify within the leaflets. Conversely, periostin is downregulated in the proximal outflow track, where cushions normally do not form valves but rather are myocardialized to form the muscular outlet septum. Uniquely, in the chick, periostin expression is also downregulated in the right atrioventricular lateral (mural) cushion, which correlates with myocardialization (muscularization) of this cushion to form a large muscular flap that acts as a tricuspid valve (perhaps an evolutionary adaptation to flight) (de La Cruz and Markwald,1998). Loss of periostin expression in cushions that do not form valves suggests that periostin may act to inhibit differentiation into myocardial cells (myocardialization) or the stability of a myocardial phenotype. The latter is suggested by the fate of myocardial cells that directly interface with cushion tissue. At such boundary interfaces, periostin expression is enhanced and, over time, the myocardial tissue is replaced by dense fibrous periostin-positive tissue that acts as the anchorage site (annuli fibrosae) for valve leaflets (Kern et al.,2005). How this subpopulation of myocardium is replaced is unknown. To date, there are no studies linking periostin directly to myocardial cell death or loss but recent studies have shown that periostin is upregulated several-fold at sites of pathological myocardial remodeling (Stanton et al.,2000; Katsuragi et al.,2004). Wang et al. (2003) reported a 40-fold increase in periostin mRNA in mouse hearts when subjected to pressure overload. In the later stages of valvulogenesis, strong periostin protein was also observed subendocardially, especially on surfaces associated with hemodynamic shear stress (Stekelenburg et al., 2005; Kern et al.,2005). Thus, modifications in periostin expression could have clinical implications that affect myocardial remodeling directly or possibly indirectly through altered hemodynamics. Stage 21 (Hamburger and Hamilton staging) AV cushion from chicken embryos grown in a collagen gel matrix for 24 (A) and 48 hr (B). Immunofluorescent staining with antiperiostin antibody is primarily nonfibrillar (A), but with time, fibrillar staining increases (B). Periostin and PLF are isoforms resulting from alternatively spliced RNAs. An analysis of the peptide differences between PLF and periostin and the exon-intron arrangement of the gene showed that nucleotides that code for the 673–699 aa region in mouse PLF (Mus musculus PLF) comprise exon 17 (present in periostin intron 16–17) on chromosome 3 and the nucleotides that code for 785–812 aa comprise exon 21 of periostin (Mus musculus periostin). A detailed comparison of the nucleotide and predicted amino acid sequence of periostin and PLF can be found in Litvin et al. (2004) and shows that differences in C-terminal peptides impart uniqueness to each isoform (Fig. 2). PLF is expressed during embryonic and neonatal heart development and at very low levels in the adult heart. Its level increases significantly under conditions of hemodynamic overload in adult cardiac myocytes and drops when the heart is unloaded by the left ventricular assist device (data not shown). While periostin is detected primarily in fibroblasts in the embryonic, neonatal, and adult heart, PLF is expressed primarily in neonatal and adult cardiac myocytes, suggesting a functional difference between the two isoforms (data not shown). We believe that PLF mediates cardiac disease through its interaction with β1-integrin (data not shown), possibly by changes in cell-cell interactions or adhesion to the extracellular matrix by outside-in signaling. Since it has a putative nuclear localization signal (Litvin et al.,2004), it may have additional functions mediated within the cell. Alignment of the predicted amino acid sequence in CDART-NCBI (Marchler-Bauer et al.,2005) of Mus musculus PLF (AY651928), Mus musculus periostin (NM 015784), Mus musculus βig-h3 (NM 009369), and Drosophila melanogaster fasciclin I (M32311). Fasciclin (repeat) domains are shown in bold and fas I domains are underlined. Diagram of PLF, periostin, βig-h3, and fasciclin I proteins. Fasciclin and Fas domains shown as red boxes and the signal peptide as a green box. The figure is a composite of data obtained by using CDART at the NCBI site (http://www.ncbi.nlm.nih.gov/Structure/lexington/lexington.cgi). The common features between periostin and PLF are their shared homologies and their expression as isoforms generated from a single gene. The biological significance of alternative splicing in the heart has recently become apparent in the studies by Xu et al. (2005) and Cooper (2005). The difference between the isoforms of periostin lies in their temporal and spatial pattern of expression, reflecting possibly different functions for each. Upregulation during development and/or in response to damage and their role in cell adhesion possibly through integrins suggests a function in tissue remodeling (Markwald et al.,2005). Modulating their expression and/or interaction with the integrins may lead to therapeutic alternatives during cardiovascular disease and/or oncogenesis.