A constitutive endogenous osteopontin production is important for macrophage function and differentiation

Protective role of osteopontin in endodontic infection

Endodontic infections are polymicrobial infections resulting in bone destruction and tooth loss. The host response to these infections is complex, including both innate and adaptive mechanisms. Osteopontin (OPN), a secreted, integrin-binding protein, functions in the regulation of immune responses and enhancement of leucocyte migration. We have assessed the role of OPN in the host response to endodontic infection using a well-characterized mouse model. Periapical bone loss associated with endodontic infection was significantly more severe in OPN-deficient mice compared with wild-type 3 weeks after infection, and was associated with increased areas of inflammation. Expression of cytokines associated with bone loss, interleukin-1alpha (IL-1alpha) and RANKL, was increased 3 days after infection. There was little effect of OPN deficiency on the adaptive immune response to these infections, as there was no effect of genotype on the ratio of bacteria-specific immunoglobulin G1 and G2a in the serum of infected mice. Furthermore, there was no difference in the expression of cytokines associated with T helper type 1/type2 balance: IL-12, IL-10 and interferon-gamma. In infected tissues, neutrophil infiltration into the lesion area was slightly increased in OPN-deficient animals 3 days after infection: this was confirmed by a significant increase in expression of neutrophil elastase in OPN-deficient samples at this time-point. We conclude that OPN has a protective effect on polymicrobial infection, at least partially because of alterations in phagocyte recruitment and/or persistence at the sites of infection, and that this molecule has a potential therapeutic role in polymicrobial infections.

Osteopontin and the skin: multiple emerging roles in cutaneous biology and pathology

Osteopontin (OPN) is a glycoprotein expressed by various tissues and cells. The existence of variant forms of OPN as a secreted (sOPN) and intracellular (iOPN) protein and its modification through post-translational modification and proteolytic cleavage explain its broad range of functions. There is increasing knowledge which receptors OPN isoforms can bind to and which signaling pathways are activated to mediate different OPN functions. sOPN interacts with integrins and CD44, mediates cell adhesion, migration and tumor invasion, and has T helper 1 (Th1) cytokine functions and anti-apoptotic effects. iOPN has been described to regulate macrophage migration and interferon-alpha secretion in plasmacytoid dendritic cells. Both sOPN and iOPN, through complex functions for different dendritic cell subsets, participate in the regulation of Th cell lineages, among them Th17 cells. For skin disease, OPN from immune cells and tumor cells is of pathophysiological relevance. OPN is secreted in autoimmune diseases such as lupus erythematosus, and influences inflammation of immediate and delayed type allergies and granuloma formation. We describe that OPN is overexpressed in psoriasis and propose a model to study OPN function in psoriatic inflammation. Through cytokine functions, OPN supports immune responses against Mycobacteria and viruses such as herpes simplex virus. OPN is also implicated in skin tumor progression. Overexpression of OPN influences invasion and metastasis of melanoma and squamous cell carcinoma cells, and OPN expression in melanoma is a possible prognostic marker. As OPN protein preparations and anti-OPN antibodies may be available in the near future, in-depth knowledge of OPN functions may open new therapeutic approaches for skin diseases.

Osteopontin enhances phagocytosis through a novel osteopontin receptor, the alphaXbeta2 integrin

Osteopontin (OPN) is a cytokine with multiple functions, including immune defense mechanisms against invading microorganisms. OPN-deficient mice are impaired in clearing intracellular pathogens, suggesting an important role of OPN during phagocytosis, but it remains to be defined how OPN may enhance this innate immune process. Here, we demonstrate that OPN binds to monocytes, but not resting T cells, NK cells, or B cells, and mediates chemoattraction of IL-1-activated human monocytes. Moreover, OPN binds in a specific manner to all known serotypes of the two bacterial species Streptococcus agalactiae and Staphylococcus aureus and opsonizes these bacteria for phagocytosis. We identify the integrin alpha(X)beta(2) (CD11c/CD18), which is highly expressed on the cell surface of monocytes, as a novel OPN receptor. To eliminate the contribution from other molecular interactions between the bacteria and the phagocyte, we show that OPN-coated synthetic beads are phagocytosed in an alpha(X)beta(2) integrin-dependent manner. The ligand recognition does not involve the RGD motif previously reported to support binding of OPN to integrins. Taken together, these data identify the alpha(X)beta(2) integrin as a novel OPN receptor that is required for OPN-mediated phagocytosis, thereby elucidating an important mechanism of an innate immune function of OPN.

Osteopontin is not crucial to protective immunity during murine tuberculosis

Upon infection with Mycobacterium (M.) tuberculosis, the development of a strong T helper 1 (Th1)-mediated adaptive immune response is considered as being most important for containment of the infection. Osteopontin (OPN) is a phosphorylated glycoprotein that is chemotactic for inflammatory cells and has been implicated in the induction of Th1 responses and granulomatous disease. We tested the hypothesis that OPN facilitates protective immunity during M. tuberculosis infection using wild-type (WT) and OPN knockout (KO) mice in a model of pulmonary tuberculosis. OPN expression was up-regulated in alveolar macrophages and lymphoid cells during M. tuberculosis infection. There were no significant differences in bacterial outgrowth, inflammation or recruitment of lymphocytes, macrophages and polymorphonuclear cells in the lungs after 2 and 5 weeks of infection. However, the numbers of CD4(+) and CD8(+) T cells were reduced in the absence of OPN 5 weeks after infection. Similar concentrations of cytokine were observed in lungs from both WT mice and OPN KO mice; however, there was a trend towards decreased levels of interferon-gamma (IFN-gamma) in OPN KO mice 5 weeks after infection. Despite an unaltered immune response in the early phase of tuberculosis, OPN KO mice had a modest survival advantage. Of note, both pulmonary bacterial loads and lung inflammation were reduced in these mice 31 weeks after infection. These data suggest that OPN is not crucial for protective immunity upon M. tuberculosis infection and during the late phase of tuberculosis may even be detrimental for the host.

Deletion of the alpha8 integrin gene does not protect mice from myocardial fibrosis in DOCA hypertension

BACKGROUND

In the heart, the alpha8 integrin chain is expressed in fibroblasts and vascular smooth-muscle cells but its functional role in the myocardium is unknown. Integrins can contribute to tissue fibrosis in several organs. We tested the hypothesis that alpha8 integrin-mediated cell-matrix interactions add to cardiac fibrotic alterations during hypertension.

METHODS

Desoxycorticosterone-acetate (DOCA)-salt hypertension was induced in mice homozygous for a deletion of the alpha8 integrin chain and wild-type mice. Histological and immunohistochemical evaluations were performed in heart tissue.

RESULTS

Blood pressure was slightly higher in DOCA-treated alpha8 integrin-deficient mice compared to DOCA-treated wild types. Expression of alpha8 integrin and its ligands fibronectin and osteopontin was increased in the hearts of DOCA-treated wild types compared to salt-loaded controls. However, relative left ventricular weights did not differ between DOCA-treated wild types and alpha8 integrin-deficient mice. Moreover, expansion of collagen I immunoreactivity and cell proliferation was similar in both groups. The number of osteopontin-positive cells was not different in DOCA-treated alpha8 integrin-deficient and DOCA-treated wild-type mice. Despite of a comparable degree of fibrosis in both groups, alpha-smooth-muscle actin and discoidin domain receptor 2 (DDR2)-positive myofibroblasts were only detected in wild-type DOCA-treated mice, not in DOCA-treated alpha8 integrin-deficient mice.

CONCLUSIONS

The results show that lack of alpha8 integrin does not reduce fibrotic changes in the hearts of DOCA-salt hypertensive mice. Our findings do not argue for a profibrotic effect of an increased alpha8 integrin expression in the myocardium in hypertension.

Elevated expression of osteopontin may be related to adipose tissue macrophage accumulation and liver steatosis in morbid obesity

OBJECTIVE

Osteopontin (OPN) plays an important role in the development of insulin resistance and liver complications in dietary murine models. We aimed to determine the expression pattern of OPN and its receptor CD44 in obese patients and mice according to insulin resistance and liver steatosis.

RESEARCH DESIGN AND METHODS

OPN and CD44 expressions were studied in 52 morbidly obese patients and in mice. Cellular studies were performed in HepG2 cells.

RESULTS

Hepatic OPN and CD44 expressions were strongly correlated with liver steatosis and insulin resistance in obese patients and mice. This increased OPN expression could be due to the accumulation of triglycerides, since fat loading in HepG2 promotes OPN expression. In contrast, OPN expression in adipose tissue (AT) was enhanced independently of insulin resistance and hepatic steatosis in obese patients. The elevated OPN expression in AT was paralleled with the AT macrophage infiltration, and both phenomena were reversed after weight loss. The circulating OPN level was slightly elevated in obese patients and was not related to liver steatosis. Further, AT did not appear to secrete OPN. In contrast, bariatric surgery-induced weight loss induced a strong increase in circulating OPN.

CONCLUSIONS

The modestly elevated circulating OPN levels in morbidly obese patients were not related to liver steatosis and did not appear to result from adipose tissue secretion. In subcutaneous AT, expression of OPN was directly related to macrophage accumulation independently from liver complications. In contrast, hepatic OPN and CD44 expressions were related to insulin resistance and steatosis, suggesting their local implication in the progression of liver injury.

In vivo osteopontin-induced macrophage accumulation is dependent on CD44 expression

In order to assess the role of osteopontin (OPN) in leukocyte accumulation in inflammatory conditions, native OPN and its thrombin cleaved form (OPN+Thr) were studied in vivo using a rodent subcutaneous air pouch model (AP). Both forms of OPN-induced macrophage infiltration into the AP in wild-type mice. In animals lacking CD44, macrophage numbers were significantly reduced within the cavity, but cells still accumulated along the subcutaneous lining. In animals lacking endogenous OPN, no differences were found in exogenous OPN-induced macrophage accumulation, although macrophage exhibited increased alpha4 integrin expression. These studies reveal that both OPN and OPN+Thr attract macrophages in vivo through CD44.

Osteopontin mediates obesity-induced adipose tissue macrophage infiltration and insulin resistance in mice

Obesity is associated with a state of chronic, low-grade inflammation characterized by abnormal cytokine production and macrophage infiltration into adipose tissue, which may contribute to the development of insulin resistance. During immune responses, tissue infiltration by macrophages is dependent on the expression of osteopontin, an extracellular matrix protein and proinflammatory cytokine that promotes monocyte chemotaxis and cell motility. In the present study, we used a murine model of diet-induced obesity to examine the role of osteopontin in the accumulation of adipose tissue macrophages and the development of insulin resistance during obesity. Mice exposed to a high-fat diet exhibited increased plasma osteopontin levels, with elevated expression in macrophages recruited into adipose tissue. Obese mice lacking osteopontin displayed improved insulin sensitivity in the absence of an effect on diet-induced obesity, body composition, or energy expenditure. These mice further demonstrated decreased macrophage infiltration into adipose tissue, which may reflect both impaired macrophage motility and attenuated monocyte recruitment by stromal vascular cells. Finally, obese osteopontin-deficient mice exhibited decreased markers of inflammation, both in adipose tissue and systemically. Taken together, these results suggest that osteopontin may play a key role in linking obesity to the development of insulin resistance by promoting inflammation and the accumulation of macrophages in adipose tissue.

Osteopontin expression in normal skin and non-melanoma skin tumors

Osteopontin (OPN) is an adhesive, matricellular glycoprotein, whose expression is elevated in many types of cancer and has been shown to facilitate tumorigenesis in vivo. To understand the role of OPN in human skin cancer, this study is designed to determine whether OPN is expressed in premalignant [solar/actinic keratosis (AK)] and in malignant skin lesions such as squamous cell carcinomas (SCC) and basal cell carcinomas (BCC), as well as in normal skin exposed or not exposed to sunlight. Immunohistochemical analyses showed that OPN is expressed in SCC (20/20 cases) and in AK (16/16 cases), which are precursors to SCC, but is absent or minimally expressed in solid BCC (17 cases). However, positive staining for OPN was observed in those BCC that manifest differentiation toward epidermal appendages such as keratotic BCC. In sunlight-exposed normal skin, OPN is minimally expressed in the basal cell layer, but in contrast to those not exposed to sunlight, OPN is more prominent in the spinous cell layer with increasing intensity toward the granular cell layer. Additionally, OPN is expressed in the hair follicles, sebaceous glands, and sweat glands of normal skin. In conclusion, these data suggest that OPN is associated with keratinocyte differentiation and that it is expressed in AK and SCC, which have metastatic potential, but minimally expressed in solid BCC.

Pathophysiological role of osteopontin in hepatic inflammation, toxicity, and cancer

Osteopontin (OPN) is a highly modified integrin-binding extracellular matrix glycophosphoprotein produced by cells of the immune system, epithelial tissue, smooth muscle cells, osteoblasts, and tumor cells. Extensive research has elucidated the pivotal role of OPN in cell signaling that controls inflammation, tumor progression, and metastasis. OPN interaction with the integrin receptors expressed on inflammatory cells through its arginine-glycine-aspartate (RGD) and non-RGD motifs promote migration and adhesion of cells. In the liver, it has been reported that hepatic Kupffer cells secrete OPN facilitating macrophage infiltration into necrotic areas following carbon tetrachloride liver toxicity. Recent work has highlighted the role of OPN in inflammatory liver diseases such as alcoholic and nonalcoholic liver disease and T-cell-mediated hepatitis. The role of OPN in hepatocellular carcinoma (HCC) has also generated significant interest, especially with regards to its role as a prognostic factor. OPN therefore appears to play an important role during liver inflammation and cancer. In this review we will present data to demonstrate the key role played by OPN in mediating hepatic inflammation (neutrophils, monocytes/macrophages, and lymphocytes) and its role in HCC. Greater understanding of the pathophysiologic role of OPN in hepatic inflammation and cancer may enable development of novel inflammation and cancer treatment strategies.