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CONCISE REVIEW |
1 Department of Clinical Biochemistry, Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel; and
2 Division of Oral Medicine, Department of Oral/Maxillofacial Surgery and Diagnostic Sciences, University of Florida College of Dentistry, PO Box 100416, Gainesville, FL 32610-0416, USA;
* corresponding author, jkatz{at}dental.ufl.edu
| ABSTRACT |
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KEY WORDS: insulin-like growth factors (IGF) IGF-I receptor oral biology salivary gland Sjögrens syndrome
| THE IGF SYSTEM |
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| ENDOCRINE vs. LOCAL ACTIONS OF IGF FAMILY MEMBERS |
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| THE CELLULAR ROLE OF IGF-I |
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One of the most important aspects of IGF-I action that allow the peptide to function as a cell survival agent is its strong anti-apoptotic activity. The capacity of IGF-I to inhibit apoptotic death has been demonstrated in multiple cellular systems, including cerebellar granule neurons, pheochromocytoma cells, hemopoietic and erythroid colony-forming cells, etc. (DMello et al., 1993; Muta and Krantz, 1993). The critical determinant for cell survival proved to be the number of cell-surface IGF-IRs. The obvious implication of these findings is that activation of the IGF-IR may rescue from apoptosis cell populations that, in the absence of IGFs, are tagged for elimination.
| IGFS AND CANCER |
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The transforming potential of the IGF-IR is further illustrated by the results of experiments demonstrating that receptor overexpression in NIH3T3 cells resulted in a ligand-dependent, highly transformed phenotype (Kaleko et al., 1990). As a corollary to the important role of the IGF-IR in malignant transformation, several experimental protocols have been designed to target the IGF-IR as a potential therapeutic approach. These methodologies include the use of peptide analogues of IGF-I, antireceptor antibodies, antisense oligonucleotides, small interference RNA, and several chemical compounds, including specific tyrosine kinase inhibitors (Bohula et al., 2003; Mitsiades et al., 2004).
The central role of the IGF system in the initiation and progression of cancer is further highlighted by the finding that IGF-I constitutes a risk factor in several malignancies, including breast, prostate, lung, and colorectal cancer (Pollak, 2000).
| THE ROLE OF THE IGF SYSTEM IN GROWTH REGULATION OF SALIVARY GLAND CELLS |
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The effects of several growth factors and cytokines on the regulation of IGF system components in salivary-gland-derived cells were recently investigated by means of the HSG cell line. Treatment of HSG cells with tumor necrosis factor-
(TNF-
) and interferon-
(IFN-
) induced a significant decrease in cellular proliferation that was associated with increased accumulation of IGFBP-3 (Katz et al., 1995). When antibodies to IGFBP-3 were used in conjunction with TNF-
and IFN-
, the inhibitory effects of the cytokines on cell growth were diminished (Katz et al., 1999). Analysis of these data supports the hypothesis that inhibitory growth factors and cytokines may exert their biological actions, at least in part, via specific induction of IGFBP-3 and, potentially, other BPs. As a result of the increment in IGFBP levels, IGF-I is able to shift from the free to the IGFBP-3-bound form, with ensuing reduction in IGF-I-mediated mitogenic action.
In addition, the inhibitory effects of TNF-
and IFN-
on HSG cell proliferation were associated with a significant decrease in IGF-IR mRNA and protein levels, and with inhibition of IGF-IR promoter activity, suggesting that both cytokines affect IGF-IR gene expression at the transcriptional level (Shalita-Chesner et al., 2001).
| THE ROLE OF THE IGF SYSTEM IN TOOTH GROWTH AND DEVELOPMENT |
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The concept that IGF-I functions in a paracrine/autocrine fashion during tooth development has been demonstrated by immunohistochemistry and in situ hybridization methods (Joseph et al., 1996). IGF-I and IGF-IR were shown to be synthesized locally by tooth-forming cells. The distribution and intensity of the hybridization signal, however, varied with the developmental stage of the rat incisor. Thus, the cells of the apical loop expressed a positive hybridization signal, but the earliest polarized odontoblasts and pre-ameloblasts did not show any positive signal. The onset of enamel secretion was accompanied by a strong hybridization signal in the secretory ameloblasts as well as in the odontoblasts. The cells of the pulp and the dental follicle were consistently negative. However, in the adjacent alveolar bone, there was a strong signal in osteoblasts and osteoclasts.
The role of IGF-I in pulp healing and reparative dentinogenesis following pulp-capping was recently investigated (Lovschall et al., 2001). For this purpose, exposures were made through the mesial pulp horn in first upper molars in several two-month-old Wistar rats. The pulp was covered with methylcellulose gel containing or lacking IGF-I (400 ng). The exposure site was closed. After 3, 7, or 28 days, molars were decalcified and processed for histological analysis. Results obtained indicate that pulp-capping of rat molars by means of IGF-I enhanced reparative dentinogenesis in comparison with vehicle-treated controls. Furthermore, the effect of IGF-I addition, in combination with platelet-derived growth factor (PDGF), to calcium hydroxide in the repair of apical perforations was recently evaluated in a canine model (Kim et al., 2001). Fifty-one premolar teeth from 4 beagle dogs were used in this study. After periapical lesions developed, root apices were artificially perforated, following which the teeth were divided into 3 groups: (1) The apical perforations were not sealed; (2) the perforated areas were obturated with calcium hydroxide; and (3) sites of perforation were sealed with calcium hydroxide plus growth factors. After 12 wks, teeth were stained with hematoxylin/eosin and osteonectin. Analysis of the results showed that, in group 3, there was no inflammatory reaction of apical tissue, and the connective tissue adjacent to the newly formed hard tissue was strongly immunostained for osteonectin. In comparison, most sections in group 1 showed no apical healing, whereas moderate healing was found in group 2. Thus, these results demonstrate that the combination of IGF-I and PDGF with calcium hydroxide significantly improved healing of apical perforations in dogs.
| THE ROLE OF THE IGF SYSTEM IN PERIODONTAL STRUCTURES |
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Increasing experimental evidence supports the notion that IGF-I may be of great importance in periodontal regeneration. Two commonly used animal models for the evaluation of putative periodontal regenerative therapies are the beagle dog model with natural periodontal disease and the non-human primate with ligature-induced attachment loss. The results of experiments comparing the healing responses to periodontal surgery with and without concurrent use of IGF-I and PDGF showed that, at 1 mo, IGF-I/PDGF administration resulted in 64% and 51% increase in new attachment formation in the primate and canine models, respectively (Giannobile et al., 1994). In comparison, control animals (surgery plus placebo) demonstrated 34% and 8% increases, respectively, in new attachment formation in either model. Likewise, enhanced regeneration of both the soft and hard components of the periodontium following PDGF and IGF-I administration in beagle dogs has been reported (Lynch et al., 1991). Taken together, combined IGF-I/PDGF treatment appears to be of significant value in promoting periodontal wound healing.
| PATHOLOGICAL ASPECTS |
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To investigate the mechanisms of tissue destruction in experimental autoimmune sialadenitis (EAS), an animal model of Sjögrens syndrome, investigators assessed the expression of the IGF-IR in the submandibular glands of an NOD mouse with this disease (Mustafa et al., 2001). Receptor-binding autoradiography revealed that IGF-IR levels in submandibular glands were significantly reduced in NOD mice with EAS. Diminished IGF-IR levels may induce a defective glandular homeostasis in the submandibular gland epithelial cells and may thus trigger EAS.
The involvement of IGF-II in the etiology of salivary gland adenoma was inferred from studies showing that IGF-II transcripts are highly expressed in adenomas overexpressing PLAG1 (Voz et al., 2000). PLAG1 is a zinc finger gene that undergoes re-arrangement in pleomorphic adenoma of the gland with 8q12 translocations. In contrast, IGF-II transcripts are not detectable in adenomas without abnormal PLAG1 expression, or in normal salivary gland tissue. Hence, these results suggest that IGF-II is one of the PLAG1 target genes. Transcriptional activation of the IGF-II gene by PLAG1 may constitute an important mechanism in the initiation and/or progression of salivary gland tumors. Further support for a potential role of the IGF system in oral squamous cell carcinoma was recently provided by the results of a study showing that genetic variation at the IGF-II receptor locus may influence cancer risk (Zavras et al., 2003).
| SUMMARY |
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Received January 21, 2004; Last revision August 27, 2004; Accepted August 31, 2004
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