Role of cyclin E and cyclin E-dependent kinase in mitogenic stimulation by cementum-derived growth factor in human fibroblasts

Are Cementoblasts a Subpopulation of Osteoblasts or a Unique Phenotype?

Experimental studies have shown a great potential for periodontal regeneration. The limitations of periodontal regeneration largely depend on the regenerative potential at the root surface. Cellular intrinsic fiber cementum (CIFC), so-called bone-like tissue, may form instead of the desired acellular extrinsic fiber cementum (AEFC), and the interfacial tissue bonding may be weak. The periodontal ligament harbors progenitor cells that can differentiate into periodontal ligament fibroblasts, osteoblasts, and cementoblasts, but their precise location is unknown. It is also not known whether osteoblasts and cementoblasts arise from a common precursor cell line, or whether distinct precursor cell lines exist. Thus, there is limited knowledge about how cell diversity evolves in the space between the developing root and the alveolar bone. This review supports the hypothesis that AEFC is a unique tissue, while CIFC and bone share some similarities. Morphologically, functionally, and biochemically, however, CIFC is distinctly different from any bone type. There are several lines of evidence to propose that cementoblasts that produce both AEFC and CIFC are unique phenotypes that are unrelated to osteoblasts. Cementum attachment protein appears to be cementum-specific, and the expression of two proteoglycans, fibromodulin and lumican, appears to be stronger in CIFC than in bone. A theory is presented that may help explain how cell diversity evolves in the periodontal ligament. It proposes that Hertwig’s epithelial root sheath and cells derived from it play an essential role in the development and maintenance of the periodontium. The role of enamel matrix proteins in cementoblast and osteoblast differentiation and their potential use for tissue engineering are discussed.

Revealing the mechanism of in vitro wound healing properties of Citrus tamurana extract

In the present investigation, we examined the effect of Hyuganatsu (Citrus tamurana) extract (HE) on skin fibroblast (TIG-119) proliferation and migration during in vitro wound healing. HE selectively inhibited proliferation of TIG-119 cells at higher concentration (>1.0 mg/mL); at lower concentrations (0.1, 0.25, 0.5, and 0.75 mg/mL), it exhibited linear and time-dependent cell proliferation. In vitro scratch wound healing studies showed that the HE also accelerated the migration of cells towards the wounded region. Cytometric analysis demonstrated that HE extract did not alter G1/0 and S phases of cell cycle in any concentration studied; however, G2/M phases of cell cycle were significantly (P < 0.05) accelerated at 0.75 mg/mL dose. RT-PCR and Western blotting analysis indicated that HE markedly overexpressed levels of Rac-1, Rho-A, and Cdc-42 mRNA and the respective proteins. Cyclin-dependent kinases (Cdk-1 and -2) gene expression activity was significantly (P < 0.05) increased, but protein content decreased during treatment with HE. The induction of Cdk-1 and -2 by HE was abolished by inhibitors, transcription (DRB), and translation (CHX), implying transcriptional regulation that required de novo protein synthesis.

Nonsteroidal anti-inflammatory drug effects on osteoblastic cell cycle, cytotoxicity, and cell death

Previous studies indicated that nonsteroidal anti-inflammatory drugs (NSAIDs) suppress bone repair, growth, and remodeling in vivo. Our previous in vitro study demonstrated that indomethacin and ketorolac inhibited osteoblast proliferation. In this study, we further investigated the influences of 4 NSAIDs on cell cycle kinetics, cytotoxicity, and cell death pattern in osteoblast cultures from rat fetal calvaria. Our results showed that NSAIDs significantly arrested cell cycle at the G(0)/G(1) phase and induced cytotoxicity and cell death of osteoblasts. Apoptosis was more pronounced than necrosis caused by NSAIDs. Among these NSAIDs, piroxicam showed the least effect to produce osteoblastic dysfunction. Moreover, we found that the cytotoxic and apoptotic effects of NSAIDs on osteoblasts might not be prostaglandin related. These results suggest that the NSAID effects on cell cycle arrest and cell death induction in osteoblasts may be one of the important mechanisms contributing to their suppressive effect on bone formation.

Immunohistochemical localization of components of the insulin-like growth factor-system in human deciduous teeth

To investigate the occurrence of components of the insulin-like growth factor (IGF) system during the resorption process of shedding human deciduous teeth, we investigated sections of 13 decalcified and paraffin-embedded deciduous teeth immunohistochemically with antibodies against IGF-I and -II, six IGF binding proteins (IGFBPs 1-6) and the IGF receptors IGF1R and IGF2R. The teeth were in different stages of resorption and all showed reparative cementum formation. It was found that acellular extrinsic fiber cementum, reversal lines and reparative cellular intrinsic fiber cementum were immunoreactive for both IGFs and various IGFBPs. Therefore, in human deciduous teeth, all subgroups of cementum, but not dentine, may represent sources of components of the IGF system. Odontoclasts did not carry IGFs or the IGF1R, but IGFBPs and the IGF2R. Therefore, these cells, in contrast to osteoclasts, may not respond to IGFs, but may be involved in the release and sequestration of IGFs from cementum during the resorption process. In contrast to odontoclasts, cementoblasts and periodontal ligament (PDL) fibroblasts carried IGF1R. The influence of the IGF system on the function of these cells with respect to periodontal matrix turnover and cementogenesis is discussed. On the behalf of the IGFBP immunoreactivities found, the PDL extracellular matrix can be considered to be a reservoir for IGF system components, where binding proteins may regulate IGF distribution and activity.

Role of D1 and E cyclins in cell cycle progression of human fibroblasts adhering to cementum attachment protein

Cementum attachment protein (CAP) is a collagenous protein present in the matrix of tooth cementum that mediates preferential attachment of some mesenchymal cell types, and CAP binding capacity is related to mineralizing tissue-forming capacity in culture. We have examined if adhesion to surfaces containing CAP as the only attachment protein permits human fibroblasts to escape G1 arrest and synthesize DNA, and if adhesion to CAP modulates the levels of cyclins D1 and E. Human gingival fibroblasts (HGFs) were serum-starved, trypsinized, and added to plates coated with CAP or bovine serum albumin (BSA). Cells were then exposed to either 10% fetal bovine serum (FBS) or to cementum-derived growth factor (CGF), an insulin-like growth factor I (IGF-I)-like molecule sequestered in tooth cementum, plus epidermal growth factor (EGF). DNA synthesis was measured as [3H]thymidine uptake, and cyclin D1 and E levels were determined by Western analysis. Cyclin E-dependent kinase (Cdk) activity was assessed in terms of H1 kinase activity in immunoprecipitates of cyclin E. Cells adhering to CAP synthesized DNA, whereas on BSA they remained unattached and did not synthesize DNA. Protein levels of cyclin D1 were higher in cells adhering to CAP in the absence and presence of growth factors. Cyclin E levels were not affected by adhesion alone, but they increased in the presence of growth factors. Cyclin E-associated kinase activity was higher in cells adherent on CAP, and it increased further in the presence of growth factors. Our results indicate that adhesion to CAP increases cyclin D1 levels and cyclin E-associated Cdk activity, and that these increases contribute to cell cycle progression. We previously observed that the signaling reactions induced during adhesion are characteristic of the CAP; together these observations indicate that specific matrix components present in the local environment can contribute to recruitment and differentiation of specific cell types for normal homeostasis and wound healing.

Cyclin E in human cancers

Regulators of the cell cycle such as cyclin E play an important part in neoplasia. The cyclin E protein forms a partnership with a specific protein kinase. This complex phosphorylates key substrates to initiate DNA synthesis. Cyclin-dependent kinase inhibitors (CKIs) are able to suppress the activity of cyclin E. Various substances (including proteins produced by oncogenic viruses) affect cyclin E directly or indirectly through an interaction with CKIs. These interactions are important in elucidating the mechanisms of neoplasia. They may also provide prognostic information in a wide range of common cancers. Cyclin E may even be a target for treatment of cancers in the future.