increase in [
3
H]dT incorporation into DNA, indicative of an enhanced cell proliferation compared to uncoated substrates (Wiens et al. 2010b). Calculation of the ratio
of cell proliferative activity and biomineralization activity for SaOS-2 cells grown
on silicatein/biosilica-coated Ca-P cover slips, uncoated Ca-P cover slips, and glass
cover slips revealed the highest ratio for cells grown on biosilica-coated Ca-P cover
slips; the lowest ratio was obtained for the glass cover slip cultures (Wiens et al.
2010b). These results provide evidence that enzymatically formed biosilica has a
mitogenic effect on bone forming SaOS-2 cells (Wiens et al. 2010b).
10.11 Effect of Biosilica on Gene Expression
The beneficial effects of orthosilicic acid/biosilica on bone metabolism have also
been demonstrated in studies of gene expression and in enzymatic studies. In
human osteoblast-like cells, orthosilicic acid was found to enhance the expression
of several key proteins involved in bone formation, including bone morphogenetic
protein-2 (BMP-2; Gao et al. 2001) and collagen type-I (COL1; Reffitt et al. 2003).
BMP-2 expression is required for differentiation of osteoblasts (Tanaka et al. 2001;
Fromigue et al. 2006; Li et al. 2007). Besides collagen type I, orthosilicic acid has
been shown to regulate the expression of alkaline phosphatase and osteocalcin
mRNA in human bone-derived osteoblasts (Arumugam et al. 2006). In SaOS-2
cells, biosilica increases the expression of amelogenin and enamelin which are
involved in enamel formation (M€ uller et al. 2007a). Administration of soluble
silicate to mice has been shown to affect, in addition to BMP-2 and collagen type I,
the expression of Runx-2 (runt-related transcription factor 2) which is involved in
the control of skeletal gene expression (Stein et al. 2004), as well as the expression
of OPG and RANKL (Maehira et al. 2008, 2009). Furthermore, silicic acid causes a
stimulation of prolyl hydroxylase activity which is involved in collagen synthesis
(Carlisle and Alpenfels 1980, 1984; Carlisle and Garvey 1982; Carlisle and Suchil
1983; Carlisle et al. 1981). Hence, soluble silicon causes a significant increase in
hydroxyproline content in mice (Maehira et al. 2009). In the tibia of silicondeficient rats, the amount of hydroxyproline was significantly lower than in silicon-supplemented animals (Seaborn and Nielsen 2002). In addition, silicon deprivation in rats results in a decreased formation of collagen in bone and a lower
activity of ornithine transaminase (involved in proline synthesis) in liver (Seaborn
and Nielsen 2002). Moreover, zeolite-A, a Si-containing compound, has been
reported to stimulate proliferation, differentiation, and protein synthesis in human
osteoblast-like cells, and to increase the production of transforming growth-factor-b
(TGF-b) in these cells (Brady et al. 1991; Keeting et al. 1992)
The effects of silicon at different stages of collagen formation and mineralization are summarized in Fig. 10.6. In addition to its effect on gene expression
and posttranslational modification (formation of hydroxyproline, mediated by
prolyl hydroxylase; see above), silicic acid affects the assembly and mineralization
of collagen fibrils. Low concentrations of orthosilicic acid promote the collagen
296
H.C. Schr€ oder et al.
3
H]dT incorporation into DNA, indicative of an enhanced cell proliferation compared to uncoated substrates (Wiens et al. 2010b). Calculation of the ratio
of cell proliferative activity and biomineralization activity for SaOS-2 cells grown
on silicatein/biosilica-coated Ca-P cover slips, uncoated Ca-P cover slips, and glass
cover slips revealed the highest ratio for cells grown on biosilica-coated Ca-P cover
slips; the lowest ratio was obtained for the glass cover slip cultures (Wiens et al.
2010b). These results provide evidence that enzymatically formed biosilica has a
mitogenic effect on bone forming SaOS-2 cells (Wiens et al. 2010b).
10.11 Effect of Biosilica on Gene Expression
The beneficial effects of orthosilicic acid/biosilica on bone metabolism have also
been demonstrated in studies of gene expression and in enzymatic studies. In
human osteoblast-like cells, orthosilicic acid was found to enhance the expression
of several key proteins involved in bone formation, including bone morphogenetic
protein-2 (BMP-2; Gao et al. 2001) and collagen type-I (COL1; Reffitt et al. 2003).
BMP-2 expression is required for differentiation of osteoblasts (Tanaka et al. 2001;
Fromigue et al. 2006; Li et al. 2007). Besides collagen type I, orthosilicic acid has
been shown to regulate the expression of alkaline phosphatase and osteocalcin
mRNA in human bone-derived osteoblasts (Arumugam et al. 2006). In SaOS-2
cells, biosilica increases the expression of amelogenin and enamelin which are
involved in enamel formation (M€ uller et al. 2007a). Administration of soluble
silicate to mice has been shown to affect, in addition to BMP-2 and collagen type I,
the expression of Runx-2 (runt-related transcription factor 2) which is involved in
the control of skeletal gene expression (Stein et al. 2004), as well as the expression
of OPG and RANKL (Maehira et al. 2008, 2009). Furthermore, silicic acid causes a
stimulation of prolyl hydroxylase activity which is involved in collagen synthesis
(Carlisle and Alpenfels 1980, 1984; Carlisle and Garvey 1982; Carlisle and Suchil
1983; Carlisle et al. 1981). Hence, soluble silicon causes a significant increase in
hydroxyproline content in mice (Maehira et al. 2009). In the tibia of silicondeficient rats, the amount of hydroxyproline was significantly lower than in silicon-supplemented animals (Seaborn and Nielsen 2002). In addition, silicon deprivation in rats results in a decreased formation of collagen in bone and a lower
activity of ornithine transaminase (involved in proline synthesis) in liver (Seaborn
and Nielsen 2002). Moreover, zeolite-A, a Si-containing compound, has been
reported to stimulate proliferation, differentiation, and protein synthesis in human
osteoblast-like cells, and to increase the production of transforming growth-factor-b
(TGF-b) in these cells (Brady et al. 1991; Keeting et al. 1992)
The effects of silicon at different stages of collagen formation and mineralization are summarized in Fig. 10.6. In addition to its effect on gene expression
and posttranslational modification (formation of hydroxyproline, mediated by
prolyl hydroxylase; see above), silicic acid affects the assembly and mineralization
of collagen fibrils. Low concentrations of orthosilicic acid promote the collagen
296
H.C. Schr€ oder et al.