Proliferation was assessed by two different approaches: (1) [^HJthymidine incorporation into newly synthesised DNA of proliferating cells; and (2) measuring total nucleic acid content of cultured cells, using a protocol based on the ‘FIuoReporter® Blue Fluorometric dsDNA Quantification Kit (F-2962)’ (Molecular Probes, Netherlands), which analyses total cellular DNA vvdth the blue-fluorescent ‘Hoechst 33258’ nucleic acid stain. The bisbenzimidazole derivative ‘Hoechst 33258’ exhibits fluorescence enhancement upon binding to A-T rich regions of double stranded DNA, which is enhanced under high ionic strength conditions. The procedure is rapid, and all manipulations can be carried out in microplate wells on cells lysed by freezing in distilled water.
[^H]thymidine incorporation
Primary rat osteoblastic cells were obtained and cultured till confluence as described above. At confluence, cells were subcultured into 24-well plates at a density of lO"^/ well in DMEM supplemented with 5% FBS. Cells were maintained for 24 h in this medium before replacement with the same medium containing nucleotides (adenosine, ADP, ATP, UTP 1-100 pM), and cultured for 2 or 5 d. Cells were labelled with IpCi/ml of [3H]thymidine (Amersham International, UK) for the final 6 h o f culture. Cell layers were then washed three times with PBS containing ImM unlabelled thymidine, treated with 0.75% (w/v) trypsin and 0.03% EDTA (w/v) for 5 min at 37°C, and precipitated
twice at 4°C overnight with 7.5% trichloracetic acid in the presence of 0.2% (w/v) bovine serum albumin (all Sigma, UK), before final digestion with 0.2 M sodium hydroxide (NaOH) and liquid scintillation counting.
DNA quantification using Hoechst 33258
Primary rat osteoblastic cells were obtained and cultured till confluence as described above. At confluence, cells were subcultured into 48-well plates at a density o f 10"^ or 2 x 10"^/well in DMEM, rendered quiescent by varying methods (see results section), and treated with nucleotides (adenosine, ADP, ATP, UTP, at 0.5 - 500 pM) for 2 - 4 d. To measure total DNA, cells were treated according to the manufacturer’s protocol for ‘FIuoReporter® Blue Fluorometric dsDNA Quantification Kit (F-2962)’ (Molecular Probes, Netherlands). Briefly, plates were emptied at the desired endpoints by overturning onto paper towels, stored at -80°C (up to 4 weeks), and thawed to room temperature. 450 pi distilled water was added per well, incubated at 37°C for 1 h, placed at -80°C until frozen, and then thawed to room temperature. 450 pi o f aqueous Hoechst 33258 (stock solution 5mg/ml in 20% dimethyl sulfoxide (DMSO) (v/v)), diluted 1:500 in TNE buffer (10 mM Tris, 2 M NaCl, 1 mM EDTA, pH 7.4) were added. Fluorescence, using excitation and emission filters centered at 360 nm and 460 nm, respectively, was measured on a fluorescence plate reader. To obtain a standard curve, a titration for known amounts of calf thymus DNA (stock solution 10 pg/ml) (Sigma, UK), diluted in TE buffer (10 mM Tris base, 1 mM EDTA, pH 7.4) to a range of 0 - 2500 ng DNA, was performed using the same protocol. A linear regression curve was fit to the values (regression coefficient: 0.979) (using ‘Sigma Plot’ version 1.0, Jandel Scientific), and DNA concentrations for experimental fluorescence values were interpolated. The standard curve is shown in Figure 4.1 (Fig. 4.1).
3500 3000 § S 2500 i 0) g 2000
i,„
u_ 1 0 0 0 - • Ç ' 500 0 500 1000 1500 2000 2500 Total DNA (ng) Figure 4.1Standard curve for DNA quantification using Hoechst 33258 dye.
Fluorescence values were plotted against known amounts o f calf thymus DNA (0-2500 ng), and a linear regression curve was fit to the values, using ‘Sigma Plot’.
RESULTS
Adenosine, ADP, ATP, UTP and 2-meSADP, at concentrations between 0.1 and 500 |j.M, were tested for their effects on bone nodule formation and osteoblast proliferation.
Bone nodule assay
Cell clusters typically started to appear first at around day 3-6, and became denser and mineralised during the course o f the culture, until the assay was usually stopped at around day 16-20. Examples of the appearance o f bone nodules during culture and stained with alizarin red, at increasing magnifications, are shown in Figure 4.2 (Fig. 4.2).
ATP and UTP had an inhibitory effect on bone nodule formation, both on the number of nodules and the total area o f nodules/well. Strong inhibitory effects, as was the case for ATP and UTP, could be easily observed by naked eye (Fig. 4.2 A). As illustrated in Figure 4.3, the inhibitory effect of ATP started at 10 pM and was maximal at 100 pM (Fig. 4.3), whereas UTP started to inhibit bone nodule formation at concentrations as low as 1 pM, again with maximal inhibition at 100 pM (Fig. 4.4).
In contrast, neither adenosine (Fig. 4.5), nor ADP (Fig. 4.6), nor 2-meSADP were found to have an effect on bone nodule formation, suggesting that ATP, and no further degradation products, exerts the effect.
Figure 4.2
Appearance of bone nodules in culture, and stained with alizarin red.
(A) Picture o f a 24-well plate, with two control rows and four rows treated with ATP from 0.1 to 100 pM (see labels); inhibitory effects of ATP on bone nodule formation were clearly visible by naked eye. (B) Appearance o f a single well, as photographed before image analysis. (C, D, E) Appearance o f bone nodules, formed by osteoblasts in culture and stained with alizarin red, at increasing magnification x2 (C), x5 (D), xlO (E). (F, G) Osteoblasts showed first signs o f bone nodule formation after ~5 d in culture by forming discrete multi-layered cell aggregations. Scale bars = 2 cm (A), 1 mm (B,C), 200 pM (D,E,F,G). (F,G) kindly provided by Siva Mahendran.
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Figure 4.3Inhibitory effect of ATP on bone nodule formation.
Concentrations o f ATP > 10 [lM significantly inhibited number of bone nodules and total area of bone nodules/well. Values are means ± SEM (n = 6). Significantly different from control: *** p < 0.001.
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to Figure 4.4Inhibitory effect of UTP on bone nodule formation.
Concentrations of UTP > 1 |iM significantly inhibited number o f bone nodules and total area o f bone nodules/well. Values are means ± SEM (n = 6). Significantly different from control: * p < 0.05, * * p < 0.01, *** p < 0.001.
600 ^ 400 0) o m 200 o E 3 Z 0.2 1 5 25 Adenosine (pIVI) 125 10 I
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Figure 4.5Lack of effect of adenosine on bone nodule formation.
Adenosine at concentrations between 0.2 - 125 jiM had no effect on number of bone nodules and total area of bone nodules/well. Values are means ± SEM (n = 6).
600 400 i
i
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0) § CO 200 0) .Q E 3 1 5 A D P (nM ) 25 125 g’ 3 (D z R c 3 Figure 4.6Lack of effect of ADP on bone nodule formation.
ADP at concentrations between 1 - 1 2 5 )iM had no effect on number o f bone nodules and total area of bone nodules/well. Values are means ± SEM (n = 6).
Proliferation assays
Effects o f nucleotides on proliferation of primary rat calvarial osteoblasts were inconsistent and changed from culture to culture. Although a variety o f cell culture conditions were tried by changing (1) the exposure time to nucleotides, (2) the total time in culture, (3) two independent methods to assess DNA synthesis and (4) different conditions to render the cells quiescent, no consistent effects were observed. Even when slight effects on proliferation (both inhibitory or stimulatory) were seen, they were only evident at relatively high concentrations (~ 100 pM). Thus, the biological significance of the proliferation studies is questionable in contrast to the clear-cut strong inhibitory effects o f low-dose nucleotides on bone nodule formation. However, for completeness, some representative results are summarised below.
[^H]thymidine incorporation
Using [^H]thymidine incorporation methods to study osteoblast proliferation, UTP at 100 pM seemed to have a mitogenic effect when osteoblasts were cultured for 2 d (Fig. 4.7 A). However, after 5 d of culture, a different pattern was seen: UTP had no effect on the number o f proliferating cells, whereas both adenosine and ATP, at 100 pM, exerted a weak inhibitory effect on cell proliferation, and ADP at 100 pM slightly stimulated proliferation (Fig. 4.7 B).
DNA quantification using Hoechst 33258
Examples o f two sets of experiments using the DNA quantification assay with the fluorescent DNA stain ‘Hoechst 33258’ are summarised in the following two tables
(Tables 4.1, 4.2). Table 4.1 shows results for one set o f experiments where cells were subcultured at a density of lOVwell into 48-well plates, rendered quiescent with DMEM/0.1% FBS for 48 h, before test media were added for a further 48 h. No treatment changed the total amount of DNA.
100 (^M) B ] Adenosine V y ///A ADP ^ 1 ATP UTP 100 (^iM) Figure 4.7 [^H]thymidine incorporation
(A, B) Effects of nucleotides on cell proliferation o f primary rat calvarial osteoblasts cultured for 2 days (A) or 5 days (B). After 2 days, UTP slightly stimulated proliferation (A), whereas after 5 days, adenosine and ATP exerted a slight inhibitory effect and ADP was slightly mitogenic (B) (all at 100 pM). Values are means ± SEM (n = 6).
Concentration Adenosine Mean ±'SËM Total DN/ ADP Mean ± SEM ^ Vwell (ng) Mean ±SEM Control 1821 ±43.5 2013 ±51.8 1921 ±25.1 1969 ±30.8 0.5 1784 ±33.6 1952 ±53.6 1868 ±34.0 1909 ±40.0 5 1768 ±78.5 1932 ±42.3 1880 ± 51.8 1942 ±28.0 Control 1818 ±28.8 1956 ±36.5 1881 ± 54.1 1942 ±39.6 50 1787 ±24.9 1916 ±66.1 1831 ± 50.5 1903 ±41.3 500 1713 ±32.9 1830 ±48.2 1752 ±37.2 1879 ±27.2 Table 4.1
Effects of nucleotides on rat osteoblast proliferation I.
Cells were cultured for 48 h (2 d) in test and control media (DMEM/0.1% FBS), after being rendered quiescent for 48 h in DMEM/0.1% FBS. Proliferation assessed by DNA quantification with Hoechst 33258. Values expressed as means ± SEM of total ng DNA/well (n - 8).
Table 4.2 shows results for a second set o f experiments, where cells were subcultured at 10“^ cells/well, cultured in DMEM/5% FBS for 24 h, before nucleotides were added in DMEM/5% FBS. The assay was stopped after 48 h (2 d). The only slight effects were observed with 100 pM adenosine and ATP, which resulted in a reduction of cell number
(Table 4.2). T otal DNA
A D P ^ ^ #
Jwell (ng)
UTP ^
Mean ± SEM Control 2557 ± 40.3 2359 ± 37.2 2523 ±11.5 2416 ±22.8 1 2399 ±64.1 2428 ± 48.5 2473 ±20.1 2423 ± 23.6 10 2340 ±51.8 2456 ±31.5 2513 ±44.4 2405 ± 9.4 100 2305 ±64.3 2290 ± 69.9 2218 ±25.7 2399 ± 25.9 Table 4.2Effects of nucleotides on rat osteoblast proliferation II.
Cells were cultured for 48 h (2 d) in test and control media (DMEM/5% FBS), after being rendered quiescent for 24 h in DMEM/5% FBS. Proliferation assessed by DNA quantification with Hoechst 33258. Values expressed as means ± SEM o f total ng DNA/well (n - 6).
DISCUSSION
Although studies on purinergic signalling in osteoblasts began some 12 years ago, the roles o f nucleotides in regulating osteoblast functions were not entirely understood. This chapter aimed at investigating the effects of nucleotides on primary rat calvarial osteoblasts studying bone nodule formation, an in vitro model for bone formation by osteoblasts, and cell proliferation, using two different approaches: [^H]thymidine incorporation, which assesses the number o f cells undergoing active proliferation, and DNA quantification with the fluorescent dye Hoechst 33258, which determines the total cell number.
Both ATP and UTP, but not adenosine, ADP or 2-meSADP, significantly and consistently inhibited bone nodule formation, whereas no clear-cut major effects of nucleotides on cell proliferation were found. The potent inhibitory actions o f ATP and UTP point to the involvement of either P2Y2 or P2Y4 receptors, since ATP and UTP are potent agonists at both receptor subtypes. However, since no evidence was found for the expression of the P2Y4 receptor subtype on bone cells (Chapter 2), it can be suggested that the P2Y2 receptor, expressed on osteoblasts, might be responsible for mediating these effects.
The inhibitory effects o f UTP and ATP started at concentrations as low as 1 pM and 10 pM, respectively, which stands in sharp contrast with an earlier paper on the effects of nucleotides on appositional bone formation (Jones et al., 1997). This earlier study employed a different in vitro model of bone formation, using the same type of primary rat calvarial cells, but seeding them on dentine slices with cut-in grooves, on which bone could be deposited. At the end of the experiment, number, length and area o f bone loci were analysed. However, only when applying relatively high concentrations o f 500 pM ATP, a significant inhibition o f bone formation could be observed, and in one experiment, ATP at 50 pM even slightly stimulated bone formation. Lower inhibitory concentrations o f 20 pM were reported for 2-meSATP and the non-hydrolysable ATP analogue ATPyS, whereas, similar to my studies, adenosine had no effect. ADP was not tested, and the effects of UTP were equivocal: in one experiment, 2 pM UTP stimulated
bone formation, whereas in another experiment, 2-20 pM o f UTP inhibited bone formation. Thus, the results presented in this chapter provide a more conclusive result: both ATP and UTP, in the low micromolar range, are strong inhibitors o f bone formation. The fact that in the earlier study ATPyS, a potent agonist at the P2Y2 receptor, was inhibitory provides further evidence for the role o f the P2Y2 receptor in this process.
ATP at much higher, millimolar concentrations has long been known to play a role in the mineralisation process. ATP (> 5 mM) inhibits Ca^^ deposition because it is a potent stabiliser o f amorphous calcium phosphate, thus inhibiting its conversion to hydroxyapatite in vitro. However, ATP at 1-5 mM can promote bone formation by providing pyrophosphate for the mineralisation process by matrix vesicles (Hsu and Anderson, 1977). This highlights the role for specific ecto-ATPases in the calcification process, which are responsible for the ATP-dependent Ca^^-and Pj-depositing activity of bone or cartilage-derived matrix vesicles (Hsu and Anderson, 1996).
However, inhibitory effects of ATP and UTP on bone nodule formation in the present study were observed in the low micromolar range. Thus, an indirect, not receptor-mediated effect on mineralisation through the mechanism described above can probably be excluded. Additionally, negative effects on bone nodule formation, as observed here for ATP and UTP, are most likely due to inhibitory effects on cell aggregation and/or collagen/matrix deposition in the earlier stages o f the culture, prior to mineralisation, and not to inhibiton of mineral deposition. From this work, it cannot be told how nucleotides signal intracellularly to inhibit bone formation. Nor was determined whether single and intermittent pulses of ATP or UTP would have the same consequence as continuous application.
In this context, it has recently been shown that induction o f osteoblast differentiation by addition o f ascorbic acid led to 3- to 4-fold increases in respiration and ATP production, resulting in a 5-fold increase in ATP content compared with that in immature, undifferentiated cells. This demonstrated that progressive osteoblast differentiation coincides with changes in cellular metabolism and mitochondrial activity, which are likely to play key roles in osteoblast fimction. An enlarged pool o f ATP might
be stored in mature osteoblasts for later release in response to appropriate stimuli, so that ATP can act as a paracrine agonist at P2 receptors and, based on my results, potentially serve as a ‘stopping signal’ for bone formation (Komarova et a l, 2000).
Although only one paper has been published on nucleotides effects on osteoblastic bone formation, more studies have been performed on ECM formation in chondrocyte cultures. In contrast to the inhibitory effects of ATP and UTP on bone nodule formation, an increased accumulation o f cartilage proteoglycan and collagen in the presence of relatively high-dose (500 pM) ATP and UTP, but not ADP, has been demonstrated in a chondrocyte pellet culture system, whereas no effect on cell number was observed. This illustrates that chondrocytes might undergo a long-term change to an anabolic, matrix- secreting phenotype in response to extracellular nucleotides. The anabolic response was only seen in the presence of serum, pointing to a cross-talk between growth factor receptor and purinergic signalling pathways (Croucher et a l, 2000).
The finding that ATP and UTP might play a role in osteoblastic activity agrees with a number of earlier studies. Kumagai et a l first reported in 1989 that ATP elevates cytosolic Ca^^ through mobilisation of [Ca^"^],, but not through Ca^^ influx, in rat osteoblastic UMR-106 cells (Kumagai et a l, 1989). In a follow-up study, a two-receptor model was suggested because ADP and UTP, in addition to ATP, also elicited a rapid transient increase in [Ca^^Jj at 1-100 pM, followed by an increase in IP3 and IP (Kumagai et a l, 1991). The presence of two functional P2Y receptors (P2Yi and P2Y2- like) on osteoblasts, coupled to internal Ca^^ release, was confirmed by further experiments (Reimer and Dixon, 1992; Yu and Ferrier, 1993b; Sistare et a l, 1994; Sistare et a l, 1995), whereas adenosine, AMP, a,p-meATP and p,y-meATP were not found to have an effect on [Ca^^],. Reimer and Dixon reported a time-dependent inactivation o f the Ca^"^ signalling pathway with continued receptor occupation by ATP (1992). This may be physiologically important as a mechanism to limit the duration of the Ca^^ signal in the continued presence of extracellular nucleotides. Additionally, activation o f PKC has been shown to selectively desensitise the P2Yi, and not the P2Y] signalling pathway in osteoblastic cells, and may thus be implicated in the negative
feedback control o f elevation (Gallinaro et a l, 1995). The avian P2Y] receptor has three consensus phosphorylation sites for PKC, which may play a role in receptor desensitisation (Webb et a l, 1993). In vivo, endocrine or paracrine factors, acting through PKC, may therefore regulate responsiveness of osteoblasts to extracellular nucleotides.
Kumagai et a l also observed a potentiated Ca^^ response to nucleotides in the presence of PTH (1991). This is consistent with later studies showing that activation of P2Yi and P2Y] receptors potentiates subsequent PTH receptor-mediated Ca^^ signalling (Kaplan et a l, 1995; Sistare et a l, 1995; Bowler et a l, 1999; Buckley et a l, 2001b). It has been suggested that PTH receptors are capable of activating adenylate cyclase through Gs proteins, but might be unable to activate PLC until cells receive a signal as a consequence of P2 receptor activation (Sistare et a l, 1995).
Signalling events involved in the synergistic effect o f PTH (PTH 1-31, but not PTH 3-34 peptide) and nucleotides (esp. ADP) in UMR-106 cells were investigated in a recent study: despite the link to Gs and adenylate cyclase, this synergy did not involve cAMP or cGMP accumulation, nor PKA activation, so that events further upstream at the level of G protein subunit interaction o f P2Yi and PTH receptors were suggested (Buckley et a l,
2001b). The significance of this potentiation can be seen in its effects on transcription factor activation and gene expression: ADP in combination with PTH, but not alone, activated the transcription factor CREB (cAMP response element-binding protein), and both synergistically activated c-fos expression, which, as mentioned before, is strongly implicated in controlling the proliferation and differentiation o f bone cells. These results contrasted with an earlier study on the human osteosarcoma SaOS-2 cell line, also predominantly expressing P2Yi, but requiring the serum response element, in addition to the Ca/CRE (Ca^VcAMP response element), of the c-fos promoter for synergy upon co stimulation with ADP and PTH (Bowler et a l, 1999). P2Y] receptors can also mediate synergistic c-fos induction by ATP/UTP and PTH in primary human osteoblasts (Bowler
et a l, 1999). These synergies suggest a potentially highly targeted mechanism through which systemic PTH could initiate bone remodelling at specific sites in the skeleton by cooperating with the localised release of nucleotides.
Whether PTH can act synergistically with ATP and UTP in inhibiting bone nodule formation has not been investigated in the present study, but presents an interesting area of future work. As mentioned above, PTH combines with UTP and ATP to induce c-fos
expression in primary human osteoblasts; thus, this synergistic effect is not only restricted to osteoblastic immortal cell lines. The action o f PTH itself on osteoblastic