Low dose Arecoline induces the proliferation of fibroblasts
Previous studies have showed that Arecoline was cytotoxic to oral fibroblasts from the concentration of 50 µg/ml up [8]. However, it is true that the areca nut chewing increases the risk of oral cancer and oral submucous fibrosis. In fact, the salivary Arecoline level during chewing were about 0.1 µg/ml, and increased to about 0.3 µg/ml after chewing [20], which is much lower than the concentration used in most experiments.
In the present study, we intend to test the effect of different dose Arecoline (0.1, 0.5, 1, 4, 8, 16, 32 or 50 µg/ml) on the proliferation of human oral fibroblasts, half of the medium was changed every 24 hours. We confirmed that high dose Arecoline (32 µg/ml and 50 µg/ml) could inhibit the proliferation of fibroblasts (Fig. 1a). Our results also showed that Arecoline promoted the proliferation of fibroblasts at the concentrations from 0.1 µg/ml to 16 µg/ml (Fig. 1b-1f), and the maximum proliferate effect was 8 µg/ml (Fig. 1e). These results showed that low dose Arecoline could promote the proliferation of human oral fibroblasts.
Arecoline Promotes Fibroblasts Proliferation By Inducing Wnt5a Expression
Earlier reports have shown that activation of Wnt/β-catenin signaling may promote the proliferation of fibroblasts by regulating the expression level of Wnt1, Wnt2, Wnt3a or Wnt5a [19]. To further study whether other Wnt isoforms take part in accommodation of human oral fibroblasts proliferation, the mRNA expression levels of all 19 Wnt gene family members in human oral fibroblasts exposed to 8 µg/ml Arecoline for 24 h were analyzed by RT-PCR. As illustrated in Table 1, Arecoline altered the transcription of ten Wnts (Wnt1, 2, 3a, 5a, 5b, 8b, 10a, 10b, 11 and 16). Of those, Wnt3a, Wnt5b, Wnt8b, Wnt10a, Wnt10b, Wnt11 and Wnt16 were expressed only at very low levels in both control and treatment group, whereas Wnt1, Wnt2 and Wnt5a expressed at higher levels. We next detected the protein expression level of Wnt1, Wnt2 and Wnt5a in 8 µg/ml Arecoline time course experiment. As expected, Arecoline significantly promoted the expression of Wnt1, Wnt2 and Wnt5a on the protein level (Fig. 2a).
Table 1
Normalized mRNA expression of Wnts
Gene Name | Control | Arecoline 8 µg/ml for 24 h |
Relative Fold | CT value | Relative Fold | CT value |
Wnt1* | 1 | 27.53 ± 0.86 | 1.92 ± 0.18 | 26.71 ± 0.69 |
Wnt2* | 1 | 29.26 ± 0.29 | 2.25 ± 0.21 | 28.12 ± 0.54 |
Wnt2b | 1 | 33.09 ± 1.37 | 1.12 ± 0.33 | 32.84 ± 1.71 |
Wnt3 | 1 | 30.41 ± 0.84 | 0.92 ± 0.22 | 31.38 ± 1.25 |
Wnt3a* | 1 | 36.17 ± 1.26 | 1.42 ± 0.27 | 35.67 ± 0.85 |
Wnt4 | 1 | 21.71 ± 0.42 | 0.87 ± 0.21 | 22.32 ± 0.62 |
Wnt5a* | 1 | 23.59 ± 0.31 | 2.57 ± 0.64 | 21.89 ± 0.78 |
Wnt5b* | 1 | 34.12 ± 1.73 | 1.63 ± 0.22 | 32.25 ± 1.47 |
Wnt6 | 1 | 29.35 ± 0.68 | 0.94 ± 0.15 | 28.46 ± 0.33 |
Wnt7a | 1 | 37.31 ± 1.79 | 1.07 ± 0.26 | 36.77 ± 1.36 |
Wnt7b | 1 | 31.64 ± 0.52 | 0.95 ± 0.25 | 32.53 ± 0.90 |
Wnt8a | 1 | 28.03 ± 1.15 | 1.17 ± 0.39 | 27.41 ± 1.17 |
Wnt8b* | 1 | 35.52 ± 1.27 | 1.78 ± 0.32 | 34.93 ± 0.83 |
Wnt9a | 1 | 26.47 ± 1.07 | 0.89 ± 0.17 | 27.05 ± 0.79 |
Wnt9b | 1 | 28.02 ± 1.33 | 0.96 ± 0.11 | 28.62 ± 0.41 |
Wnt10a* | 1 | 35.74 ± 2.28 | 1.44 ± 0.27 | 34.62 ± 1.49 |
Wnt10b* | 1 | 36.17 ± 1.41 | 1.75 ± 0.48 | 34.91 ± 1.58 |
Wnt11* | 1 | 34.86 ± 1.73 | 2.63 ± 1.39 | 33.17 ± 1.37 |
Wnt16* | 1 | 36.77 ± 1.03 | 1.81 ± 0.42 | 35.82 ± 0.73 |
The values represent the mean ± S.E. of three independent experiments. *P < 0.05 |
To determine if Wnt1, Wnt2 or Wnt5a was required for Arecoline to promote the proliferation of fibroblasts, recombinant Wnt1, Wnt2 or Wnt5a protein and Wnt1, Wnt2 or Wnt5a antibody were used to treat human oral fibroblasts. Results showed that the proliferation of fibroblasts was not affected by Wnt1, Wnt2 protein or their antibody (Fig. 2b and 2c); recombinant Wnt5a protein could increase the proliferation of fibroblasts (Fig. 2d); and the Arecoline-induced cell proliferation was inhibited by Wnt5a antibody (Fig. 2d). Furthermore, we found the specific siRANs Wnt5a inhibited the Arecoline-induced cell proliferation, but the proliferation of fibroblasts was not affected by Wnt1 or Wnt2 siRNAs (Fig. 2e). Together, these results demonstrated that Wnt5a mediated the Arecoline-induced fibroblasts proliferation.
Egr-1 Is Necessary For The Expression Of Wnt5a
Previous studies have identified many transcription factor binding sites in the human Wnt5a promoter, such as NF-κB, FOXO1, Smad2, Smad3 and Egr-1 [21, 22]. To identify signaling mechanisms regulating Wnt5a expression, specific siRNAs for NF-κB p65, FOXO1, Smad2, Smad3 or Egr-1 were used. The results revealed that NF-κB p65, FOXO1, Smad2 or Smad3 siRNAs could not affect the promoter activity and the expression of Wnt5a (Fig. 3a and 3b), demonstrating an NF-κB p65, FOXO1, Smad2 or Smad3-independent manner of regulating Wnt5a expression.
To confirm if Egr-1 is involved in Wnt5a regulation, fibroblasts were transfected with Egr-1 siRNAs. Arecoline induced Wnt5a expression was effectively blocked by siEgr-1 (Fig. 3a and 3c), demonstrating that Egr-1 is involved in Wnt5a regulation. In support of this result, Egr-1 siRNAs significantly suppressed Wnt5a protein expression (Fig. 3c). Therefore, we concluded that Egr-1 is essential for transcriptional induction of Wnt5a in human oral fibroblasts.
Inhibition Of Egr Activity Prevents Arecoline Induced Fibroblast Proliferation
We next assessed the roles of Egr-1 in fibroblasts proliferation. Our results showed that Egr-1 knockdown inhibited the Arecoline-induced proliferation of fibroblasts (Fig. 4a). Furthermore, Mithramycin A (MMA) and Chromomycin A3 (CHA) were used to treat cells (MMA and CHA repress transcription by selectively displacing GC-rich DNA binding transcription factors, such as Egr-1 [23, 24]). The results showed that MMA or CHA treatment blocked Arecoline induced Wnt5a upregulation and fibroblasts proliferation (Fig. 4b, 4c and 4d). These results indicated that the expression and activity of Egr-1 were required for Arecoline induced fibroblasts proliferation.