One of the pathways that induces angiogenesis as a physiological or defence response in tissues is mediated by HIF-1α and growth factors such as ANG1, ANG2, and its receptor TIE2 [11]. The present study measured the expression of these proteins in dental pulps from immature and mature apexes teeth, which are involved at the initial step of the angiogenesis.
HIF-1α was evaluated, as this protein is oxygen-dependent and its expression indicates that the dental pulp tissue is under hypoxic conditions, because HIF-1α during normoxia is vulnerable to proteasomal degradation [3]. It is well known that for HIF-1 transcription, both activators HIF-1α and β are needed. HIF-1β is a constitutive protein, which remains inactive inside the cell until it binds to HIF-1α in order to transcript HIF-1 to trigger the angiogenic activity [3, 8].
The presence of higher HIF1-α levels in mature apexes teeth indicates lower oxygen concentration in those pulp tissues; and also the possible function of HIF-1 triggering ANG2/TIE2 and VEGFA/VEGFR2 or VEGFA/NRP1 bond in order to start angiogenesis in mature teeth [1]. On the contrary, lower HIF-1α levels in immature apexes teeth could be indicating a lower rate in the angiogenic activity when compares with mature teeth. This suggests two types of cellular activities, proliferation and synthesis. In one hand, cell proliferation, which requires low oxygen concentrations in immature apexes teeth [19] and in the other hand, mineralized tissue formation which requires new blood vessel generation to increase oxygen concentration needed to the metabolic reactions in mature teeth.
A high level of HIF-1α has been observed in teeth under orthodontic movements [20], these findings could be applied to the results of the present research, where stimuli such as dental trauma and mastication in pulps of teeth with mature apexes could raise HIF-1α levels with respect to pulps of teeth with immature apexes that are not under occlusal contact.
It is well known that blood vessel stability is produced via TIE2 phosphorylation by ANG1 [16, 17]. On the other hand, ANG2 has an antagonistic effect over TIE2 inhibiting its phosphorylation which leads to vascular destabilization starting the angiogenic process. Also, it is important to mention that despite of ANG1 is expressed in high levels (11:1 ratio), ANG2 has more affinity to TIE2 and will bind to it [1, 16]. In this research both ligands ANG1 and ANG2 and its receptor TIE2 are significantly higher in dental pulps from teeth with mature apexes than in immature ones, which lead to consider that in mature apexes teeth there are more blood vessels in destabilizing process which suggest a higher angiogenic activity when it compared to the immature apexes teeth.
VEGF is the primary pathway for initiating angiogenesis in dental pulp tissue [13, 21]. However, ANG2 has been studied as a way to initiate the angiogenic response in the dental pulp [1]. In the mentioned study, ANG2 was expressed in both dental pulps from teeth with immature and mature apexes without significant differences. While, in the present study, the expression of ANG2 is significantly increased in mature apex teeth, which could be due to a variation in the inclusion criteria of immature apex teeth group. In the present study, in the immature apex group, teeth with an apical orifice diameter greater than 5 mm were included, while in the study by Gomez-Sosa et al. [1], the group of teeth from the immature apex group consisted of teeth with an apical orifice between 3 and 5 mm. A possible explanation for this difference is that the bigger the apical opening, the less hypoxic dental pulp tissue would be, which supports the findings in HIF-1α in the present investigation. As mentioned before, TIE2 is the receptor of both ligands ANG1 and ANG2 [16], this receptor was found significatively higher in mature apexes teeth than immature ones, which could be due to the increased levels of ANG1 and ANG2 [1] suggesting greater angiogenic activity in matures teeth.
The increased potential for angiogenic activity in mature apexes teeth could be due to intense metabolic activity in order to produce more dentine in teeth with a smaller pulp cavity and denser pulp tissue than immature apexes teeth [1, 12]. This is in accordance with the findings by Gomez-Sosa et al. [1] where an increased potential angiogenic activity was observed in mature apexes teeth associated with all occlusal activities such as mastication, occlusal trauma and bruxism. In this study mature teeth were in occlusal contact while immature ones were not.
Due to a limitation of the present research, in which immunohistochemical assays were not performed, it was not possible to establish which dental pulp cells express each protein. An ELISA test was carried out instead of RT2-PCR because the purpose of the study was to verify the expression of these proteins in the dental pulp, not the genic capacity to synthesize them.
More research is needed to understand the angiogenic process in the dental pulp, its response to different therapeutic alternatives at different root development stages, as well as the different pathways of vascular destabilization and stabilization.