1. Introduction
Cancer is a group of diseases caused by various alterations in oncogenes and suppressor genes, causing tumors associated with oncogenic signaling pathways [
1], its high morbidity and mortality impact socially and economically in the world. It is known that there is a wide range of existing therapies, however, their relative efficiency and their various adverse reactions cause the search for alternatives to treat this pathology to continue.
Some plants have antitumor active ingredients that can create a window of opportunity for cancer patients whose economic conditions do not allow them to obtain affordable treatments, patients in whom existing treatments are not effective or generate too many adverse reactions and in general to improve the quality of life during the disease-therapy process. Within phytotherapy, green tea has attracted worldwide attention, its production and consumption have increased each year, it is estimated that the production of green tea will increase by 7.5% per year to reach 3.6 million tons in 2027. One of the reasons for this increase are the attributed therapeutic qualities [
2]. Among its secondary metabolites are polyphenols such as flavan-3-ols, which represent up to 30% of the dry weight. (─)- Epigallocatechin gallate (EGCG)
Its structure is made up of four aromatic rings joined by the C ring, which is a pyran, giving the molecule a concave shape.
Figure 1.
EGCG structure. Made up of four rings joined by the C pyran ring. Based on Du, G.-J.,
et al. 2012 [
3].
Figure 1.
EGCG structure. Made up of four rings joined by the C pyran ring. Based on Du, G.-J.,
et al. 2012 [
3].
It is considered responsible for a large part of the benefits provided by green tea, this metabolite is also found in strawberries, apples, cocoa, among others, however, it is highly commercialized of green tea which makes EGCG a very important, highly consumed and affordable phytochemical worldwide. It has been shown that among its most striking properties are antioxidants, cardiovascular protection, antimutagenic, antiviral and anticancer [
3], to validate this last effect as an antineoplastic,
in vitro and
in vivo investigations have been carried out in different cancer models, for example that of the pancreas [
5], lung [
6], breast [
7,
8], bladder [
9] and ovary [
10].
The antineoplastic action mechanisms attributed to it are inhibition of initiation, development, proliferation and apoptosis through various oncogenic pathways that continue to be explored [
11].
The problem so far for its use in prevention or treatment at the clinical level is that the data obtained in humans and
in vivo models have a lot of variability and are inconsistent and incompatible with the
in vitro results [
12], which has led to a conflict to advance towards clinical trials [
13].
It is necessary to provide more evidence of the in vivo antitumor effect of EGCG alone or in combination to obtain better efficacy in treatments, reduce adverse reactions or resistance to chemotherapy. This work team is aware of the importance of investigating the antitumor effect of EGCG alone or in combination, observing its effect in vivo alone and combined with vincristine sulfate in a solid murine lymphoma model.
3. Discussion
Despite the research and efforts made, until now the treatments used in cancer have a very narrow therapeutic index and resistance. Among these drugs, vincristine is commonly used as an important clinical agent for lymphomas, leukemias, and testicular cancer.
To achieve less aggressive and more effective treatments, more alternatives must be explored. Considering the above, in this work the effect of EGCG alone or combined with vincristine sulfate was observed in a solid murine lymphoma model. The results in general showed that when EGCG or VCR/EGCG was administered, the tumor volume was smaller and the life time was longer with a better life prognosis than the control group, interestingly the greatest inhibition in tumor development was with the VCR/EGCG group, significantly both VCR and EGCG and the combination presented an increase in p53 and a decrease in the expression of the antiapoptotic protein Bcl2, observing a greater decrease with concomitant treatment than with independent treatments.
To obtain these results, an aggressive murine lymphoma was used, which caused the death of 100% of the animals within 15 days. Among the antineoplastics to which it responds is vincristine sulfate [
16].
It has been seen that this alkaloid acts in a dose-dependent manner [
17]. In this study, only 0.30 mg/kg of vincristine presented smaller tumors and a better life expectancy in relation to the control group, while 0.05 and 0.15 mg/kg showed no difference with said group. It has already been reported that at a dose of 0.15 mg/kg no effect of vincristine was observed on survival [
18]. And although in antitumor therapy the ideal is to use the highest possible dose to obtain the best result, with this alkaloid the problem is that increasing the dose in a patient does not provide sufficient benefits in the benefit/toxicity balance, in fact to the doses normally used, the patient must accept some adverse reactions that do not justify the reduction of the dose, such as the first sensory changes [
19] and it is even accepted that the treatment can lead to axonal degeneration [
20], so it is important to find substances that can increase the effect of this drug, to reduce the doses achieving the antitumor effect with fewer adverse reactions.
It is interesting that in a positive way within natural products it has been reported that the consumption of polyphenols contained in
Camellia sinensis provides an antineoplastic benefit, among these phytochemicals there is an abundance of Epigallocatechin gallate (EGCG), which has an effect on different types of tumors
in vitro and
in vivo, however importantly there is controversy in the results reported
in vivo, which could be due to the model used, dose, and the low availability of EGCG in tumors [
12].
In this model, EGCG showed a significant antineoplastic effect by negatively modulating the development of tumor volume and increasing survival in BALB/c mice with solid L5178Y tumor; among the different doses used, only the 5 mg/kg dose was significantly different from the control, obtaining 43 % in size reduction, this is probably due to the fact that the results showed a high variability. Notably, the inhibition of tumor development by EGCG was similar to the effect obtained with the highest dose of vincristine used in this work (0.30 mg/kg). The lowest dose used in this work (0.05 mg/kg) was selected to test the adjuvant effect of EGCG. In this sense, when using both substances concomitantly, the antineoplastic effect was favorably different from the control and VCR alone, which did not happen when using only the alkaloid at this dose and in relation to survival, it was also higher, however, when comparing these data with the result provided by EGCG in survival, the effect of the combination was similar to that provided by catechin alone, so it can be deduced that this is given by catechin and that vincristine does not interfere with said effect. The EGCG result with this dose coincides with that reported in a 4T1 study with breast cancer, who used 5, 10 and 20 mg/kg, with an inhibition of tumor growth in all administered doses of 20, 31 and 34% respectively [
21].
Unfortunately, there is a very wide range of doses reported in the literature, for example, a delay in tumor development in a mouse lung cancer model is mentioned with 10 mg/kg [
22]; with 25 mg/kg in breast cancer positive to Estrogen Receptors [
23] and in leukemia [
24], with 30 mg in osteosarcoma [
25], 50 mg in triple negative breast [
26], 57 mg in prostate carcinoma [ 27] and ovarian [
10], an effect has even been observed with doses as high as 100 mg/kg, as happened in bladder cancer where the tumor was reduced by 63% [
28].
Therefore, it can be said that the results so far are variable in terms of doses and the presence of the effect, which is not always observed as in the case of the investigation carried out with a xenograft of squamous cell carcinoma of the tongue in mouse [
29], nor when using lung cancer A549 with 20 mg/kg EGCG [
30].
In vitro results have also been controversial, which have been attributed to the concentration, cell type, and aging of the cell culture [
31].
The importance of finding the effect at a small dose (5 mg/kg) was that this dose is well below the tolerable oral dose of EGCG, which is 67.8 mg/kg daily for 14 days [
32]. And that this dose also coincides with the administration in patients with breast cancer and radiotherapy (400 mg / day / 8 weeks) where inhibition of the PI3K/Akt pathway was observed and arrest in G1 with reduction of metastatic cells [
33].
Inhibiting tumor growth with EGCG in this model results in the fact that tumor size is one of the main prognostic factors in cancer, therefore volume reduction is one of the therapeutic strategies used, which may have an impact on longer life. As was observed in this study, the groups with EGCG that reduced the volume had a higher probability of survival. These effects led to the belief that EGCG could be used as an adjuvant in order to improve the antineoplastic effect of vincristine. Various natural products have already given favorable results when used concomitantly with other chemotherapeutics [
34], in the case of EGCG an increase in the sensitivity to cisplatin was observed in ovarian cancer cells [
38] and administered with taxanes has contributed to inhibit the growth of prostate tumor cells favoring their apoptosis through increasing p53, among other proteins [
36], among other antineoplastics such as capecitabine [
37], 5-fluorouracil and doxorubicin [
38].
In this sense, coinciding with this work, it is reported that the combination with vincristine had a favorable effect by inhibiting tumor growth in squamous cell carcinoma with VCR at doses of 0.46 mg/kg and EGCG 10, 20 and 40 mg/kg [
29].
These results could be partially attributed to an increase in p53, since in this work it was observed that while the control group maintained low p53 positivity, this increased in the treated groups, with vincristine being the group that presented the highest immunopositivity to this protein and is that although VCR has been mainly related to the intervention in the mitotic process by binding to tubulin, preventing the assembly of microtubules and therefore the separation of chromosomes in metaphase, it has also been observed that it blocks cell proliferation by drive the cell to apoptosis, importantly VCR treatment has been reported to express p53 [39, 40].
Therefore, the effect observed in this work could be partly due to the increase in the expression of p53. The increase of this protein was also present in EGCG, in relation to this, it has been suggested that EGCG causes cell cycle arrest and induces apoptosis by different mechanisms, including inhibiting angiogenesis, proliferation, migration and metastasis. Among the different proposed pathways is the increase in the tumor suppressor p53, which is partly attributed to the inhibition of MDM2, the main negative regulator of p53, thus preventing its ubiquitination. [41, 42].
This increase in p53 was also observed in the group with the EGCG/VCR combination, without being different from the treatments independently, however it is important that despite not being a summative effect, it continues to be present in the same proportion which means that the presence of p53 is not decreased by using both treatments concomitantly.
And it is that the presence of said tumor suppressor is very important, several actions have been attributed to it, including as a transcription factor that expresses several genes, is an inducer of apoptosis, favors genomic stability and regulates the cell cycle in relation to this it is mentioned that it can stop the cycle in the G2 phase, but its main effect is in the G1 phase, in which it occurs mainly by the transcriptional activation of p21, with a decrease in Cyclin D1 and CDk4 and kinases. CDk6 and inducing senescence in the cell and if the damage is not corrected, it causes apoptosis as a consequence [
43].
Cyclin D1, is produced by a proto-oncogene, mainly phosphorylates and inactivates the retinoblastoma (RB) protein, it is known as an oncogenic protein because it is commonly overexpressed in cancer due to defective regulation at the post-translational level, increasing its presence and favoring the proliferation. Its importance has made it a target of therapeutic interest [
44]. It is mentioned that vincristine has a slight effect on decreasing CD1 but it is not enough to stop the cycle in the G1-S phase [
45].
While EGCG presents cell arrest mainly in G1 caused, among other ways, by an increase in p21 and a decrease in CD1 by destabilizing the protein, causing its ubiquitination [
46]. Therefore, in this work the effect of the different treatments in the presence of CD1 was observed. The results showed that contrary to what is mentioned in the bibliography, said decrease of CD1 in the tissues was not observed with any of the treatments used in this model, this leads to suppose that the effect observed in the inhibition of tumor development was given by another way than CD1, such as an increase in apoptosis.
Due to the results obtained previously, in this work the effect of the different treatments on the Bcl2 protein was also observed, since p53, in addition to delaying the cell cycle, leads cells with damaged DNA to apoptosis, this process is normally regulated by an equilibrium between proapoptotic and antiapoptotic factors, however, by inhibiting antiapoptotic factors, the balance may tip to cell death. Among the proteins that are antiapoptotic, Bcl2 is distinguished, the results show that both VCR and EGCG decreased the presence of said protein, but interestingly, the effect was greater in the VCR / EGCG combination, this was possibly because both substances decrease Bcl2. It is mentioned that to prevent apoptosis, Bcl2 binds to Bax, preventing the permeabilization of the outer mitochondrial membrane and the release of cytochrome C, inhibiting apoptosis. This effect has already been described for VCR in breast cancer cells, where Bcl2 hyperphosphorylation prevents its binding to Bax [
47]. It has also been reported for EGCG to downregulate the expression of Bcl2 mRNA [
48], another mechanism described is due to an allosteric effect, it has been observed that to prevent the function of Bcl2 the gallate group binds with high affinity to the Bcl2 hydrophobic grooves selectively [
49]. Therefore, different mechanisms are reflected for Bcl2, one most likely induced through p53, which acts in various stages of mitochondrial membrane permeabilization [
50] and could be given by both VCR and catechin and the second provided only by EGCG, which can explain the decrease in tumor volume and the increase in the survival prognosis observed.
Author Contributions
Conceptualizatión, G.A., G.A.V. and R.M; Methodology, G.A., G.A.V., B.M., E.M. and R.M.; Validation, G.A.V., M.E.M., O.M., A.H. and A.F.; Formal analysis, O.M. and E.M.; Investigation, G.A., E.M.M., M.B.F., B.M., A.F., E.M. and R.M.; Resources, G.A., G.A.V., R.M.: Data curation, G.A. and G.A.V; writing—original draft preparation, M.B.F., A.H. and A.F.; Writing—review and editing, G.A., G.A.V., E.M.M., M.B.F., B.M., O.M., A.H., A.F., E.M. and R.M.; Visualization, G.A. and O.M Supervision, G.A. and R.M.; Project administration, A.H.