The present study investigated the effects of physiological and pharmacological epinephrine concentrations on AGS (Stomach Cancer Cells-SCCs) and U87 (Brain Glioblastoma Cancer Cells- BGCCs) cell lines, which included cell proliferation, adhesion, and viability in two levels of proliferation thresholds. (High-speed proliferation rate for AGS cell line, with a survival rate less than one year, and Low-speed proliferation rate for U87 cell lines, with a survival rate more than ten years). The results have shown that epinephrine enhanced the proliferation of AGS and U87 cells at physiological concentrations. However, pharmacological concentrations potentially decreased cell proliferation. High concentrations of epinephrine have demonstrated toxic effects that inhibit the proliferation of both cell lines in-vitro and reduce the tumor size in-vivo. It appears that by increasing epinephrine concentrations more than 64µmol/L, oxidative stress leads to the creation of hydrogen peroxide and reactive oxygen species (ROS), which can justify epinephrine cytotoxicity.
The results of the present study are consistent with Behonick et al. and Costa et al. studies that reported epinephrine might have toxic effects at doses above physiological levels (25, 26).
This indicates the blocking effect of propranolol through β adrenergic receptors that could provide a reverse reaction to epinephrine in both low and high concentrations of the agonist. Dong et al. reported that, after adding norepinephrine, in a concentration-dependent manner to glioma LN229 and U251 cells, due to the expression of both beta-1 and β2-adrenoceptors, proliferation was significantly enhanced and blocked by propranolol as a nonspecific beta-adrenergic receptor blocker, appeared with a specific time and concentration-dependence (27).
Wong et al. demonstrated the effect of epinephrine on the proliferation of HT-29 adenocarcinoma cells was created through both β1 and β2 adrenergic receptors (28). Yamanaka et al. reported that increased epinephrine levels (10µg/Ml,100µg/Ml) delayed scratch closure among oral squamous carcinoma cancer cells through the inhibition of intracellular cAMP (29). Also, Sivamani et al. stated that under stress conditions, increased epinephrine levels and expression of beta-adrenergic receptors on keratinocytes result in impaired cellular epithelialization and delayed wound healing. On the other hand, treatment with beta-adrenergic antagonists (timolol) significantly increases the epithelial level of the wound (30). Djelic et al. used six experimental concentrations of adrenaline on human lymphocytes (0.01–200µmol/L) and reported that lower epinephrine concentrations had no genotoxic effect on sister chromatid exchange and micronucleus. However, higher concentrations (5µmol/L,50µmol/L, 150µmol/L, and 200µmol/L) decrease the mitotic index and delay the cell cycle due to the production of ROS (31). Rosenberg et al. showed that catecholamines, including norepinephrine (NE), dopamine, epinephrine, and glia at a concentration of 25µmol/L, were toxic on neurons. Oxidative degradation of catecholamines and hydrogen peroxide production and adrenochrome have suggested that endogenous catecholamines may play a role in normal and abnormal cell death. The toxicity of noreppinephrine is blocked by catalase (32).
The present study results showed that in the presence of propranolol, the adhesion of U87 cells increased, and metastasis decreased. The findings of the present study show that at physiological concentrations, epinephrine increases tumor growth and prevents its metastasis; however, its pharmacological concentrations are likely to decrease U87 cell proliferation and further enhance the metastatic state of tumors by increasing ROS. Numerous studies confirm these results. Palm et al. showed that the migration of breast, prostate, and colon cancer cells was enhanced by stress-related neurotransmitters, norepinephrine (NE = 10µmol/L) in vitro, and this effect was restrained by the inhibitor, β-propranolol (10µmol/L) (33). Pu et al. reported that in a study of the PANC-1 pancreatic cancer cell model, epinephrine promotes migration in a dose-dependent manner and contributes to the stress-induced metastasis in PANC-1 cells. Cell migration was significantly reduced by blocking the β-adrenoceptor β2 (34).
The present study showed that propranolol reduced cell viability at low concentrations of epinephrine and decreased the toxic effect of epinephrine at higher concentrations. The data show that in physiological concentrations of epinephrine, although it increases the viability of U87 cells, in pharmacological concentrations, it decreases the cell viability and has toxicity effects. Other studies confirm our findings. Patri et al. indicated that in both brain tumor cell lines, such as neuroblastoma and glioma C6, norepinephrine increased cell viability by restoring the G2 phase of the cell cycle and decreasing the percentage of cell death (35). Zhou et al. reported the toxic effects of epinephrine induced by extracellular chemicals entering cells and disrupting cellular homeostasis and activating mitochondrial signaling cascades due to stress, results in increased steady-state levels of ROS and activation of Bax, caspases, and cellular damage. This can ultimately lead to cell death and reduced survival rates (36). Uchida et al. showed that catecholamines, such as epinephrine above concentrations of 60µmol/L, decreased the number of living cells in the Human Oral Squamous Cell Carcinoma lines due to ROS production and cell cytotoxicity. However, this phenomenon was not seen in non-catecholamines, such as dexmedetomidine. Catalases also reduced the toxicity of this effect in adrenergic agonists (37). As the data in this study indicate the dual effects of high concentrations of epinephrine due to the potential impacts of its products, this is consistent with the study of Calvani et al. Since ROS production is a long-standing issue in cancer, Its toxic threshold can be an effective strategy to reduce tumor cell viability.
On the other hand, cancer cells increase signaling activation by maintaining moderate intracellular ROS concentration called "mild oxidative stress," enhancing tumor progression by enhancing cell viability and dangerous tumor phenotype. Many chemotherapy treatments kill the cell by increasing the concentration of ROS in the cell (38). Similar to the present study results, Ciccarese et al. showed that ROS acts as a double-edged sword in cancer cells. Increased mtROS production leads to increased mitogenic signals, oncogenic transformation, genomic instability, and evasion of cell cycle inspections. On the other hand, excessive accumulation of H2O2 leads to irreversible protein modification, oxidative damage to lipids and nucleic acids, blockade of proliferative signaling, and ultimately cell death. Therefore, elevated ROS levels may reflect the Achilles' heel of cancer cells that can be therapeutically abused because it may overcome the toxic threshold by a slight increase in ROS levels, leading to mitochondrial crest regeneration and apoptotic cell death. High levels of ROS in cancer cells balance with increased antioxidant defense (39).
In the present study, epinephrine was represented as a double blade sort that has dual effects. It caused tumor destruction at high concentrations and spread tumor cells to other tissues by declining adhesion in low concentrations, increasing tumor size, and inhibiting cell invasion and metastasis. However, this dual effect of epinephrine and its precise mechanism requires further investigation.
In the early stages of the diagnosis, epinephrine toxicity at pharmacological concentrations reduced the growth and proliferation of glioblastoma-derived brain tumors and reduced the invasive probability. As well as using beta-blockers such as propranolol at lower stages of the tumor.