**4. Melatonin and immune system**

The immunological role of melatonin was first reported by Maestroni et al. in 1987 [26]. In the study, it was observed that immune functions were suppressed in conditions where melatonin formation was inhibited by continuous exposure to light or the administration of β-adrenergic receptor blockers at night. This effect of melatonin is not evident under normal conditions. However, the effect becomes evident in cases where the immune system is suppressed, such as aging, viral diseases,

corticosteroid use, or acute stress [27–29]. In another study, they reported that suppression of immune functions as a result of soft tissue trauma and hemorrhagic shock in mice was reversed with melatonin, and that chronic melatonin treatment increased natural killer cell activity in humans [30]. The effects of melatonin against immunosuppression or enhancing immune functions are related to its binding to specific receptors on T-helper lymphocytes. The binding of melatonin to these receptors increases the secretion of gamma-interferon, IL-2, or opioid peptides. In addition, the administration of melatonin in tumor-formed mice protected blood cells from the toxic effects of chemotherapeutic drugs [29]. Also, some studies reported the melatonin anti-inflammatory effect in periodontitis [31–36]. However, melatonin also shows an immunodepressant effect in relation to the dose. At high pharmacological doses (> 100 mg/kg BW), melatonin suppresses antibody formation [29]. The inhibitory effect of melatonin on the immune response and its antioxidant effect suggest that melatonin may be beneficial in organ transplantation. In addition, the lack of toxicity supports that this agent is an agent that can be used safely in transplantation [27].

### **5. Mechanisms for a relation cardiovascular disease**

Melatonin stimulates the phospholipase C pathway by activating MT1 and MT2 receptors via the G inhibitor protein. This results in an increase in Ca++ concentration and leads to phosphorylation of protein kinase C (PKC). PKC activates the protein/ activation transcription factor cAMP-responsive element-binding protein and activating transcription factor (CREB-ATF). This pathway immediately regulates early gene transcription and thus gene transcription regulation and antioxidant enzyme levels. The production of reactive oxygen species (ROS) stimulates the expression of genes involved in inflammatory processes in the cell. Thus, the transcription of nuclear factor kappa (NF-kB) increases the expression of these inflammatory genes. Inactive NF-kB resides in the cytoplasm due to an inhibitory subunit [I-kB]. I-κB is phosphorylated, and NF-κB is translocated into the nucleus via stimulation of the cells by oxidative stress. PKC may also activate NF-κB, and it binds to the κB response element in its target genes' enhancer and promoter regions. Some of these are located in the promoter regions of the major antioxidant enzymes. Thus, an early cellular response to oxidative stress activates the antioxidant systems [37–39]. The role of melatonin on the protein kinase C (PKC) and activating transcription factor (CREB-ATF) pathway is described in **Figure 2**.

Recent studies have reported the effects of melatonin [receptor-dependent and non-receptor-mediated] on the cardiovascular system. Melatonin causes vasoconstriction in cerebral arteries and vasodilation in peripheral vascular beds. Myocardial infarction (MI) risk, coronary heart patients with sudden death risk, high LDL-cholesterol levels, and also in hypertensive patients, melatonin levels were found to be low [40–42]. The vasodilator effect of melatonin also plays an important role in inducing sleep through thermoregulation. The effects of free radicals play an important role in oxidant damage, especially in the cardiovascular system, caused by high blood pressure [43]. In addition, a decrease was observed in echocardiographic measurements, biochemical parameters in the myocardium, and measured tissue damage parameters in experimental hypertension with melatonin administration [44] (**Figure 2**). It is known that inflammation plays an important role in coronary heart diseases including atherosclerosis. Active compounds released by immune cells that

**Figure 2.**

*MT 1 and MT 2 melatonin receptor signaling. PKA, protein kinase a; cAMP response element-binding protein; MT, melatonin receptor; Akt, threonine protein kinase B [PKB; also known as Akt]; cyclic adenosine monophosphate; cGMP, cyclic guanosine monophosphate; CREB, IP3, inositol trisphosphate; MAPK, mitogenactivated protein kinase.*

become dominant in the early stages of atherosclerosis cause the progression of both atherosclerotic lesions and inflammation. An increased incidence of MI and sudden death in coronary heart muscles have been associated with decreased melatonin levels in these patients. In animals with hyperlipidemia fed with high cholesterol, melatonin administration was found to be protective of the aorta by increasing antioxidant enzyme activities in these animals [45].
