Introduction: 

Melatonin is known to inhibit the proliferation of (ER)-positive (MCF)-7 human breast cancer cells. In previous studies on different systems it has been shown that melatonin affects Ca²⁺/Calmodulin signaling by either affecting intracellular calcium ([Ca²⁺]_i) by activating its G-protein coupled receptor, or by binding directly to calmodulin. Using ATP which increases [Ca²⁺]_i via the G protein-coupled P_2y-purinoceptor and the phospholipase C (PLC) pathway we studied whether melatonin effects [Ca²⁺]_i itself, or whether melatonin modulates ATP induced [Ca²⁺]_i transients.

Methods: 

MCF-7 cells were plated for 24 hours and loaded with Fura-2 for an hour prior to the experiment. Drugs were delivered via a bath perfusion system and images were acquired using Fura-2 imaging system saved for off-line analysis.

Results: 

1. Melatonin itself did not change basal [Ca²⁺]_i levels at all concentrations tested (100mM - 100pM) 2. ATP characteristically induced [Ca²⁺]_i transients that were concentration dependent (peak rise in [Ca²⁺]i; 1mM, 10mM and 100mM induced a peak rises of [Ca²⁺]_i of 24±6.2%, 100±4.5% and 180±5.2% compared to basal [Ca²⁺]_i. 3. Pretreatment with different concentrations of melatonin followed by evoking a 10mM ATP induced transient resulted in a concentration dependent reduction in the amplitude of the transient. A bell shaped concentration-response relation was obtained with a maximum reduction at 1nM melatonin (18.2±5% p< 0.05).

Conclusions: 

Our study shows that melatonin blocks ATP induced [Ca²⁺]_i transients hence suggesting that it interferes with the P2Y-PLC pathway. Understanding the mechanism of how melatonin changes [Ca²⁺]_i signaling will open new possibilities to understand melatonin's anti-cancer effects.