Monitoring intracellular melatonin levels in human prostate normal and cancer cells by HPLC

Melatonin (N-acetyl-5-methoxytryptamine) is a potent endogenous antioxidant and free radical scavenger that has attracted much attention as a consequence of its multiple biological functions. In addition to other physiological properties, it has clear antiproliferative activity in several types of cancer cell. The concentration of melatonin necessary to inhibit cell growth is much higher than its blood physiological concentrations in some tumor types. For years its indolic nature has impeded proper monitoring, by molecular or immunological techniques, of its uptake by cancer cells. In this work we developed a simple, rapid, and validated analytical method for detection and quantification of MEL inside normal and cancer cells. For this purpose we performed high-performance liquid chromatographic analysis after liquid–liquid extraction of the indole from biological samples. The method was validated, and the correlation coefficient for amounts from 0.125 to 1.25 μg was higher than 0.999, with a range of recovery near 100%. Precision was evaluated as repeatibility, and for intermediate precision, the relative standard deviation was less than 5%. The method was used to study the stability of the indole in solution and to determine intracellular melatonin concentrations in normal (PNT1A) and several cancer (LNCaP, DU-145, PC-3) prostate cell lines. Intracellular LOQ/LOD were 7.23/2.83, 23.17/9.07, 4.03/1.83, and 6.51/2.53 nmol L−1, or 1.82/4.66, 0.56/1.45, 3.26/8.34, and 2.02/5.17 attogram in each cell in PNT1A, LNCaP, DU145, and PC-3 cells, respectively. Because there was no information about intracellular levels of melatonin inside normal or tumor prostate cells after treatment with the indole, nor a relationship between its antiproliferative activity and its intracellular concentration, this is the first time that, by using an analytical method combined with measurement of cellular volume by flow cytometry, the intracellular concentration of MEL has been estimated. Also, data obtained here explain why the antiproliferative properties of MEL vary in different cell types. This is, moreover, the first time that by increasing the intracellular concentration of melatonin, its antitumor properties have been promoted in prostate cancer cells. This process can be monitored by the method developed here.