Pharmacological doses of melatonin induce alterations in mitochondrial mass and potential, bcl-2 levels and K+ currents in UVB-exposed U937 cells.

Apoptosis is observed in 'actively' dying cells after the exposure to cell stressors such as ultraviolet light irradiation. Since melatonin has been proposed to act under stressful conditions as cell protection factor, in this study we examined the potential of this molecule when used at pharmacological concentrations to control mitochondrial damage and apoptotic signalling of UVB irradiated U937 human leukaemic cells. Moreover, the effect of melatonin treatment on electrophysiological properties and membrane K(+) currents of irradiated U937 cells was investigated as functional aspects relevant to the anti-apoptotic role of melatonin. The general effect is associated with the restoration of mass, number and membrane potential of mitochondria, with a lower caspase activation and bcl-2 upregulation. In the presence of the caspase inhibitor ZVAD-Fmk, melatonin seems to drive UVB stressed cells to follow the mitochondrial intrinsic pathway, interfering just at the mitochondrial level. Moreover, treatment with melatonin, as well as ZVAD-Fmk, prevented the K(+) current reduction observed late following the UVB insult application, by sparing cells from death; this result also indicates that the decrease of K(+) leakage currents could represent a functional feature of apoptotic process in UV-exposed U937 cells.

Defective Autophagy, Mitochondrial Clearance and Lipophagy in Niemann-Pick Type B Lymphocytes.

Niemann-Pick disease type A (NP-A) and type B (NP-B) are lysosomal storage diseases (LSDs) caused by sphingomyelin accumulation in lysosomes relying on reduced or absent acid sphingomyelinase. A considerable body of evidence suggests that lysosomal storage in many LSD impairs autophagy, resulting in the accumulation of poly-ubiquitinated proteins and dysfunctional mitochondria, ultimately leading to cell death. Here we test this hypothesis in a cellular model of Niemann-Pick disease type B, in which autophagy has never been studied. The basal autophagic pathway was first examined in order to evaluate its functionality using several autophagy-modulating substances such as rapamycin and nocodazole. We found that human NP-B B lymphocytes display considerable alteration in their autophagic vacuole accumulation and mitochondrial fragmentation, as well as mitophagy induction (for damaged mitochondria clearance). Furthermore, lipid traceability of intra and extra-cellular environments shows lipid accumulation in NP-B B lymphocytes and also reveals their peculiar trafficking/management, culminating in lipid microparticle extrusion (by lysosomal exocytosis mechanisms) or lipophagy. All of these features point to the presence of a deep autophagy/mitophagy alteration revealing autophagic stress and defective mitochondrial clearance. Hence, rapamycin might be used to regulate autophagy/mitophagy (at least in part) and to contribute to the clearance of lysosomal aberrant lipid storage.