Morphofunctional study of 12‐O‐tetradecanoyl‐13‐phorbol acetate (TPA)‐induced differentiation of U937 cells under exposure to a 6 mT static magnetic field

Abstract

This study deals with the morphofunctional influence of 72 h exposure to a 6 mT static magnetic field (SMF) during differentiation induced by 50 ng/ml 12-O-tetradecanoyl-13-phorbol acetate (TPA) in human leukaemia U937 cells. The cell morphology of U937 cells was investigated by optic and electron microscopy. Specific antibodies and/or molecules were used to label CD11c, CD14, phosphatidylserine, F-actin and to investigate the distribution and activity of lysosomes, mitochondria and SER. [Ca(2+)](i) was evaluated with a spectrophotometer. The degree of differentiation in SMF-exposed cells was lower than that of non-exposed cells, the difference being exposure time-dependent. SMF-exposed cells showed cell shape and F-actin modification, inhibition of cell attachment, appearance of membrane roughness and large blebs and impaired expression of specific macrophagic markers on the cell surface. The intracellular localization of SER and lysosomes was only partially affected by exposure. A significant localization of mitochondria with an intact membrane potential at the cell periphery in non-exposed, TPA-stimulated cells was observed; conversely, in the presence of SMF, mitochondria were mainly localised near the nucleus. In no case did SMF exposure affect cell viability. The sharp intracellular increase of [Ca(2+)](i) could be one of the causes of the above-described changes.

Comparative Analysis of Biological Effects Induced on Different Cell Types by Magnetic Fields with Magnetic Flux Densities in the Range of 1–60 mT and Frequencies up to 50 Hz

“…For example, a 50 Hz, 1-50 mT, ELF-PMF induces non-linear changes in both cell membrane conductivity and permittivity in in vitro-cultivated primary embryonic chicken myoblasts after 1 h of exposure; conversely, when the same cells are exposed for 1 h to a DC-induced SMF of 1, 3, or 5 mT, i.e., values comparable with those of the ELF-PMF used in the same study, no change in the same membrane electrical parameters was observed in comparison to control values [3,4]. MF exposure also induces variations in the inner mitochondrial transmembrane potential of differentiated U937 (6 mT, for up to 72 h) and SH-SY5Y cells (1.4 mT, 50 Hz, ELF-PMF, for up to 4 h) [16,17]. Interestingly, these experimental observations have been mathematically rationalized by Ye et al, (2010).…”

“…Therefore, the alterations induced by the MF exposure are the result of alterations affecting the cytoskeletal components (actin filaments, microtubules and intermediate filaments) which help the cells to maintain their shape and internal structures. In fact, a de-arrangement of the F-actin microfilaments, with their depolymerisation and marginalization from the centre towards the cell membrane, was found in differentiated U937, as well as in Raji or HaCaT cells exposed to a 6 mT SMF (for up to 72 h) or to a 2 mT, 50 Hz, ELF-EMF (for up to 72 or 96 h), respectively [16,19,31]. Dini et al, (2005 and found that some substances (i.e., 12-0-tetradecanoyl-13-phorbol acetate (TPA), low concentration of glutamine, dimethyl sulfoxide) induce U937 cell differentiation through a progressively lower degree.…”

“…Dini et al have demonstrated that a moderate SMF (6 mT) upregulates [Ca 2 )] i in different cell types (human peripheral blood lymphocytes (PBLs), murine thymocytes, 3DO, U937, HeLa, HepG2, FRTL-5 cells). In particular: (i) SMF affects the release of calcium from the endoplasmic reticulum, as well as its cellular influx, by calcium channels present in the plasma membrane that, in turn, influence the apoptosis onset by triggering (sometimes) or counteracting it (very often) [2,11,12,16,27]; (ii) the SMF modulation of [Ca 2 )] i depends on cellular aging. For instance, with respect to control values and after 24 h of 6 mT SMF exposure, [Ca 2 )] i increased by 30% in freshly-isolated PBLs, whereas it doubled in 5 day-old samples [27].…”

“…These shape changes, which involve the whole cell or specialized components of it, depend on the kind of MF, on the cell type considered, as well as on their corresponding mode of growth [7,11,12,16,19,27,31,32]. For example, PBLs lose their round shape and became irregularly elongated [11], whereas the surface microvilli of U937 cells, which are commonly few and short, become abundant and lamellar-shaped under exposure conditions, i.e., after 24 h exposure to a SMF of 6 mT [11,16,32]. Similarly, Raji cells exposed for up to 72 h to a 2 mT, 50 Hz, ELF-PMF show a more elliptical shape but exhibit morphological alterations in the plasma membrane, such as a complete loss of microvilli [19].…”

“…Exposure to a SMF of 6 mT for up to 72 h decreases the degree of differentiation induced by the same differentiating molecules, with a consequent fall in cell adhesion and increased polarization of pseudopodia and cytoplasmic protrusions. As a result of the decrease in differentiation, a delay in engulfing dying cells was also observed [16,32]. Interestingly, when Manni et al, (2002) investigated the effects on HaCaT cells of exposure for 96 h to an ELF-EMF of 2 mT and 50 Hz, they found that both the percentage of adherent cells and the progression of cell differentiation status increased.…”