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Multidrug resistance (MDR) is the phenomenon in which cultured tumor cells, selected for resistance to one chemotherapeutic agent, simultaneously acquire resistance to several apparently unrelated drugs. The MDR phenotype is multifactorial. The best-studied mechanism involves the expression of a membrane protein that acts as an energy-dependent efflux pump, known as P-glycoprotein (Pgp), capable of extruding toxic materials from the cell. In this work, resistance to UVA radiation, but not to UVC nor UVB, was observed in an MDR leukemia cell line. This cell line overexpresses Pgp. To study the role of Pgp in the resistance to UVA radiation, two MDR modulators or reversing agents (verapamil and cyclosporin A) capable of blocking Pgp activity were used. Cell viability was assessed and the techniques of flow cytometry and fluorescence microscopy were employed to measure the extrusion of rhodamine 123 by the efflux pump. The results show that MDR modulators did not modify the resistance to UVA radiation. Furthermore, although cell viability was not significantly altered, Pgp function was impaired after UVA treatment, suggesting that this glycoprotein may be a physical target for oxidative damage, and that other factors may be responsible for the UVA resistance. In agreement with this, it was found that the resistant cell line presented a higher catalase activity than the parental (non-MDR) cell line.

Ouabain, a known inhibitor of the Na,K-ATPase, has been shown to regulate a number of lymphocyte functions in vitro and in vivo. Lymphocyte proliferation, apoptosis, cytokine production, and monocyte function are all affected by ouabain. The ouabain-binding site occurs at the alpha subunit of the enzyme. The alpha subunit plays a critical role in the transport process, and four different alpha-subunit isoforms have been described with different sensitivities to ouabain. Analysis by RT-PCR indicates that alpha1, alpha2, and alpha3 isoforms are all present in murine lymphoid cells obtained from thymus, lymph nodes, and spleen. In these cells ouabain exerts an effect at concentrations that do not induce plasma membrane depolarization, suggesting a mechanism independent of the classical inhibition of the pump. In other systems, the Na,K-ATPase acts as a signal transducer in addition to being an ion pump, and ouabain is capable of inducing the activation of various signal transduction cascades. Neither resting nor concanavalin A (Con A)-activated thymocytes had their levels of phosphorylated-extracellular signal-regulated kinase (P-ERK) modified by ouabain. However, ouabain decreased p38 phosphorylation induced by Con A in these cells. The pathway induced by ouabain in lymphoid cells is still unclear but might vary with the type and state of activation of the cell.

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Lymphocytes activated by mitogenic lectins display changes in transmembrane potential, an elevation in the cytoplasmic Ca 2+ concentrations, proliferation and/or activation induced cell death. Low concentrations of ouabain (an inhibitor of Na + ,K + -ATPase) suppress mitogen-induced proliferation and increases cell death. To understand the mechanisms involved, a number of parameters were analyzed using fluorescent probes and flow cytometry. The addition of 100nM ouabain to cultures of peripheral blood lymphocytes activated with 5µg/ml phytohemagglutinin (PHA) did not modify the increased expression of the Fas receptor or its ligand FasL induced by the mitogen. However, treatment with ouabain potentiated apoptosis induced by an anti-Fas agonist antibody. A synergy between ouabain and PHA was also observed with regard to plasma membrane depolarization. PHA per se did not induce dissipation of mitochondrial membrane potential but when cells were also exposed to ouabain a marked depolarization could be observed, and this was a late event. It is possible that the inhibitory effect of ouabain on activated peripheral blood lymphocytes involves the potentiation of some of the steps of the apoptotic process and reflects an exacerbation of the mechanism of activation-induced cell death.

Multidrug resistance (MDR) is the phenomenon in which cultured tumor cells, selected for resistance to one chemotherapeutic agent, simultaneously acquire resistance to several apparently unrelated drugs. The MDR phenotype is multifactorial. The best-studied mechanism involves the expression of a membrane protein that acts as an energy-dependent efflux pump, known as P-glycoprotein (Pgp), capable of extruding toxic materials from the cell. In this work, resistance to UVA radiation, but not to UVC nor UVB, was observed in an MDR leukemia cell line. This cell line overexpresses Pgp. To study the role of Pgp in the resistance to UVA radiation, two MDR modulators or reversing agents (verapamil and cyclosporin A) capable of blocking Pgp activity were used. Cell viability was assessed and the techniques of flow cytometry and fluorescence microscopy were employed to measure the extrusion of rhodamine 123 by the efflux pump. The results show that MDR modulators did not modify the resistance to UVA radiation. Furthermore, although cell viability was not significantly altered, Pgp function was impaired after UVA treatment, suggesting that this glycoprotein may be a physical target for oxidative damage, and that other factors may be responsible for the UVA resistance. In agreement with this, it was found that the resistant cell line presented a higher catalase activity than the parental (non-MDR) cell line.