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Mechanism of the cytolytic action of corticosteroids and its relationship to corticosteroid-resistance in malignant lymphocytes

University of British Columbia

When cells of the thymus or mouse lymphosarcoma P1798S are treated in vitro with 0.27 µM Cortisol in tissue culture medium with 10% serum or 0.5% human albumen, nuclear damage ending with karyorrhexis occurs. The steroid-resistant subline P1798R does not show these changes. The corticosteroid-sensitive lymphocytes which undergo lysis differ in several respects from the resistant subline. Most critical, apparently, is the capacity for oxidation of free fatty acids (FFA). This was determined by the evolution of ¹⁴CO₂ from labelled substrates, and isotope dilution with unlabelled substrate. Corticosteroid treatment in vivo for 2 hours raised the FFA in thymus and P1798S cells by 77% and 58%, respectively, while decreasing the resistant subline P1798R by 26%. In vitro, Cortisol had no effect on uptake of ¹⁴C-palmitic acid but decreased oxidation by 46% and 17% in thymus and P1798S, respectively, while increasing 9% in P1798R. After incubation of sensitive cells in medium containing FFA calculated to be equivalent to that accumulated after steroid treatment, electronmicroscopy revealed that certain effects of corticosteroids could be reproduced by fatty acids of chain length C-9 and higher: nuclear edema, focal dissolution and disintegration of the nuclear membrane and ultimately karyolysis. Steroid-resistant cells show cytological changes only at tenfold higher concentrations of FFA. On the basis of these results the following scheme is proposed as the mechanism by which cytolysis occurs in corticosteroid-sensitive lymphoid tissues:[See Thesis for Diagram] In an attempt to render resistant cells sensitive to the cytolytic actions of corticosteroids, studies were carried out where the oxidation of free fatty acids was inhibited in an attempt to induce an accumulation of FFA. Inhibition of oxidation by deoxycarnitine in the steroid-resistant P1798R, L5178Y, and L1210 tumors results in some lysis. Much more effective, however, are compounds with branched structures, which cannot undergo 3-oxidation but can be oxidized to mono- or dicarboxylic acids which are lytic. In thymocytes, P1798S and P1798R, FFA did not affect the in vitro incorporation of ¹⁴C-leucine and ³H-uridine into a 5% TCA insoluble precipitate. The incorporation of ³H-thymidine was only slightly reduced in the steroid-responsive cells. This suggests that some steroid effects, for example, those of immunosuppression, might be dissociable from those involving accumulation of FFA which lead to nuclear damage and cytolysis of sensitive cells.

Text

2011-03-28

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http://hdl.handle.net/2429/32998

Biochemistry and Molecular Biology

University of British Columbia

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