Ayyub, Mohammad; (1993) Actions of antioxidants on human T lymphocytes and leukaemia cells. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The roles of leukotrienes (LTs) in the m ediation of specific cellular responses in im m unity and inflam m ation are well established. Some authors have suggested th a t the endogenous generation of LTs m ay play a role in the regulation of the proliferation of both norm al and m alignant haemopoietic cells. I have therefore investigated effects of 5-lipoxygenase (LO) inhibitors on the proliferation of norm al and m alignant haemopoietic cells. Piriprost, nordihydroguiaretic acid (NDGA), and BW755C, inhibitors of LO, inhibited DNA synthesis (as m easured by the incorporation of [^H] thymidine) in freshly isolated leukaem ia blasts and in the leukaem ia cell lines HL60, K562, and Ju rk a t. However, DNA synthesis and proliferation (as m easured by counting trypan blue-excluding cells) of the above cell lines were not affected by the novel, highly specific LO inhibitors MK 886 and BWA4C in concentrations in excess of those required for substantial inhibition of leukotriene generation in whole blood. Furtherm ore, MK886 and BWA4C, did not im pair DNA synthesis in PHA-stim ulated T lymphocytes and norm al haemopoietic cells. The data suggest th a t endogenous leukotriene production does not play a role in regulating the proliferation of either norm al haemopoietic cells or leukaem ia cell lines. I have also studied effects of LO inhibitors on m itogen-stim ulated inositol lipid breakdown, one of the specific signal transduction pathw ays which regulate the proliferation of haemopoietic cells. Piriprost, NDGA, and BW755C inhibited PHA- stim ulated breakdown of inositol lipid in hum an T lymphocytes. However, inositol-lipid breakdown in PHA- and CD3 monoclonal antibody (McAb)- stim ulated T lymphocytes and Ju rk a t leukaem ia cells was not affected by the lipoxygenase inhibitors, BWA4C and MK886 in concentrations which were shown in parallel studies to substantially inhibit leukotriene generation in whole blood. Furtherm ore, leukotrienes generation (as m easured by radioim unoassay) was not detected following PHA stim ulation of T lymphocytes. These results imply th a t inhibition of m itogen-stim ulated inositol lipid breakdown by NDGA, BW755C, and piriprost m ust be due to a mechanism other th a n inhibition of lipoxygenase. Because these LO inhibitors also have antioxidant properties, I studied the effects of several general antioxidants on growth regulatory signal transduction pathw ays. Butylated hydroxytoluene (BHT), dithiothreitol, and N-acetylcysteine abrogated PHA- and CDS McAb- stim ulated inositol lipid breakdown in T lymphocytes. The effects of antioxidants on inositol-lipid breakdown were not attributable to irreversible damage to cellular structures or inhibition of P td ln s4,5-P2-PLC. These observations imply th a t an antioxidant-sensitive step was obligatory in coupling cell-surface receptors to the inositol signalling system in T lymphocytes. By contrast, antioxidants did not perturb PHA- or CDS McAb- stim ulated inositol lipid breakdown in Ju rk a t T acute lymphoblastic leukaem ia cell line. Therefore, the biochemical mechanisms which couple cell-surface receptors to the inositol lipid signalling system are different in norm al peripheral blood T lymphocytes and in Ju rk a t leukaem ia cells.

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