Secreted cystatins decrease proliferation and enhance apoptosis of human leukemic cells

Samar Hunaiti, Hanna Wallin, Mia Eriksson, Marcus Järås, Magnus Abrahamson
FEBS Open Bio 2020 August 18
Cysteine proteases are implicated in proteolysis events favouring cancer cell growth, spread and death by apoptosis. Herein, we have studied whether the net growth and survival of the leukemic cell lines Jurkat, U937 and HL-60 are affected by external addition of five proteins acting as natural cysteine protease inhibitors. None of the cystatins examined (A, C, D and E/M) or chagasin showed consistent effects on Fas-induced apoptosis when evaluated at 1 µM. In contrast, when the intrinsic apoptosis pathway was activated by hydrogen peroxide, addition of cystatin D augmented caspase-3-like activity within all three cell lines. Flow cytometric analysis of U937 cells also showed increased numbers of annexin V positive cells when hydrogen peroxide was used to initiate apoptosis and cells were cultured in the presence of cystatin D or C. Moreover, stimulation of hydrogen peroxide-induced apoptotic U937 cells with either cystatin C or D resulted in a dose-dependent decrease of the number of cells. Cell viability was also decreased when U937 cells were cultured in the presence of cystatin C or D (1-9 µM) only, demonstrating that these cystatins can reduce cell proliferation by themselves in addition to enhancing apoptosis induced by oxidative stress. These effects on U937 cells were paralleled by internalization of cystatins C and D, indicating these effects are caused by down-regulation of intracellular proteolysis. External addition of cystatins C and D to HL-60 and Jurkat cells demonstrated similar degrees of cystatin D uptake and decreased viability as for U937 cells, indicating that these effects are general for leukemic cells.

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