STIM1 but not STIM2 is an essential regulator of Ca influx-mediated NADPH oxidase activity in neutrophil-like HL-60 cells

Extracellular Ca²⁺ entry, primarily mediated through store-operated Ca²⁺ entry (SOCE), is known to be a critical event for NADPH oxidase (NOX2) regulation in neutrophils. While defective NOX2 activity has been linked to various inflammatory diseases, regulatory mechanisms that control Ca²⁺ influx-induced NOX2 activation are poorly understood in SOCE. The role of STIM1, a Ca²⁺ sensor that transduces the store depletion signal to the plasma membrane, seems well established and supported by numerous studies in non-phagocytic cells. Here, in neutrophil-like HL-60 cells we used a siRNA approach to delineate the effect of STIM1 knock-down on NOX2 activity regulated by Ca²⁺ influx. Because the function of the STIM1 homolog, STIM2, is still unclear, we determined the consequence of STIM2 knock-down on Ca²⁺ and NOX2. STIM1 and STIM2 knock-down was effective and isoform specific when assayed by real-time PCR and Western blotting. Consistent with a unique role of STIM1 in the regulation of SOCE, STIM1, but not STIM2, siRNA significantly decreased Ca²⁺ influx induced by fMLF or the SERCA pump inhibitor thapsigargin. A redistribution of STIM1, originally localized intracellularly, near the plasma membrane was observed by confocal microscopy upon stimulation by fMLF. Inhibition of STIM1-induced SOCE led to a marked decrease in NOX2 activity while STIM2 siRNA had no effect. Thus, our results provide evidence for a role of STIM1 protein in the control of Ca²⁺ influx in neutrophils excluding a STIM2 involvement in this process. It also places STIM1 as a key modulator of NOX2 activity with a potential interest for anti-inflammatory pharmacological development.