Regulation of store-operated Ca²⁺ entry by CD38 in human airway smooth muscle

The ectoenzyme CD38 catalyzes synthesis and degradation of cyclic ADP ribose in airway smooth muscle (ASM). The proinflammatory cytokine TNFα, which enhances agonist-induced intracellular Ca²⁺ ([Ca ²⁺]_i) responses, has been previously shown to increases CD38 expression. In the present study, we tested the hypothesis that the effects of TNFα on CD38 expression vs. changes in [Ca²⁺]_i regulation in ASM cells are linked. Using isolated human ASM cells, CD38 expression was either increased (transfection) or knocked down [small interfering RNA (siRNA)], and [Ca²⁺]_i responses to sarcoplasmic reticulum depletion [i.e., store-operated Ca²⁺ entry (SOCE)] were evaluated in the presence vs. absence of TNFα. Results confirmed that TNFα significantly increased CD38 expression and ADP-ribosyl cyclase activity, an effect inhibited by CD38 siRNA, but unaltered by CD38 overexpression. CD38 suppression blunted, whereas overexpression enhanced, ACh-induced [Ca²⁺]_i responses. TNFα-induced enhancement of [Ca²⁺]_i response to agonist was blunted by CD38 suppression, but enhanced by CD38 overexpression. Finally, TNFα-induced increase in SOCE was blunted by CD38 siRNA and potentiated by CD38 overexpression. Overall, these results indicate a critical role for CD38 in TNFα-induced enhancement of [Ca²⁺]_i in human ASM cells, and potentially to TNFα augmentation of airway responsiveness.