TLR3-/4-Priming Differentially Promotes Ca2+ Signaling and Cytokine Expression and Ca2+ -Dependently Augments Cytokine Release in hMSCs

In human mesenchymal stem cells (hMSCs), toll-like receptor 3 (TLR3) and TLR4 act as key players in the tissue repair process by recognizing their ligands and stimulating downstream processes including cytokine release. The mechanisms of TLR3-and TLR4-mediated cytokine releases from hMSCs remain uncertain. Here, we show that exposure to the TLR3 agonist polyinosinic-polycytidylic acid (poly(I:C)) or incubation with the TLR4 agonist lipopolysaccharide (LPS) increased the mRNA expression levels of TLR3, TLR4 and cytokines in hMSCs. Poly(I:C) exposure rather than LPS incubation not only elevated inositol 1,4,5-triphosphate receptor (IP3R) expression and IP3R-mediated Ca2+ release, but also promoted Orai and STIM expression as well as store-operated Ca2+ entry into hMSCs. In addition, we also observed that 21 Ca2+ signaling genes were significantly up-regulated in response to TLR3 priming of hMSCs by RNA sequencing analysis. Both poly(I: C) and LPS exposure enhanced cytokine release from hMSCs. The enhanced cytokine release vanished upon siRNA knockdown and chelation of intracellular Ca2+. These data demonstrate that TLR3-and TLR4-priming differentially enhance Ca2+ signaling and cytokine expression, and Ca2+ -dependently potentiates cytokine release in hMSCs.