Intracellular Ca2+ is not essential for SHH signaling but is promoted by Shh ligand in embryonic fibroblasts

BACKGROUND: Sonic Hedgehog (SHH) signaling pathway controls cell proliferation, differentiation, and organ formation. The changes in Ca2+ and their regulatory role in the context of activated SHH signaling have not been fully examined. RESULTS: In this study, we induced activation of the SHH signaling pathway using activators, including Full-length Shh protein (Shh) and N-terminal Shh (ShhN) ligands or a Smoothened agonist (SAG), while concurrently manipulating calcium level by either increasing extracellular Ca2+ concentration or employing BAPTA to chelate intracellular Ca2+. We found that the activation of SHH target genes by the three activators was not affected by low intracellular Ca2+ but inhibited by excess Ca2+. Although both Shh and SAG activate Ca2+ influx via the store-operated calcium entry (SOCE), their effects differ in the presence of BAPTA: Shh directly increases intracellular Ca2+ concentration, whereas the effect of SAG is enhanced by high extracellular Ca2+. CONCLUSIONS: Excessive extracellular Ca2+ impairs SHH signal transduction, whereas intracellular Ca2+ levels have a negligible effect. At normal physiological extracellular Ca2+ concentrations, the depletion of intracellular Ca2+ does not affect SHH signal transduction.