The canonical transient receptor potential 6 channel as a putative phosphatidylinositol 3,4,5-trisphosphate-sensitive calcium entry system

We previously reported that phosphatidylinositol 3,4,5-trisphosphate (PIP3), a lipid product of phosphomositide 3-kinase (PI3K), induced Ca2+ influx via a noncapacitative pathway in platelets, Jurkat T cells, and RBL-2H3 mast cells. The identity of this Ca2+ influx system, however, remains unclear. Here, we investigate a potential link between PIP3-sensitive Ca2+ entry and the canonical transient receptor potential (TRPC) channels by developing stable human embryonic kidney (HEK) 293 cell lines expressing TRPC1. TRPC3, TRPC5, and TRPC6. Two lines of evidence support TRPC6 as a putative target by which PIP3 induces Ca2+ influx. First, Fura-2 fluorometric Ca2+ analysis shows the ability of PIP3 to selectively stimulate [Ca2+](i) increase in TRPC6-expressing cells. Second, pull-down analysis indicates specific interactions between biotin-PIP3 and TRPC6 protein. Our data indicate that PIP3 activates store-independent Ca2+ entry in TRPC6 cells via a nonselective cation channel. Although the activating effect of PIP3 on TRPC6 is reminiscent to that of 1-oleoyl-2-acetyl-sn-glycerol, this activation is not attributable to the diacylglycerol substructure of PIP3 since other phosphoinositides failed to trigger Ca2+ responses. The PIP3-activated Ca2+ entry is inhibited by known TRPC6 inhibitors such as Gd3+ and SKF96365 and is independent of IP3 production. Furthermore, we demonstrated that TRPC6 overexpression or antisense downregulation significantly alters the amplitude of PIP3- and anti-CD3-activated Ca2+ responses in Jurkat T cells. Consequently, the link between TRPC6 and PIP3-mediated Ca2+ entry provides a framework to account for an intimate relationship between PI3K and PLCgamma in initiating Call response to agonist stimulation in T lymphocytes.