Selective down-regulation of IP3receptor subtypes by caspases and calpain during TNF α -induced apoptosis of human T-lymphoma cells

Abstract

There are at least three types of inositol 1,4,5-trisphosphate receptor (IP3R) [IP3-gated Ca2+channels], which are expressed in different cell types and mammalian tissues. In this study, we have identified three IP3R subtypes in human Jurkat T-lymphoma cells. All three subtypes have a molecular mass of about 260 kDa, and display Ca2+channel properties in an IP3-dependent manner. We have also demonstrated that TNFα promotes the activity of different proteases (e.g. caspase-8, caspase-3 and calpain), alters the TCR-mediated Ca2+response and subsequently induces apoptosis in Jurkat cells. During the first 6 h of incubation with TNFα, several IP3R subtype-related changes occur (e.g. proteolysis of IP3R subtypes, inhibition of IP3binding and impairment of IP3-mediated Ca2+flux) concomitantly with an elevation of protease (caspase-8, caspase-3 and calpain) activity. Furthermore, the caspase inhibitor, Z-VAD-fmk, significantly reduces TNFα-mediated perturbation of IP3R1 and IP3R2 (but not IP3R3) function; whereas the calpain inhibitor I, ALLN, is capable of blocking the inhibitory effect of TNFα on IP3R3 function. These findings suggest that IP3R1 and IP3R2 serve as cellular substrates for caspases, and IP3R3 is a substrate for calpain. We propose that the selective down-regulation of IP3R subtype-mediated Ca2+function by caspase-dependent and calpain-sensitive mechanisms may be responsible for the early onset of the apoptotic signal by TNFα in human T-cells.