Selective down-regulation of IP3 receptor subtypes by caspases and calpain during TNFα-induced apoptosis of human T-lymphoma cells

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 TNF alpha 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 TNF alpha, several IP3R subtype-related changes occur (e.g. proteolysis of IP3R subtypes, inhibition of IP3 binding 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 TNF alpha-mediated perturbation of IP(3)R1 and IP(3)R2 (but not IP(3)R3) function; whereas the calpain inhibitor 1, ALLN, is capable of blocking the inhibitory effect of TNF alpha on IP(3)R3 function. These findings suggest that IP(3)R1 and IP(3)R2 serve as cellular substrates for caspases, and IP(3)R3 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 TNF alpha in human T-cells. (C) 2000 Harcourt Publishers Ltd.