Selective down-regulation of IP3 receptor subtypes by caspases and calpainduring TNF alpha-induced apoptosis of human T-lymphoma cells

There are at least three types of inositol 1,4,5-trisphosphate receptor (IP 3R) [IP3-gated Ca2+ channels], which are expressed in different cell types and mammalian tissues. In this study, we have identified three IP3R subtype s in human Jurkat T-lymphoma cells. All three subtypes have a molecular mas s 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-re lated changes occur (e.g. proteolysis of IP3R subtypes, inhibition of IP3 b inding and impairment of IP3-mediated Ca2+ flux) concomitantly with an elev ation 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 th e calpain inhibitor 1, ALLN, is capable of blocking the inhibitory effect o f 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 mechanis ms may be responsible for the early onset of the apoptotic signal by TNF al pha in human T-cells. (C) 2000 Harcourt Publishers Ltd.