Calcium-dependent Signaling Pathways in T Cells

Abstract

Engagement of ␤1 integrin receptors initiates an increase in intracellular calcium concentrations in T cells, potentially affecting calcium-sensitive signaling pathways. The calcium-activated cysteine protease, calpain, regulates a variety of cell functions by calcium-dependent limited proteolysis. To investigate the function of calpain in T cells, we sought to determine the role of this protease in calcium-dependent signaling events. Subsequent to elevations in intracellular calcium concentrations induced by ionomycin or adherence to fibronectin, calpain activity translocated to the cytoskeletal/membrane fraction of T cells. In addition, stimulation of T cells with these agents initiated the proteolytic cleavage of protein tyrosine phosphatase 1B by calpain. Enzymatic cleavage of protein tyrosine phosphatase 1B occurs near the endoplasmic reticulum-targeting sequence and results in the generation of an enzymatically active form of the phosphatase. Furthermore, we show that both the native and the cleaved forms of protein tyrosine phosphatase 1B interact with p130Cas in T cells. This interaction may serve to relocate protein tyrosine phosphatase 1B to sites of focal contact resulting in potential interactions with substrates previously inaccessible to the endoplasmic reticulum-associated phosphatase. Thus, we describe a novel calcium-dependent signaling pathway in T cells that may mediate signals generated by ␤1 integrin adherence to the extracellular matrix.Calpain (calcium-activated cysteine protease; EC 3.4.22.17) is ubiquitously expressed in mammalian cells, yet its precise physiologic function in T cells remains to be determined. The majority of calpain is localized in the cytosol of cells (1-4). However, raising intracellular calcium concentrations ([Ca 2ϩ ] i ) 1 in cells induces calpain translocation to the plasma membrane, the cytoskeleton, and points of attachment between cells and the extracellular matrix (ECM) (5-9). Furthermore, interactions between integrins and ECM proteins in some cell types, induces the translocation and activation of calpain in response to elevated [Ca 2ϩ ] i (5, 10 -12). Calpain can induce the proteolytic modification of proteins associated with multiple signaling cascades, such as protein kinase C, phospholipase C, pp60Src , and focal adhesion kinase (13-16) as well as the limited proteolysis of cytoskeletal proteins including talin, actinbinding protein, and paxillin (17-19). Thus, calpain is implicated in cytoskeletal reorganization as well as signal transduction pathways in response to elevations in [Ca 2ϩ ] i . This suggests that calpain has a crucial role in integrin-induced alterations in cellular physiology and function. Interaction of integrins with their ligands activates cellular signaling pathways and reorganization of the cytoskeletal system (20 -24). An increase in [Ca 2ϩ ] i represents one second messenger that is quantitatively altered as a consequence of these interactions. This would further suggest the possibility that calpain is activated b...

Differential expression and translocation of protein tyrosine phosphatase 1B-related proteins in ME-180 tumor cells expressing apoptotic sensitivity and resistance to tumor necrosis factor: potential interaction with epidermal growth factor receptor

“…The precise mechanism of PTP1B truncation and cellular tra cking in apoptotically responsive cells is unknown but may be related to processing pathways for PTP1B previously described (Frangioni et al, 1993). Calcium-responsive proteases and other proteolytic enzymes were able to recognize and cleave PTP1B at its C-terminus and calpain inhibitors were able to prevent cleavage of PTP1B protein in platelets and activated T-cells (Frangioni et al, 1993;Rock et al, 1997). However, while these inhibitors (calpeptins) were ine ective in modifying the intrinsic level of PTP1B-like proteins in Sen cells, other agents which alter Ca 2+ stores (A23187, thapsigargin) e ect PTP1B-like proteins in Sen cells (Donato, et al, manuscript in preparation) demonstrating that Ca 2+¯u x can e ect PTP1B-like protein expression in Sen cells but its mechanism of action may be distinct from that which controls intrinsic PTP1B-like protein levels in ME-180 cells.…”

“…One possible mechanism may involve the uncoupling of tyrosine phosphorylated receptor signaling events from activation of down stream cell growth or survival pathways. However, PTP1B-like truncated proteins may also associate with other signaling proteins in a phosphotyrosine independent fashion and mediate changes in other signaling events (Liu et al, , 1998Rock et al, 1997). Analysis of the preferred substrate interaction of the 50, 42 and 36 kDa proteins may provide additional information about the role of these molecules in signal transduction and apoptosis.…”

Essential Tyrosine Residues for Interaction of the Non-receptor Protein-tyrosine Phosphatase PTP1B with N-cadherin

“…The carboxyl terminus of PTP1B directs its localization to the cytoplasmic face of the endoplasmic reticulum, thus restricting the number of potential interactors (20). In platelets and activated T-cells, proteolytic cleavage in the ER targeting domain results in translocation of PTP1B to the cytoskeletal/membrane fraction (27)(28)(29). This cleavage is dependent on integrin engagement, resulting in increased Ca 2ϩ levels and, consequently, activation of calpain.…”

Turn‐off, drop‐out: Functional state switching of cadherins

“…The carboxy terminus of PTP1B directs its localization to the cytosolic face of the endoplasmic reticulum (Frangioni et al, 1992). In platelets and activated T cells, proteolytic cleavage in the ER targeting domain results in translocation of PTP1B to the cytoskeletal/membrane fraction (Frangioni et al, 1993;Ezumi et al, 1995;Rock et al, 1997). This cleavage is dependent on integrin engagement, resulting in increased Ca 2ϩ levels and, consequently, activation of calpain.…”