Thrombospondin 1 binding to calreticulin‐LRP1 signals resistance to anoikis

Abstract

Anoikis, apoptotic cell death due to loss of cell adhesion, is critical for regulation of tissue homeostasis in tissue remodeling. Fibrogenesis is associated with reduced fibroblast apoptosis. The matricellular protein thrombospondin 1 (TSP1) regulates cell adhesion and motility during tissue remodeling and in fibrogenesis. The N-terminal domain of TSP1 binds to the calreticulin-LRP1 receptor co-complex to signal down-regulation of cell adhesion and increased cell motility through focal adhesion disassembly. TSP1 signaling through calreticulin-LRP1 activates cell survival signals such as PI3-kinase. Therefore, we tested the hypothesis that TSP1 supports cell survival under adhesion-independent conditions to facilitate tissue remodeling. Here, we show that platelet TSP1, its N-terminal domain (NoC1) as a recombinant protein, or a peptide comprising the calreticulin-LRP1 binding site [amino acids 17-35 (hep I)] in the N-terminal domain promotes fibroblast survival under anchorage-independent conditions. TSP1 activates Akt and decreases apoptotic signaling through caspase 3 and PARP1 in suspended fibroblasts. Inhibition of PI3K/Akt activity blocks TSP1-mediated anchorage-independent survival. Fibroblasts lacking LRP1 or expressing calreticulin lacking the TSP1 binding site do not respond to TSP1 with anchorage-independent survival. These data define a novel role for TSP1 signaling through the calreticulin/LRP1 co-complex in tissue remodeling and fibrotic responses through stimulation of anoikis resistance.-Pallero, M. A., Elzie, C. A., Chen, J., Mosher, D. F., Murphy-Ullrich, J. E. Thrombospondin 1 binding to calreticulin-LRP1 signals resistance to anoikis.

Clinicopathological significance of calreticulin in breast invasive ductal carcinoma

“…40 Matricellular protein thrombospondin 1, when bound to calreticulin, induces resistance to anoikis in fibroblasts. 41 Cancer cells are known to acquire enhanced survival mechanisms, which include regulating calcium homeostasis in the endoplasmic reticulum 30 to counter hypoxia and endoplasmic reticulum stress. 42 Calreticulin, being the main alleviator of endoplasmic stress, 43 could therefore have a key role in the survival of cancer cells.…”

Calreticulin promotes cell motility and enhances resistance to anoikis through STAT3–CTTN–Akt pathway in esophageal squamous cell carcinoma

“…In combination, these results suggest that CRT overexpression elevates the expression of CTTN protein, promotes activation of the PI3K/Akt pathway and finally increases resistance to anoikis in ESCC. It has recently been reported that CRT-LRP1 receptor complex has a critical role in thrombospondin 1 signaling to increase cell survival of fibroblasts in anoikis by downregulating apoptotic signaling and stimulating Akt activity (Pallero et al, 2008). However, in human glioblastoma U251MG cells, overexpression of CRT-enhanced radiation-induced apoptosis by suppressing Akt signaling (Okunaga et al, 2006).…”

“…CRT enhances cell migration and wound healing (Gold et al, 2006), and influences apoptosis positively or negatively via modulation of intracellular calcium stores (Mesaeli and Phillipson, 2004;Vanoverberghe et al, 2004;Chen et al, 2005). Recent evidence has shown that CRT protects fibroblasts from anoikis through binding to LRP1 and TSP-1 (Pallero et al, 2008). Akt activation or inactivation has been reported to play an important role in CRT-associated apoptosis (Kageyama et al, 2002;Okunaga et al, 2006;Lim et al, 2008).…”

The Calreticulin-Binding Sequence of Thrombospondin 1 Regulates Collagen Expression and Organization During Tissue Remodeling

“…[12][13][14] TSP1 can be proapoptotic to endothelial cells, but it also stimulates cell survival by signaling resistance to anoikis. 15 These diverse and sometimes paradoxical activities can be ascribed to its interactions with multiple receptors, including integrins, syndecans, 13,16 CD47, 17 CD36, 18 low-density lipoprotein receptor-related protein 1 (LRP1), 19 and calreticulin (CRT). 20 To date, the role of TSP1 in tissue remodeling has largely been studied through injury models in TSP1-null mice.…”

“…Furthermore, TSP1 binding to aa19-36 in the NTD of CRT is necessary for TSP-CRT binding and induction of signaling, and cells lacking this site in CRT do not respond to TSP1. 15,30,31 The role of TSP1 binding to the CRT-LRP1 complex in vivo is unknown. Based on previous studies, we hypothesized that local expression of the secreted CRT-binding sequence of TSP1 at sites of injury in vivo would signal intermediate cell adhesion and migration of CRT-expressing cells to increase cellularity of wounds.…”