Targeting endoplasmic reticulum protein transport: a novel strategy to kill malignant B cells and overcome fludarabine resistance in CLL

Abstract

Previous studies showed that chronic lymphocytic leukemia (CLL) cells exhibit certain mitochondrial abnormalities including mtDNA mutations, increased superoxide generation, and aberrant mitochondrial biogenesis, which are associated with impaired apoptosis and reduced sensitivity to fludarabine. Here we report that CLL cells and multiple myeloma cells are highly sensitive to brefeldin A, an inhibitor of endoplasmic reticulum (ER) to Golgi protein transport currently being developed as a novel anticancer agent in a prodrug formulation. Of importance, brefeldin A effectively induced apoptosis in fludarabine-refractory CLL cells. Disruption of protein trafficking by brefeldin A caused the sequestration of the prosurvival factors APRIL and VEGF in the ER, leading to abnormal ER swelling and a decrease in VEGF secretion. Such ER stress and blockage of secretory protein traffic eventually resulted in Golgi collapse, activation of caspases, and cell death. Notably, the cellular sensitivity to this compound appeared to be independent of p53 status. Taken together, these findings suggest that malignant B cells may be highly dependent on ER-Golgi protein transport and that targeting this process may be a promising therapeutic strategy for B-cell malignancies, especially for those that respond poorly to conventional treatments. IntroductionChronic lymphocytic leukemia (CLL) is a common adult leukemia with unique characteristics in that the disease progression involves the persistent accumulation of apparently quiescent B cells, primarily due to defects in apoptosis rather than increased cell proliferation. 1 The apoptotic resistance of CLL cells has been attributed, in part, to high expression of antiapoptotic proteins such as Bcl-2, Bcl-X L , and Mcl-1. [2][3][4][5][6][7][8] Our previous work demonstrated that CLL cells contain multiple mitochondrial defects including mtDNA mutations, aberrant mitochondrial biogenesis, and increased generation of the oxygen radical superoxide, which is predominantly produced in the mitochondria. These abnormalities are associated with alterations in sensitivity to anticancer agents, suggesting that they may compromise the execution of the mitochondrial apoptotic machinery. [9][10][11] Fludarabine and alkylating agents have significantly improved clinical outcomes of CLL treatment. However, there are currently limited effective therapeutic options for patients that are refractory to these agents. 1,12,13 Thus, identification and evaluation of novel agents for the treatment of refractory CLL are important and challenging tasks. While investigating the mechanistic basis of aberrant mitochondrial biogenesis activity in primary CLL cells, we observed that in addition to increased mitochondrial content, CLL cells also appear to have a more extensive endoplasmic reticulum (ER) network than normal B lymphocytes. 10 The abundance of ER membranes in CLL cells has also been noted in earlier investigations and suggests that ER function may be very important for their survival. Consequently, this or...

γ‐Secretase inhibitor I induces apoptosis in chronic lymphocytic leukemia cells by proteasome inhibition, endoplasmic reticulum stress increase and notch down‐regulation

“…23 Importantly, CLL cells have a functional ER stress-associated apoptotic signaling, and are susceptible to both pharmacologic and genetic ER targeting strategies. 24,25 Based on all the above observations and our previous findings that GSI I increased CLL cell apoptosis and inhibited Notch activation, 9 we investigated whether an inhibited proteasome activity and an increased ER stress signaling are also involved in GSI I-induced CLL cell apoptosis, and examined how these events and Notch inhibition contribute to this apoptosis. Clarifying the effects of GSI I on these three critical regulators of CLL cell survival may be important to design more effective therapies for this leukemia.…”

Differential Noxa/Mcl-1 balance in peripheral versus lymph node chronic lymphocytic leukemia cells correlates with survival capacity

“…Currently, there is much interest in application of novel, p53-independent drugs to treat B-CLL. [51][52][53] Our data provide new insight into the regulation of the apoptotic behavior of B-CLL cells and also afford new clues for therapeutic intervention by targeting Noxa expression.…”

Tetraspanin-induced death of myeloma cell lines is autophagic and involves increased UPR signalling

“…Targeting protein synthesis and secretion in MM has been addressed previously (Carew et al, 2006;Nakamura et al, 2006;Davenport et al, 2007;Patterson et al, 2008). Several drugs abrogating ER -Golgi stability have been used in vitro to induce MM cell death.…”