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Alpha-integrin expression and function modify chemoresistance and immunogenicity of acute lymphoblastic leukemia

University of British Columbia

The overall survival rate for pediatric Acute Lymphoblastic Leukemia (ALL) is >85%, achieved mainly via multi-agent chemotherapy. However, therapeutic options remain limited for those experiencing relapse, thus understanding the causes for treatment failures remains an important priority. In this thesis, I investigate the underlying mechanisms that allow leukemic cells to escape chemotherapy. Specifically, I evaluate how integrin-mediated cell adhesion promotes tumor cell survival by increased pro-survival signaling, enhanced resistance to chemotherapeutics, and decreased presentation of immunogenic cell death (ICD) markers. I show that T-lymphoblast adhesion via α4β1-integrin promotes chemoresistance to doxorubicin-induced apoptosis. Expression of α4δ, a tail-truncated α4-integrin with GFFKR as the cytoplasmic motif, promotes chemoresistance in a manner independent of integrin-mediated adhesion. The adhesion-independent chemoresistance is reproduced by expression of Tacδ, a non-integrin transmembrane receptor fused to the cytosolic GFFKR motif. Additionally, the GFFKR motif-mediated chemoresistance is associated with enhanced Akt activation, Ca²⁺ influx, and drug efflux. GFFKR is a conserved motif found in α-integrins and previously shown to interact with calreticulin, a calcium-binding endoplasmic reticulum chaperone protein. I found that α4-calreticulin interaction was enhanced by cell adhesion, while α4δ-calreticulin interaction occurred in an adhesion-independent manner. Since cell surface calreticulin is a pro-phagocytic marker for cells undergoing ICD, the impact of integrin function on surface calreticulin in lymphoblasts treated with ICD-inducing agents was evaluated. Engagement of integrins via adhesion, or expression of the minimal GFFKR motif as α4δ or Tacδ, was sufficient to reduce the levels of surface calreticulin. Furthermore, surface calreticulin was also reduced for cells co-treated with a β1-integrin activating antibody. The resulting integrin-mediated decrease in surface calreticulin significantly reduced engulfment of the target lymphoblasts by macrophages. Calreticulin expression in lymphoblasts was nullified to assess its role in integrin-mediated chemoresistance. Chemosensitivity was partially restored in calreticulin-null Tacδ cells under non-adherent conditions, and in calreticulin-null wildtype cells under adherent conditions. The affect was partly attributed to calreticulin’s role as a regulator of Ca²⁺ influx and drug efflux. Calreticulin was also implicated as a mediator of cytokine-dependent STAT proliferative signaling. This thesis provides evidence for integrin function and cell adhesion as a physiological pro-survival mediator for T-lymphoblasts.

Text

2017-02-10

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/60560

Experimental Medicine

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/