Abstract
A distinguishing feature of tumor cells is their upregulation of glucose metabolism. This metabolic change from oxidative phosphorylation to aerobic glycolysis creates a selective window that can be exploited for anticancer therapy. Here we investigate mechanisms by which pancreatic cancer cells respond to therapeutic (2-deoxy-D-glucose, 2-DG) and physiologic (glucose starvation, GS) forms of glucose restriction. 2-Deoxy-d-Glucose (2-DG) is a sugar analog of both glucose and mannose and thus has established activities inhibiting glycolysis and interfering with glycosylation, respectively. Due to 2-DG’s dual functions, it has classically been thought of as a glucose deprivation mimetic. Additionally, these roles render 2-DG an attractive anticancer agent with potential to kill both hypoxic and normoxic tumor cells. In order to anticipate and understand resistance mechanisms that may arise when 2-DG is used clinically, here we investigate survival mechanisms in cell lines that are either selected for resistance or intrinsically resistant to the sugar analog. Generally, the resistant cell lines examined display muted induction of the unfolded protein response (UPR) when treated with 2-DG. Additionally, we find multiple mechanisms upstream of ER stress/UPR that appear to contribute to resistance including: lower 2-DG uptake and reduced glycosylation interference. Due to 2-DG’s inhibitory activities, it is thought to mimic glucose starvation (GS). Surprisingly, cell lines that were resistant to 2-DG were found to be more sensitive to GS than the 2-DG sensitive cell line, 1420. Sensitivity to GS coincided with reduced breakdown of glycogen to glucose, which correlated with lowered levels of glycogen phosphorylase (PYGB). Inhibition of PYGB prevented glycogen breakdown and restored sensitivity to GS in 1420. Overall the data demonstrate that the manner in which glucose is restricted in tumor cells, i.e. therapeutic or physiologic, leads to differential biological responses involving distinct glucose metabolic pathways. Moreover, in evolving tumors where glucose restriction occurs, the identification of PYGB as a metabolic target may have clinical application.