Extracellular matrix regulation of metabolism and implications for tumorigenesis.

Attachment to extracellular matrix (ECM) is required for the survival and proliferation of normal epithelial cells. Epithelial tumor cells, however, often acquire "anchorage independence," a property that may contribute to their ability to invade and grow in foreign environments. Although apoptosis is the most rapid and effective mechanism that causes the death of matrix-detached cells, it has become apparent that detachment from matrix alters other aspects of cell physiology prior to commitment to cell death and that some of these alterations can lead to cell death under conditions where apoptosis is suppressed. This report provides an overview of death processes that contribute to the death of matrix-detached normal cells and describes mechanisms that confer anchorage independence, with a focus on ECM regulation of cell metabolism. Loss of matrix attachment leads to metabolic stress characterized by reduced nutrient uptake, decreased ATP production, and increased levels of reactive oxygen species (ROS). The decrease in ATP levels is prevented by either constitutive activation of the PI3K/Akt pathway or exogenous antioxidants. Additionally, decreased Erk signaling in matrix-detached cells causes a disproportionate decrease in flux through pyruvate dehydrogenase (PDH), leading to decreased entry of glucose carbons into the citric acid cycle. Interestingly, forced overexpression of a PDH inhibitor suppresses de novo lipogenesis and proliferation, highlighting the importance of mitochondrial metabolism in supplying intermediates for biosynthetic processes required for proliferation. Thus, ECM attachment is a key regulator of cellular metabolism, and alterations in metabolism owing to changes or loss of ECM engagement during tumorigenesis may serve important tumor-suppressive functions.

Autophagy is essential to suppress cell stress and to allow BCR-Abl-mediated leukemogenesis.

Hematopoietic cells normally require cell extrinsic signals to maintain metabolism and survival. In contrast, cancer cells can express constitutively active oncogenic kinases such as BCR-Abl that promote these processes independent of extrinsic growth factors. When cells receive insufficient growth signals or when oncogenic kinases are inhibited, glucose metabolism decreases and the self-digestive process of autophagy is elevated to degrade bulk cytoplasm and organelles. Although autophagy has been proposed to provide a cell-intrinsic nutrient supply for mitochondrial oxidative metabolism and to maintain cellular homeostasis through degradation of damaged organelles or protein aggregates, its acute role in growth factor deprivation or inhibition of oncogenic kinases remains poorly understood. We therefore developed a growth factor-dependent hematopoietic cell culture model in which autophagy can be acutely disrupted through conditional Cre-mediated excision of the autophagy-essential gene Atg3. Treated cells rapidly lost their ability to perform autophagy and underwent cell cycle arrest and apoptosis. Although Atg3 was essential for optimal upregulation of mitochondrial oxidative pathways in growth factor withdrawal, this metabolic contribution of autophagy did not appear critical for cell survival, as provision of exogenous pyruvate or lipids could not completely rescue Atg3 deficiency. Instead, autophagy suppressed a stress response that otherwise led to p53 phosphorylation and upregulation of p21 and the pro-apoptotic Bcl-2 family protein Puma. Importantly, BCR-Abl-expressing cells had low basal levels of autophagy, but were highly dependent on this process, and rapidly underwent apoptosis upon disruption of autophagy through Atg3 deletion or treatment with chemical autophagy inhibitors. This dependence on autophagy extended in vivo, as Atg3 deletion also prevented BCR-Abl-mediated leukemogenesis in a cell transfer model. Together these data demonstrate a critical role for autophagy to mitigate cell stress, and that cells expressing the oncogenic kinase BCR-Abl appear particularly dependent on autophagy for cell survival and leukemogenesis.