Humans possess elaborate mechanisms to maintain a supply of nutrients to organs despite intermittent energy consumption. Blood vessels deliver key nutrients to tissues, such as glucose, oxygen, and lipids. Further, cells regulate their metabolism to match nutrient availability in order to maintain homeostasis. In solid tumors, cancer cells must also adapt to maintain growth; in this case, to a hypoxic, low nutrient environment because tumor expansion outpaces blood supply. Hypoxia-inducible factor (HIF) transcription factors are central regulators of the adaptive response to low oxygen in both normal and cancer cells. HIFs are upregulated by low oxygen and respond to tumor hypoxia by altering cell proliferation, metabolism, and stimulating angiogenesis; all of which support tumor growth and metastasis. Consequently, HIF is a primary target for the development of anti-cancer therapeutics. Lipids such as cholesterol and fatty acids are essential cell building blocks required for tumor cell growth. Cells obtain lipid either by uptake from circulating lipoproteins outside the cell or through de novo synthesis. Like oxygen, lipid is limiting in poorly vascularized, hypoxic solid tumors due to the reduced supply of serum lipoprotein and because synthesis of cholesterol and unsaturated fatty acids requires oxygen. How lipid supply is maintained to support growth in hypoxic tumors is not understood. During studies of how human pancreatic ductal adenocarcinoma tumor cells maintain lipid supply, we discovered that removal of serum lipoprotein activates both HIF1a and HIF2a in the presence of oxygen. Based on these findings, we hypothesize that lipoprotein regulates activity of HIF transcription factors through a pathway distinct from oxygen to control lipid homeostasis. To understand how lipoprotein regulates HIF and how HIF activation affects cell homeostasis, we propose the following specific aims:
Aim 1. To identify the lipoprotein component that regulates HIF signaling.
Aim 2. To define the mechanism of lipoprotein regulation of HIFa.
Aim 3. To define HIF-dependent transcriptional and metabolic programs in response to lipoprotein depletion.
Aim 4. To identify genes required for lipoprotein signaling to HIF. These experiments will define lipoprotein as a distinct regulator of HIF that is independent from oxygen, supporting a new model in which lipoprotein and oxygen coordinately control glucose metabolism, erythropoiesis, angiogenesis, and cancer metabolism. Furthermore, these studies will define functions for HIF in lipid homeostasis, providing insight into how cancer cells acquire lipid in the nutrient-deprived environment of a solid tumor. In this proposal, we focus on cultured pancreas cancer cells and perform the first analysis of HIF-dependent gene expression in the pancreas. Future studies will investigate lipoprotein regulation of HIF in other cancer cell types, primary cells, and animals.
Hypoxia-inducible factor transcription factors are master regulators of oxygen homeostasis in humans, functioning to regulate glucose metabolism, erythrogenesis, cell survival, and angiogenesis. To date, oxygen is thought to be the principal signal controlling activity of these transcription factors, but here we describe equivalent regulation by lipoproteins. This proposal seeks to understand the upstream signals and downstream effects of HIF activation under conditions of low lipoprotein supply.
