The serine synthesis pathway: an unexplored metabolic pathway in endothelial cells, regulating angiogenesis?

The control of metabolism is a fundamental requirement for all life, with perturbations of metabolic homeostasis underlying numerous pathologies. Even though cellular metabolism has been studied for over a century, endothelial cell (EC) metabolism has been receiving growing attention only during the last few years.

The endothelium, a monolayer of endothelial cells lining the blood vessel lumen, is in direct contact with the blood and forms an important selective barrier between the circulating blood cells, proteins, metabolites and the surrounding tissue. Blood vessel-forming ECs display a remarkable behavioral plasticity during the process of vessel sprouting or angiogenesis; while quiescent for years, ECs can switch almost instantaneously to an activated, highly proliferative, and migratory state in response to growth factor stimuli, primarily through vascular endothelial growth factor-A (VEGFA) signaling. These growth factors can trigger active physiological (embryonical development, wound healing) or pathological angiogenesis (e.g. cancer, inflammation). Tumors for instance, highly upregulate pro-angiogenic factors to stimulate vessel sprouting in order to meet the continuous high demand for oxygen and nutrients, which enables the tumor to expand.

Current anti-angiogenic treatment in cancer therapy focuses on starving tumors from their vascular supply by destroying newly formed blood vessels through targeting pro-angiogenic growth factors. Despite prolongation of survival of cancer patients by a few months, current anti-angiogenic therapies suffer from insufficient efficacy, refractoriness and resistance. Thus, there is a strong clinical need to discover conceptually distinct anti-angiogenic strategies that bypass these resistance mechanisms and/or can be used in combination with the existing anti-angiogenic therapies.

Since the capacity to proliferate upon a pro-angiogenic stimulus implies that ECs are able to generate the required energy and molecular building blocks for biomass duplication, it has been postulated that a metabolic switch accompanies the angiogenic switch. However, little is known about which metabolic pathways ECs rely on to form new vessels. The host lab has recently published papers on the role of glycolysis and fatty acid oxidation (FAO), which confirmed that metabolism drives vessel sprouting in parallel to well-established growth factor-based (genetic) signaling. Aside from glycolysis and FAO, (endothelial) cells employ various other metabolic pathways, including the metabolism of the amino acid serine.

In this thesis, I explored to which extent serine metabolism is essential for angiogenesis. I report that phosphoglycerate dehydrogenase (PHGDH), the first and rate-limiting enzyme of the serine synthesis pathway (SSP), is critical to support angiogenic function of endothelial cells. Remarkably, neonatal mice with EC-specific gene knockout of PHGDH (PHGDHECKO) displayed severe vascular impairment and multi-organ angiogenic defects resulting in death within days of gene-inactivation. Complementary in vitro functional analyses confirmed that blockade of PHGDH caused vascular sprouting defects, due to impaired EC proliferation. Exposure to SSP products such as serine, glycine or a-ketoglutarate (aKG) did not rescue the proliferation defect of PHGDHKD ECs, indicating that endogenous serine synthesis must have a critical role in supporting proliferation of these cells. Mechanistically, impaired serine synthesis in ECs reduced nucleotide synthesis and impaired reactive oxygen species (ROS) scavaging capacity. In addition, insufficient heme production due to cellular serine depletion impaired electron-transport-chain enzyme activities. Mitochondrial respiration defects resulted in the inhibition of the mitochondrial pyrimidine synthesizing dihydroorotate dehydrogenase and elevated total and mitochondrial reactive oxygen species (ROS) levels. Subsequently, this impaired mitochondrial homeostasis, leading to EC apoptosis.  Supplementation of hemin in PHGDHKD ECs restored electron transport chain (ETC) function and largely rescued the apoptosis and angiogenesis defects, both in vitro and in vivo.

These results provide novel insights regarding the metabolic profile of ECs and present PHGDH as an important metabolic gatekeeper for nucleotide synthesis, mitochondrial homeostasis and survival via heme synthesis. Therefore, I provided valuable information and my study might prime interest in targeting EC metabolism as an alternative anti-angiogenesis approach.