Werner Kovacs PD Dr.
Werner is a senior scientist in the Aceto lab. He obtained his PhD in Biochemistry/Biotechnology from the Vienna University of Technology. As a postdoctoral research fellow in Prof. Skaidrite Krisans´ group at San Diego State University, he used the peroxisome-deficient Pex2-/- mouse, a model for Zellweger syndrome, to define the importance of peroxisomes in maintaining cholesterol homeostasis. They showed that peroxisome deficiency activates ER stress pathways, especially the integrated stress response, leading to dysregulation of the endogenous sterol response mechanism and SREBP-2 pathway. In 2007, he joined the group of Prof. Wilhelm Krek as senior scientist (Oberassistent), where he studied how hypoxia signaling affects hepatic metabolism and organelle homeostasis. He was responsible for teaching duties in the group of Prof. Krek (Courses in Molecular Cell Biology, Molecular Medicine, Cancer Cell Signalling, Fundamentals in Biology; Lectures in Cell Biology, Biochemistry and Pathobiochemistry) and the principal study director in charge of animal experiments. After the decease of Prof. Krek in 2018, he joined the Aceto lab as senior scientist.
The overarching goal of our research is 1) to understand the molecular mechanism of HIF-2a-mediated pexophagy and 2) to understand the role of altered peroxisome abundance in tumorigenesis.
Control of peroxisome abundance and function in health and disease
Peroxisomes are ubiquitous and highly dynamic organelles that play a central role in the metabolism of lipids (i.e., b- and a-oxidation of fatty acids, etherphospholipid synthesis, cholesterol and isoprenoid metabolism, bile acid synthesis) and reactive oxygen species. They also act as intracellular signaling platforms in redox, lipid, inflammatory, and innate immune signaling. Their importance is stressed by the existence of genetic peroxisome biogenesis disorders, leading to complex developmental and metabolic phenotypes. A remarkable feature of peroxisomes is their ability to proliferate or to be degraded in response to nutritional and environmental stimuli. Their abundance is controlled through a balance of biogenesis and degradation. Peroxisomes are linked to a number of human health concerns including cancer, neuropathology, aging, heart disease, obesity, and diabetes. Understanding the plasticity of peroxisomes, their different functions and the details of their biogenesis/turnover is important for determining the contributions the organelle makes to many human disorders and for developing treatments for these conditions.
Peroxisome abundance is reduced in various cancers, but the cause and consequence of its reduction was unclear. Lack of peroxisomal metabolism leads to extensive metabolic disarray [e.g., accumulation of very long-chain (VLC) fatty acids (FAs) and abnormal VLC polyunsaturated fatty acids (PUFAs), deficiency of etherlipids]. Reprogramming of metabolism is a hallmark of cancer, and lipid metabolism alterations are increasingly being recognized to play an important role in tumor development. Hypoxia or von Hippel-Lindau (VHL) tumor suppressor loss stabilizes hypoxia-inducible factors a (HIF-1a and -2a), which are key mediators of the adaptive response to hypoxia in pathophysiological conditions such as cancer and ischemic diseases. Since peroxisomal function depends highly on molecular oxygen (O2), we hypothesized that regulation of peroxisome homeostasis may be directly linked to O2 availability and the regulatory system of O2 signaling. We examined the effect of HIF signaling on peroxisome abundance and metabolism in liver-specific Vhl, Vhl/Hif1a, and Vhl/Epas1 knockout mice. We discovered that HIF-2a, but not HIF-1a, augments peroxisome turnover by selective autophagy (pexophagy) and thereby changes lipid composition reminiscent of peroxisomal disorders (e.g., accumulation of VLCFAs and VLC-PUFAs). These lipids are potent activators of PPARa, a nuclear receptor that induces peroxisome proliferation. HIF-2a signaling represses this ligand-dependent activation of PPARa and by this means prevents PPARa-mediated peroxisome proliferation and consequential restoration of peroxisome homeostasis. The autophagy receptors Nbr1 and Sqstm1 localize to peroxisomes and are likewise degraded by HIF-2a-mediated pexophagy.
Figure 1. Model for Hif-2a-mediated decrease in peroxisome abundance (Walter, Schönenberger et al., Cell Metabolism 2014).
It is an open question how HIF-2a induces pexophagy at the molecular level, but there is emerging evidence that pexophagy, similar to other selective autophagy pathways, is regulated by ubiquitination and phosphorylation. We proposed three alternative models for how Hif-2a might trigger pexophagy (Figure 2).
Figure 2. Three alternative models illustrating how Hif-2a might trigger pexophagy (Schönenberger and Kovacs, Front. Cell Dev. Biol. 2015).
The aim of our work is to:
- Identify mammalian autophagy adaptor proteins and posttranslational modifications (g., ubiquitination, phosphorylation) of peroxisomal membrane proteins and autophagy receptors involved in HIF-2a-induced pexophagy
- Identify E3 ubiquitin ligases, kinases and/or phosphatases regulating HIF-2a-induced pexophagy
- Determine the role of ubiquitin-binding autophagy receptors in HIF-2a-mediated pexophagy
The role of pexophagy in cancer development and therapy
Clear cell renal cell carcinoma (ccRCC) accounts for ~75% of all kidney cancers. VHL inactivation occurs in ~90% of ccRCC, leading to stabilization of hypoxia-inducible factors (HIF-1a and -2a). Oncogenic metabolism and epigenetic reprogramming are central features of ccRCC, and HIF-2a is a driver oncoprotein for ccRCC. ccRCC is regarded as a metabolic disease due to the diverse array of metabolic defects and perturbations. We showed that peroxisome abundance is reduced in ccRCC that display elevated HIF-2a levels (Walter, Schönenberger et al., Cell Metabolism 2014), revealing an important link between peroxisome abundance and HIF-2a levels in cancer. Interestingly, peroxisome abundance is reduced more frequently in well-differentiated tumors.
The aim of our work is to:
- Explore the role of pexophagy and peroxisomal metabolism in ccRCC
- Identify regulators of HIF-2a-induced pexophagy as therapeutic candidates for ccRCC