The problem of glioblastoma
Figure 1. Magnetic resonance images of a 78-year-old male with confusion and gait instability. T1-weighted axial (left) and coronal (right) images with contrast demonstrated a heterogeneously enhancing mass in the right temporal and parietal lobes. The patient underwent an open biopsy, which revealed glioblastoma, WHO Grade IV.
Glioblastoma is the most common malignant brain tumor in adults. Despite aggressive surgical resection, chemotherapy and radiation therapy, this disease remains challenging with no cure. A subpopulation of glioblastoma cells called cancer stem cells (also glioblastoma stem-like cells) drives tumor growth and has been implicated in the development of treatment resistance and tumor recurrence. We are interested in the molecular mechanisms that control key features of glioblastoma stem-like cell biology, including self-renewal and tumor initiation. The elucidation of these mechanisms is critical for our understanding of brain cancer behavior and will also advance our knowledge of normal nervous system development. The long-term goal of my laboratory is to leverage these insights for the rational design of novel strategies against this formidable tumor. We are currently examining three major mechanistic themes in the laboratory: protein homeostasis, metabolism and epigenetics.
Generation of glioblastoma stem cells
Figure 2. Experimental design for the investigation of glioblastoma stem cell biology.
The rapid turnover of selective proteins, particularly in response to cellular cues, is critical to the function of cells and is in large part governed by the ubiquitin-proteasome system (UPS). The UPS has been identified as a strategic target in human cancer. Specificity in the UPS is largely governed by catalytic E3 ubiquitin ligases, which represent an important and unexplored family of enzymes that can potentially be targeted therapeutically. Our goal is to identify catalytic ubiquitin signaling proteins that control glioblastoma stem-like cell identity, invasiveness and tumorigenicity.
The major cell cycle-regulatory ubiquitin ligase Anaphase–Promoting Complex (APC) recruits the coactivator CDC20 to drive mitosis in cycling cells. CDC20 mRNA and protein levels have been shown to be increased in glioblastoma tumors and cell lines (Kidokoro et al. Oncogene, 2008; Marucci et al. Virchows Arch, 2008), but little is known about the functions of CDC20-APC in this brain cancer. Given our previous insights into the critical role of CDC20-APC in nervous system development (Kim et al., Cell, 2009; Puram, Kim et al., Cell Reports, 2011), we hypothesized that CDC20-APC controls key biological properties of glioblastoma stem-like cells, including invasiveness and tumorigenesis, with implications for the development of novel E3 ligase-directed therapies against human glioblastoma.
We recently found that CDC20-APC is required for self-renewal, invasiveness and tumorigenesis in vivo (Mao, Gujar et al, Cell Reports, 2015). Mechanistically, CDC20-APC controls glioblastoma stem-like cell invasion and self-renewal by stabilizing the protein levels of pluripotency transcription factor SOX2, a molecule known to be required for tumor-initiation. These data provide the rationale for the development of additional pharmacological agents that manipulate the CDC20-APC-SOX2 pathway, which may represent a new and meaningful therapeutic strategy against glioblastoma.
Nicotinamide adenine dinucleotide (NAD+) has a well-known role in oxidation-reduction reactions in cellular metabolism and is an essential component of signaling pathways as a co-factor for NAD+-dependent enzymes. These enzymes, which include sirtuin deacetylases and poly-ADP ribosyl transferases, regulate aging, inflammation, type 2 diabetes mellitus and cancer. NAD+-dependent signaling has been shown to be important for cancer-related biological processes such as DNA repair, apoptosis and cell cycle progression.
Nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme in mammalian NAD+ biosynthesis, influences these processes through regulation of the intracellular NAD+ pool. NAMPT is overexpressed in several cancers and is critical for the maintenance of key characteristics of cancer cells such as invasion, migration, anchorage-independent growth, survival and tumor growth. Recent studies show that NAD+-dependent enzymes control transcription factor activity and chromatin structure, highlighting the crucial role of NAD+ in transcription. However, little is known about the biological role of NAMPT as well as the transcriptional program governed by NAD+ in glioblastoma. We are currently examining the signaling pathways both upstream and downstream of NAMPT in human glioblastoma.
Accumulating evidence indicates glioblastoma stem-like cells represent a dynamic cell state influenced by cell-intrinsic and -extrinsic cues. Our laboratory is interested in finding ways to disrupt the epigenetic state that functionally defines these tumor-initiating cells. These investigations have two components: 1) understanding the epigenetic state that causes malignant behavior and 2) identifying specific molecules or environments that alter this epigenetic state. We are pursuing a variety of approaches, including high-throughput screening, pharmacology, biochemistry, genetics and material engineering, to accomplish these goals.