Grantee: Vanderbilt University Medical Center, Nashville, TN, USA
Grant Title: Hedgehog and hippo signaling as drivers of medulloblastoma and cell division-associated metabolic choices
Grant Type: Research Award
Program Area: Researching Brain Cancer
Duration: 3 years
Essay Neuro-developmental origins of pediatric brain cancer
During vertebrate development, precursor cells respond to extracellular signals promoting their proliferation or directing them to leave the cell cycle and differentiate. Direct and indirect interactions between mitogenic and maturation pathways influence central nervous system (CNS) precursor cell decisions to divide or commit to a terminal differentiation program [1, 2]. Alterations in the balance between pro-proliferative and pro-differentiation signals may set the stage for cancer, by establishing conditions conducive for cell transformation, or cause precocious cell cycle exit, leading to developmental disorders. Indeed, it is becoming increasingly evident that aberrant regulation of signaling pathways controlling normal development promotes cancer [3, 4]. Such dysregulation, resulting from mutations or altered activity of genes encoding signal transduction pathway components, can cause tumor cells-of-origin to increase proliferation, fail to exit the cell cycle, or fail to die in the event of genetic damage, thereby acquiring new mutations that may enhance their survival capacity.
The childhood brain tumor medulloblastoma is an example of a cancer resulting from developmental mis-cues for cell division, or failure to undergo growth arrest. This tumor, which is the most common pediatric malignancy and a leading cause of cancer deaths in children, arises in the cerebellum during post-natal development . Current treatments for medulloblastoma, which include radiation, surgery, and chemotherapy, fail to distinguish between the tumor cells and the normal cells of the still-developing brain, leaving survivors with life-long devastating side-effects, including behavioral and cognitive disorders. Insufficient radiation or incomplete resection are associated with tumor recurrence and spread thoughout the cerebral spinal fluid to distal sites in the central nervous system. However, development of more optimal treatments has been hampered by a lack of understanding of the basic mechanisms contributing to medulloblastoma cell-of-origin transformation, tumor expansion, and recurrence/spread. The overall goals of my research program are (a) to characterize how mitogenic/oncogenic signaling pathways cooperate at the cell biological and molecular genetic level to promote medulloblastoma formation and growth; (b) use primary cultures, brain slice explants, and genetically engineered mice to validate such nodes of cooperativity as potential targets for new medulloblastoma therapies; (c) to apply the insights gained from my basic research to the clinic, in collaborative translational research.
The cerebellum regulates movement and posture. Cerebellar granule neuron precursors (CGNPs) proliferate rapidly during a post-natal expansion phase, before terminally differentiating into glutamatergic interneurons . CGNPs are proposed cells-of-origin for certain classes of medulloblastoma [7, 8], particularly those associated with aberrant activation of the pathway stimulated by the secreted morphogen Sonic Hedgehog (Shh) . Such tumors comprise approximately 30% of medulloblastoma cases, as determined by gene expression profiling. An increased understanding of how Shh signaling triggers neural precursor cell cycle progression, by interacting with pathways that regulate cell size, new protein synthesis, and metabolic control, will lead to identification of new medulloblastoma therapy targets as well as improve our understanding of basic brain developmental biology and how its dys-regulation prompts tumor formation.
In the event of an incomplete resection or an effort to reduce radiation doses to spare side effects such as intellectual impairment, tumors recur and metastasize as a result of activation of so-called "tumor re-initiating cells". These cells are known to reside adjacent to blood vessels in tumors, a region known as the "peri-vascular niche (PVN). Signaling in this region is proposed to generate a unique microenvironment that supports quiescence of these cells until they are stimulated to divide and re-grow the tumor. This unique microenvironment involves signaling by Shh and our preliminary studies also show that there are tumor-specific metabolic choices taken by these cells, which could be exploited in therapies aimed at eradicating these cells and enabling reduced radiation levels during medulloblastoma treatment.
In agreement with our preliminary studies, recent studies have demonstrated that cancer cells acquire and become dependent upon unique metabolic pathways that cause them to exhibit fuel preference choices and regulation of intracellular lipid synthesis distinct from that of normal cells . Defining how essential metabolic pathways are altered or co-opted in medulloblastomas, which to date remain poorly understood, may lead to new therapeutic modalities. My co-investigator Dr. Zaher Nahle (Weill Cornell Medical College)and I have made the novel observation that Shh signaling is tightly coupled to the reprogramming of mitochondrial bioenergetics; specifically that Shh inhibits the critical process of fatty acid oxidation (FAO, or β-oxidation) while driving the increase in fatty acid synthesis (FAS), which is required for lipid synthesis. The effects of Shh on modifying lipid metabolism are seen in normal development -primary cultures of CGNPs -as well as in Shh-mediated mouse medulloblastomas. Thus, our studies link developmental and oncogenic Shh signaling to the basal metabolic machinery regulating fuel production and biosynthetic processes associated with cell proliferation.
Hedgehog, Hippo, and medulloblastoma growth and recurrence
The objective of my James S. McDonnell Foundation proposal is to identify mechanisms through which the developmentally critical Sonic hedgehog (Shh) and newly-described Hippo signaling pathways interact to regulate CGNP proliferation and medulloblastoma growth, survival, and recurrence. The Shh pathway is highly conserved through evolution from Drosophila to mammals, and functions by activating a signal transduction pathway leading to expression and activation of transcription factors whose target genes determine the outcome of pathway activation. For example, activation of Shh signaling in the ventral spinal cord results in induction of specific motor neuron subtypes. In contrast, Shh pathway activation in the developing cerebellum and several other progenitor populations causes proliferation.
Like the Shh signaling pathway, the Hippo pathway is highly evolutionarily conserved. This pathway is tumor suppressive, and promotes growth arrest when activated, by causing phosphorylation and degradation of a transcriptional regulator known as yes-associated protein-1 (YAP1). YAP1 has been identified as an oncogene in breast and liver cancers, and is amplified in several tumors including glioma. YAP1 operates by binding to specific transcription factor partners, recruiting proteins to activate the gene-regulatory properties of these transcription factors. Target genes regulated by YAP1 are therefore determined by its interacting partners, and studies in cell line systems have identified YAP1 targets including genes involved in proliferation, survival, and apoptosis.In the developing chick spinal cord, YAP1 has been shown to promote maintenance of an undifferentiated neural precursor state . However, little is known of its function in the developing mammalian nervous system. We have recently shown that in the developing cerebellum, Shh induces expression, stabilization, and nuclear localization of YAP1. YAP1 activity was required for CGNP proliferation and, indeed, YAP1 was able to drive CGNP proliferation independent of Shh. Moreover, we found that YAP1 was up-regulated and amplified in human medulloblastomas belonging to the Shh subclass, indicating a conservation of mechanism beween cerebellar development and medulloblastoma formation . The interaction between Shh signaling and Hippo suppression/YAP1 activation plays essential roles in CGNP proliferation. Mechanisms through which this pathway cross-talk takes place, whether it can be generalized to other neural precursor populations which themselves may be cells-of-origin for other pediatric cancers, how YAP1 specifically contributes to medulloblastoma initiation and the role played by YAP1 in cell cycle regulation and Shh-mediated lipid metabolism in medulloblastoma recurrence remain to be determined. These questions form the core of our proposed JSMF research. Our ultimate goal is to determine whether interactions between Shh, Hippo/YAP1, and metabolic pathway may be targeted in new medulloblastoma therapies aimed at reducing current required levels of radiation and chemotherapy, while preventing tumor recurrence. Moreover, since metabolic de-regulation is found in many highly proliferative tumors, including some with neuronal origins, and hedgehog pathway activity has been implicated in a number of neoplasms, including those of lung, prostate, and pancreas, our research has the potential to be applicable to a broad spectrum of tumors.
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