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Drosophila as a model system for studying cancer

The fruitfly, Drosophila melanogaster, is a popular model system used by many labs to probe various aspects of cell biology, cell signaling, and embryonic development. It's popularity stems from a number of different intrinsic characteristics, including the fact that development occurs outside of the adult organisms, which allows one to obtain samples with out the need to sacrifice the adults; an extensively defined collection of defined mutants, and a sequenced genome. In addition, most of the key signaling pathways that play a role in regulating cell proliferation, cell differentiation, anf programmed cell death are evolutionarily conserved. To this end, I use Drosophila to study various aspects of the JNK signaling pathway and how it induces programmed cell death in response to genotoxic stress.

Research Interests

Being trained as a developmental biologist, I am taking genetic and biochemical approaches to analyze key signaling pathways that orchestrate development, yet are co-opted during oncogenesis. As these key pathways are evolutionarily conserved, I use the fruit fly, Drosophila melanogaster, to study various aspects of the Jun N-terminal kinase (JNK) signaling pathway and how it influences the induction of programmed cell death in response to genotoxic stress.

Cell fate decisions depend on the duration and/or peak amplitude of JNK activation; thus, strict modulation of JNK activity directs phenotypic decisions. The main antagonists of JNKs are the MAPK phosphatases (or MKPs). At least nine distinct genes exist in vertebrates. However, selective inactivation of these genes fails to affects organism viability, suggesting functional redundancy exists between vertebrates MKPs. In contrast, I have demonstrated that inactivation of Puckered (Puc), the sole JNK-specific MKP in Drosophila, induces apoptosis. Thus, Puc is required autonomously in all cells to promote cell viability by restraining basal JNK activity (anti-apoptotic).

Furthermore, I have demonstrated that JNK signaling plays an important role in genotoxic stress-induced apoptosis. g-irradiation stimulates both JNK activity and puc expression in a p53-dependent manner, whereas JNK-dependent apoptosis can be suppressed by p53 inactivation. Nevertheless, we understand very little about the integration of these two signaling pathways. To this end, one focus of my lab is to define the function relationships between the p53 and JNK signaling pathways.

Finally, I have demonstrated that Puc-mediated antagonism of JNK is required during normal development. However, if cell death is prevented, JNK activation can trigger tissue overgrowth. Thus, Puc is a key regulator of cell proliferation and functions similar to a tumor suppressor gene during development. To this end, I am using Drosophila genetics to characterize both the signals that promote JNK activation as well as the downstream effectors that support proliferation in the absence of apoptosis.

JNK Signaling The Jun N-Terminal kinase (or JNK) signaling cascade is an evolutionarily conserved signaling pathway that is capable of directing cells to proliferate, differentiate, or undergo programmed cell death. Cell fate decisions depend on the duration and/or peak amplitude of JNK activation; thus, demonstrating that regulation of JNK activity directs cell fate decisions.	MAPK phosphatases The main antagonist of MAPKs are the MAPK phosphatases (or MKPs). At least nine distinct genes exist in vertebrates. However, inactivation of these genes fails to have significant affects on organismal viability. Together with their overlapping substrate specificity, these results suggest that functional redundancy exists between vertebrates MKPs. In contrast, MKPs in Drosophila function with respect to distinct substrates and their inactivation produces distinct phenotypes.
JNK-dependent apoptosis JNK is known to regulate the induction of apoptosis. However, multiple mechanisms for JNK-mediated apoptosis have been described. For example, JNK-dependent upregulation of either Fas ligand or TNF-alpha can promote apoptosis via an autocrine loop. Alternatively, direct phosphorylation of BIDXL, a BH3-only protein, by JNK promotes neuronal apoptosis in response to neurotrophin receptor activation, whereas phosphorylation of 14-3-3 by JNK promote stimulates apoptosis via translocation of Bax, a proapoptotic Bcl-2 family member, from the cytoplasm to the mitochondrial membrane.	p53p53 is the most frequently mutated tumor suppressor gene in solid tumors.p53 is thought to primarily act as a transcription factor, whose activity is dependent on maintenece of genome integrity. In vertebrates, activation of p53 can result in either cell cycle arrest or the induction of programmed cell death. The Drosophila ortholog of p53 regulates programmed cell death; yet, is not required for cell cycle arrest in response to genotoxic stress. To this end, Drosophila has emerged as a valuable model system for understanding how p53 induces programmed cell death.
Planar Cell Polarity In addition to having an apical - basal polarity, epithelial cells often times exhibit a polarity within the plane of the epithelia. Classic examples on planar cell polarity (PCP) include orientation of hairs on the Drosophila wing and coordinated rotation of omatidia in the developing eye. Genetic screens have identified a number of genes involved in this process in Drosophila, while more recent studies have demonstrated the importance of PCP signaling during convergent extension and neural tube closure iin vertebrates. Evolutionary conservation of the PCP signaling pathways between invertebrates and vertebrates has allowed the use of forward genetics in identifying genes that influence early embryonic development.	CytoskeletonOne possible mechanism whereby the “core” planar cell polaritymachinery might initiate cytoskeletal remodeling is through the localized activation of JNK. Consistent with this process, dishevelled¹, an allele that selectively affects planar cell polarity in Drosophila, fails to interact with JNK. Similarly, engineered mutations in vertebrate Dishevelled block gastrulation movements, while vertebrate Dishevelled is a potent stimulator of JNK activity. As JNK is known to directly phosphorylate and potentially regulate both actin- and microtubule-associated proteins, the Frizzled/Dishevelled complex may function as a scaffold that promotes localized activation of JNK in response to Wnt/Wg ligands.

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Donald G. McEwen, Ph.D.

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Drosophila as a model system for studying cancer

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