PI: Aimee L. Edinger, VMD/PhD
aedinger@uci.edu

The Edinger Lab studies how regulated nutrient transporter expression controls cell growth and survival. This research has important implications for cancer biology and treatment.

Role of nutrient transporter proteins in growth control and cancer

Cells become highly autophagic upon ceramide exposure. Rapid and profound nutrient transporter loss causes starvation even in rich media as the cells cannot access the abundant extracellular nutrients.

My lab is investigating the hypothesis that mammalian cell growth is regulated at the level of nutrient transporter expression. Although the bloodstream constantly supplies mammalian cells with nutrients, signal transduction cascades regulate access to these nutrients by modulating the expression of nutrient transport systems in the cell membrane. By identifying the proteins regulating nutrient transporter expression and trafficking, we expect to gain insight into normal cell biology and identify novel therapeutic targets in cancer. Normal cells become quiescent when restricted for nutrients. Cancer cells, on the other hand, have activated oncogenes that promote growth regardless of the extracellular conditions and deleted the tumor suppressor proteins that allow them to switch to catabolism. Thus, drugs that limit nutrient transporter expression are likely to be selectively toxic to tumor cells.

Cells protected from apoptosis by Bcl-XL expression die by necrosis following ceramide-induced nutrient transporter loss. PI is a vital dye that does not enter live cells. Hoechst allows the visualization of nuclear morphology in all



In order to pursue this idea, my lab has amassed expertise and reagents that allow us to examine how alterations in bioenergetics characteristic of cancer cells interact with conditions that alter nutrient transporter expression. One area of particular interest is autophagy—the adaptive cellular response to the starvation induced by nutrient transporter loss. Through autophagy, (literally, eating one’s self) cells recycle their constituents to provide essential nutrients. We have shown that cells undergoing autophagy in the presence of abundant extracellular nutrients are often cells that have reduced nutrient transporter expression. We have also found that blocking apoptosis (programmed cell death) does not stop nutrient transporter loss from killing cells. The ability of nutrient transporter down-regulation to kill cells necrotically could be useful in cancer therapy as most tumor cells have disabled apoptotic pathways. These studies are also highly relevant to the fields of diabetes and aging.

How is nutrient transporter turnover regulated?

Nutrient transporter proteins are trafficked to the lysosome following growth factor withdrawal through a process that requires Rab7. When Rab7 levels are decreased by RNAi (shRab7), nutrient transporter proteins (red) are shunted away from the lysosome (green) and into the default recycling pathway back to the cell

Most scientists are surprised that very little is known about the signals that regulate nutrient transporter expression and trafficking. Although the trafficking of the transferrin receptor (TfR), the LDL receptor (LDLR), and GLUT4 (the insulin sensitive glucose transporter found in adipose and muscle tissue) has been exhaustively studied, amino acid transporters and the broadly expressed GLUT1 are not nearly as well studied. Because the TfR and LDLR deliver their cargo by endocytosis, their regulation is likely to be very different from channel-type transporters like amino acid transporters and GLUT1. Studies of GLUT4 may provide more clues, but this protein trafficks in and out of a special compartment in the cytoplasm and is likely to utilize pathways distinct from those travelled by the vast majority of transporters. My lab has identified signal transduction cascades including the serine-threonine kinase Akt and mammalian TOR (mTOR) as key players in this process. In addition to these kinases, we have identified a downstream regulator of nutrient transporter expression, the small GTPase Rab7. Rab7 promotes membrane fusion events between late endsomes and lysosomes, including those required for nutrient transporter degradation. When Rab7 function is disrupted, it has a dramatic effect on the ability of growth factors to regulate cell growth and survival. Recently, we have made the exciting discovery that ceramide kills mammalian cells by causing a rapid and profound down-regulation of nutrient transporter proteins. Ceramide activates a variety of signaling molecules, and we are investigating which ones might be responsible for the effect of ceramide on nutrient transporter proteins. As ceramide has been linked to cell cycle arrest, death, differentiation, and senescence, our findings may have broad implications for cell biology.

What could I work on in the lab?
If you are interested in these ideas and are considering joining in the lab, please email or call Dr. Edinger to make an appointment to discuss possible projects in detail. Current areas of active investigation include: the biochemical mechanisms by which mTOR regulates transporter turnover, investigation of Rab7 as a potential tumor suppressor protein, elucidation of the mechanism by which sphingolipids such as ceramide control nutrient transporter expression and trafficking, evaluation of chemotherapeutic agents whose MOA may include the control of transporter turnover. These problems are approached using molecular and biochemical techniques, multicolor flow cytometry, mammalian tissue culture, and mouse model systems.

DR. EDINGER’S HISTORY
After growing up in San Diego, I did my undergraduate work at UC Davis where I originally intended to go to vet school. I signed up to work with UC Davis veterinary researchers studying the effects of altitude on exercise physiology in horses and emus and enjoyed research so much that I ended up doing a senior research project (Bio199) in a molecular biology lab. These experiences convinced me that I should enter the combined veterinary/PhD training program at the University of Pennsylvania. After graduating from vet school in 1996, I completed my PhD in the lab of Dr. Bob Doms studying the envelope proteins of HIV and SIV (the simian form of the virus). I ended up staying at PENN for my postdoc, but switched fields entirely to work on growth factor regulation of cell growth and survival in the lab of Dr. Craig B. Thompson who had just moved to PENN from the University of Chicago. Working in the Thompson lab fostered my interest in cellular bioenergetics and has granted a unique perspective to our work on nutrient transporter trafficking.

SELECTED PUBLICATIONS ON THESE TOPICS

Romero KR, Peralta EP, Guenther GG, Wong SY, and AL Edinger (2009). Rab7 activation by growth factor withdrawal contributes to the induction of apoptosis. Molec. Biol. of the Cell 20:2831-40.
http://www.ncbi.nlm.nih.gov/pubmed/19386765?itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum&ordinalpos=1
This paper demonstrates that Rab7 activity is regulated by growth factors and that activating Rab7 can kill cells.

AL Edinger (2008). Starvation in the midst of plenty: making sense of ceramide-induced autophagy by analyzing nutrient transporter expression. Biochem Soc Trans. 2009 Feb;37(Pt 1):253-8.
http://www.ncbi.nlm.nih.gov/pubmed/19143642?itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum&ordinalpos=4
This review helps to place our recent studies of the effect of ceramide on nutrient transporter expression in context.

Guenther GG, Peralta EP, Romero KR, Wong SY, Siskind, LJ and AL Edinger (2008). Ceramide starves cells to death by down-regulating nutrient transporter proteins. Proc. Natl. Acad. Sci. 105, 17402–17407.
http://www.ncbi.nlm.nih.gov/pubmed/18981422?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum
This paper establishes for the first time that ceramide kills cells through a bioenergetic mechanism.

Edinger, AL (2007). Controlling cell growth and survival through regulated nutrient transporter expression. Biochem. J. 406:1-12.
http://www.ncbi.nlm.nih.gov/pubmed/17645414?ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum
Comprehensive review of regulated nutrient transporter expression and its role in cellular growth control.

Edinger, AL and CB Thompson (2004). Death by design: apoptosis, necrosis, and autophagy. Curr. Opin. Cell Biol. 16:663-669.
http://www.ncbi.nlm.nih.gov/pubmed/15530778?ordinalpos=5&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum
Describes how apoptosis, necrosis, and autophagy are interelated.

Edinger AL, Cinalli RM, and CB Thompson (2003). Rab7 prevents growth factor-independent survival by inhibiting cell-autonomous nutrient transporter expression. Dev. Cell 5:571-582.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=14536059
Preview: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=14580329
If you only read one paper about what we do, this should be it! This work is exciting because there are very few examples of molecules not directly involved in signal transduction that have such a profound effect on cell growth and survival. In addition, Rab7 had never been linked to transformation. This work suggests that Rab7 may in fact represent a whole new class of tumor suppressor proteins.

Edinger AL and CB Thompson (2002). Akt maintains cell size and survival by increasing nutrient uptake via an mTOR-dependent mechanism. Molec. Biol. of the Cell 13:2276-2288.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12134068
Akt has long been studied by the diabetes field for its ability to control glucose transporter expression. This paper shows for the first time that growth factors and Akt regulate not just glucose uptake, but global nutrient transporter protein expression and that this activity is important for Akt-mediated growth factor-independent survival. In addition, these effects of Akt are identified as dependent on mTOR activity.

Edinger AL, Linardic CM, Chiang GG, Thompson CB, and RT Abraham (2003). Differential effects of rapamycin on mTOR signaling functions in mammalian cells. Cancer Research 63:8451-8460.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=14679009
It has long been known that yeast TOR possesses an essential function that is not inhibited by the drug rapamycin. mTOR also has rapamycin-insensitive activities. As rapamycin analogs are currently in stage III clinical trials as cancer chemotherapeutics, it is critically important to understand which mTOR-dependent functions are inhibited by this drug.

Edinger AL and CB Thompson (2002). Antigen presenting cells control T cell proliferation by regulating amino acid availability. Proc. Natl. Acad. Sci., USA 99:1107-1109.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11830651
This commentary highlights the ways in which immune cell function is modulated at the level of nutrient access.