Autophagy was the subject of a State of the Art presentation given by James Bradner, M.D., Ph.D., at the 2010 American College of Veterinary Internal Medicine Forum in Anaheim. Literally, “autophagy” means “self-cleaning.” Bodies can’t live without their autophagy systems in place. Autophagy encompasses the automatic disposal systems of the cell. Interestingly, when autophagy adapts to starvation, certain mechanisms are activated that also have anti-aging and anti-cancer effects. We learned that our cells void metabolic waste and the kidneys extract it from serum and eliminate the waste through the urine. However, there was no clear understanding of autophagy or how the cellular cleansing phenomenon operates on a molecular level until recently. Read on and be amazed at the basic miraculous efficiency with which our bodies function every second of every day. Where It Begins The flesh of our cells is called cytoplasm. It contains all the microproteins and chemicals and structures needed to operate. Waste products are created in the cytoplasm and that waste must be digested and transported outside the cell. How does this happen? Scientists have observed that cell debris—proteins and organelles—gets encapsulated by tiny rearrangements of membranes and moved into empty spaces called vacuoles. The transportation of the cell debris is a pathway now called “cytoplasm-to-vacuole targeting,” or the Cvt pathway. Autophagy is the sequestration of the cargo material, bulk cytoplasm or specific organelle within double-membrane structures and its delivery to the vacuole for further degradation. The major cellular pathways for protein and organelle turnover are autophagy and proteasome-mediated degradation. Proteasomes are large protein complexes that degrade unneeded, misfolded or damaged proteins by breaking down peptide bonds (proteolysis). The enzymes that do this work are called proteases. Be aware that every new protein found may serve as a target for future therapy against cancer and illness. Autophagy and proteasome degradation are vital processes. They are important to maintain a well-controlled balance between anabolism (constructive metabolism) and catabolism (destructive metabolism) for normal cell growth and development and apoptosis (programmed cell death). Proteasomes are barrel shaped and have a hollow core that waits for waste to enter. Studies have found that a small protein called ubiquitin tags the waste proteins, forcing them into the barrel to be degraded. The tagging reaction is catalyzed by enzymes called ubiquitin ligases. Once a protein is tagged with a single ubiquitin molecule, this is a signal to other ligases to attach additional ubiquitin molecules. The result is a polyubiquitin chain that is bound by the proteasome, allowing it to degrade the tagged protein. Regulatory proteins serve as gates and caps to signal ubiquitin on the target protein, which must enter the central pore of the proteasome barrel before it is degraded. Some regulatory proteins are called ligands and ligases. Ligands are organic molecules that act as “keys.” Ligands enter receptors as they donate their electrons to form covalent bonds with metallic ions. Ligands also react to form complexes with another molecule such as ubiquitin to form the polyubiquitin chain that tags waste proteins for degradation in proteasome barrels. Ligases act as enzymes that catalyze (enhances or slows) the joining together of two molecules as regulatory proteins. Historic Discoveries In 2004, the Nobel Prize in Chemistry was awarded to Aaron Ciechanover, Avram Hershko and Irwin Rose for elucidating the important role of ubiquitin in proteolytic pathways and linking how these basic cellular functions affect the cell cycle, the regulation of gene expression and responses to oxidative stress. Remember that I said that every protein may serve as a target for therapy? Researchers can create proteasome inhibitors to induce apoptosis in rapidly dividing cells. New chemotherapy agents are available that inhibit proteins and enzymes involved in malignant and disease pathways. These are exciting days because we will have some of these new therapies available in our practice lifetime! We already have gene therapy with the Canine Melanoma Vaccine, which targets tyrosine kinase. We also have a new class of small molecules such as Palladia and Masivet. These drugs inhibit tyrosine kinase enzymes that target c-Kit (a proto-oncogene that has wild and mutated types) to cause cell cycle arrest and apoptosis (programmed cell death) of mast cells. Some small molecules also will inhibit growth factor receptors such as platelet derived growth factor (PDGF) and fibroblast growth factor (FGR), which are over-expressed in certain cancers. What It All Means It is exciting to know more about our cells and how they function and the common pathways that can be disturbed by disease. For example, multiple myeloma shows a huge increase in misfolded proteins that are not being eliminated by the autophagy systems. Treatment with Gleevec has changed that and given people a much longer survival. It has always amazed me that dogs with huge lymph nodes can remain well while their body goes into remission, breaking down and self-digesting up to five or six pounds of tumor tissue! It is all thanks to autophagy and proteasome, our self-cleaning systems. With all this new molecular knowledge, we can look forward to the development of new, less harmful drugs. Instead of cutting, poisoning and burning tumors, we will be able to target the regulation of gene expression, affect ligands and small enzymes and proteins that are involved in the cell cycle pathways to safely inhibit cancer’s fatal agenda. Alice Villalobos is a past president of the American Assn. of Human-Animal Bond Veterinarians and is president of the Society for Veterinary Medical Ethics.
