(left to right) Jill Konowich, MD/PhD student; Olga Kovalenko, MD, PhD, post-doc; Janine Hertzog Santos, PhD, assistant professor, Department of Pharmacology and Physiology,UMDNJ-New Jersey Medical School; Haamid Sharif, BS, technician; Matthieu Caron, BS, technician
Telomerase in mitochondria: friend or foe?
The very ends of chromosomes are called telomeres. These are complex structures that ultimately help safeguard the integrity of the DNA. Telomeres are known to shorten with every cell division until telomere length becomes critically short. At that point, cells stop dividing and enter a state called replicative senescence. Thus, telomeres function as a ‘biological clock’ in cells. Telomerase is an enzyme that elongates telomeres with every cell division, thus allowing them to preserve sufficient length for further divisions. The expression of telomerase is high in stem cells as they have to divide several times, and very little or completely absent in most normal adult somatic cells. However, highly proliferative cells, such as those in the bone marrow that give rise to white blood cells in the circulation, and the majority (90%) of all human cancers express telomerase to varying degrees. A couple of years ago, I
discovered that telomerase is not only found in the nucleus, where the telomeres are, but it is also present and active in mitochondria. These structures within the cell are responsible for generating energy for cell function, and are also involved in apoptosis, a programmed event that triggers cell death to warrant system integrity. We showed that inside mitochondria, telomerase increases apoptosis when levels of toxic byproducts of oxygen metabolism are high, an effect completely abolished when telomerase works exclusively in the nucleus and not in mitochondria. These observations suggest that telomerase may help kill cells, which seems quite counterintuitive. However, we believe this effect may be another facet of telomerase to safeguard cell/tissue integrity.
Telomerase is an enzyme well known for its function in the maintenance of telomeres, structures at the end of linear chromosomes that are ultimately responsible for the proliferative potential of cells. Telomerase is composed essentially of two different subunits, a protein core (hTERT) and an associated RNA component (hTR); its enzymatic activity can be detected in normal somatic cells that have high proliferation rates and in the vast majority of cancers. Telomerase activity appears to be one of the principal features that distinguishes mortal from immortal cells, and also one of the primary factors in regulating cellular lifespan.
Because telomerase is expressed in so many types of cancer, and is believed to be the feature that allows for active proliferation of tumor cells, lots of attention has been devoted to counteracting its enzymatic function in cancers.
In recent years, telomerase has been suggested to be involved in other cellular functions that are unrelated to the maintenance of telomeres. For instance, it has been shown to modulate DNA damage response, a process that is vital for the maintenance of integrity of our genetic information and ultimately for cell survival. In line with the idea that telomerase has extra nuclear function(s), I identified a few years ago that hTERT is not only localized to the nucleus, where the chromosomes are, but also to the mitochondria. Using several different techniques, we demonstrated that the presence of hTERT inside the mitochondria sensitizes both the mitochondrial genome as well as the whole cell to oxidative damage, leading eventually to cell death.
| Fig 1: hTERT localizes to mitochondria requiring a function mitochondrial-targeting signal. (A) Sequence of the N-terminus of hTERT showing in bold amino acids of the mitochondrial targeting-signal. Two amino acids were mutated in order to abolish the mitochondrial localization of hTERT (shown in red). (B) Confocal imaging of cells expressing EGFP alone (first row), wild type hTERT fused to EGFP (middle row) and hTERT with a mutated mitochondrial localization sequence fused to EGFP (third row). Red reflects mitochondria stained with Mitotracker red; third column shows cells expressing EGFP constructs, and last column shows the merge of images from columns 2 and 3. Yellow denotes colocalization of the green and red fluorescent signals.
Mitochondria are structures within the cells involved in various metabolic processes (such as calcium and iron homeostasis) but primarily responsible for energy production. Because of that, they are considered the powerhouses of the cell. However, as a byproduct of energy generation, mitochondria are also known for being the main source of endogenous reactive oxygen species (ROS). These byproducts of oxygen metabolism can damage various cell structures such as proteins and lipids, as well as the DNA. Thus, maintenance of mitochondria integrity is key for normal cellular function. The role of mitochondria as the executioner of apoptosis is also well established.
In the past decades, many human disorders have been linked to alterations in structure or function of mitochondrial and/or nuclear-genes that encode mitochondrial proteins. For instance, mutations in the mitochondrial DNA polymerase gamma, the only protein responsible for replication and repair of the mitochondrial genome, have been associated with progressive external ophthalmoplegia and Alper's syndrome in humans, and with accelerated aging in mice. Mutations in the mitochondrial genome have also been implicated in Parkinsonism, Alzheimer's disease and in Leber's hereditary optic neuropathy. Underlying these and other neurodegenerative diseases are increased levels of ROS, altered mitochondrial function and cell death.
The discovery that hTERT is also present in the mitochondria, and that only when inside the organelle it sensitizes the cells to ROS-provoked cell death, suggests that the subcellular localization of hTERT may play a role in the response of cells to the toxic byproducts of oxygen metabolism. While at first this effect seems to be detrimental to the cell, we believe that by doing so, hTERT is acting as a mitochondrial quality control—leaving in the population only cells that carry good-quality and properly functioning mitochondria. Because telomerase is normally expressed in early stages of fetal development and in adult stem cells, these observations raise the possibility that the mitochondrial function of telomerase may be required for normal development. Furthermore, it is possible that malfunctioning or altered subcellular localization of hTERT during development or in stem cells may have an impact in normal tissue function and possibly in how well we age. Interestingly, little is known about telomerase in development besides its function at the telomeres.
My group is interested in understanding what the overall function of telomerase in normal cells is, specifically what role it plays in the mitochondria. Using various biochemical, molecular biology and cell imaging strategies, we are addressing questions related to the basic biology of the enzyme in the organelle, and what are the metabolic effects to the mitochondria once hTERT is present or not. Currently, we are investigating whether it binds to the mitochondrial genome, which mitochondrial proteins it interacts with, and how exactly it is involved in the promotion of apoptosis. In collaboration with Andrew P. Thomas, PhD, chair of the Department of Pharmacology and Physiology at UMDNJ-New Jersey Medical School, we are also investigating if mitochondrial metabolic function (such as oxygen consumption, ATP generation and/or calcium handling) is modulated by telomerase. Our ultimate goal is to elucidate how this protein functions in the normal cellular context.
As mentioned before, telomerase has received alot of attention because of its role in the carcinogenic process. As of now, this protein is believed to “fuel” tumor cell proliferation by maintaining telomeric length. In fact, abolishing telomerase activity in some types of cancers has been demonstrated as an effective means to limit the proliferative potential of tumor cells. Interestingly, this does not seem to occur in all cases. How then do we reconcile these observations with our findings that mitochondrial hTERT is pro-apoptotic? We believe that either the proliferation rate of a tumor is higher than the pro-apoptotic function of telomerase in mitochondria or that, alternatively, tumor cells lose the mitochondrial function of telomerase. Regardless of the mechanism, it is clear that elimination of telomerase as a whole from the cells may not be the most effective or the best strategy to treat all cancer cases. Understanding the normal localization of hTERT in tumors and the physiological relevance of hTERT in mitochondria of normal and cancer cells will not only give us deeper knowledge of the overall function of this protein but will also facilitate the development of therapeutic strategies to more effectively counteract tumors.
Janine Hertzog Santos earned her bachelor’s degree in biology in 1993 from Pontificia Universidade Catolica do Rio Grande do Sul, Porto Alegre, Brazil, and her PhD in genetics and molecular biology in 1999 from Universidade Federal do Rio Grande do Sul in Porto Alegre. She was a post-doctoral fellow from 2000 to 2005 at the Laboratory of Molecular Genetics, National Institute of Environmental and Health Sciences (NIEHS), Research Triangle Park, North Carolina, and joined the faculty of the Department of Pharmacology and Physiology at UMDNJ-New Jersey Medical School in January 2006.