Project summary/abstract
Awareness of the toxic effects of drugs and pollutants on mitochondria is growing. Our long-term goal is to
understand the molecular mechanisms by which pollutants induce mitochondrial dysfunction and to apply this
information to the development of new therapeutic strategies. The widespread exposure of the general
population to phthalates has raised significant public concern. Phthalates are added to plastics widely found in
food packaging, toys, medical devices, pharmaceuticals, furniture, and cosmetics and they leach out of these
products into the food, water, and air. Di-2-ethylhexyl phthalate (DEHP) is abundantly used, and inside the gut,
it is hydrolyzed to the active metabolite mono-(2-ethylhexyl) phthalate (MEHP). Recent evidence indicates that
phthalates are mitochondrial DNA (mtDNA) toxicants that alter mtDNA copy number. Toxic contaminants that
disrupt mitochondrial function may cause mitochondrial disease or place individuals with genetic variants
affecting mitochondria at greater risk. This is a very significant concern, as adult mitochondrial disease affects
1 in 4,300 individuals. Furthermore, nucleoside reverse transcriptase inhibitors (NRTIs) used to treat human
immunodeficiency virus infection cause mitochondrial toxicity and mimic mitochondrial disease by causing
severe mtDNA depletion. The proposed work will address gaps in our knowledge about the molecular
mechanisms by which these mtDNA toxicants induce mitochondrial dysfunction. The goal of this project is to
determine the molecular mechanisms driving the toxicant-induced variability in mtDNA degradation that we
have observed in our preliminary studies of undifferentiated and differentiated cells treated with the NRTI 2',3'-
dideoxycytidine (ddC). The function of undifferentiated cells in an organ is to replace cells lost under steady-
state conditions and during tissue repair. We predict that mitochondrial pollutants can trigger or contribute to
disease by targeting undifferentiated cells. We hypothesize that upon exposure to a mtDNA toxicant, a
mitophagy-mediated mtDNA degradation pathway is specifically up-regulated in undifferentiated cells causing
significantly higher mtDNA depletion compared to that in differentiated cells. There are two specific aims for
this work. First, the disruption of mitochondrial homeostasis in cells treated separately with either MEHP or
ddC will be quantified. Following toxicant exposures, we will determine whether undifferentiated cells have
increased mtDNA degradation and bioenergetic defects relative to their differentiated counterparts. Also, the
extent of toxicant-induced mtDNA deletions and mutations will be determined. Second, the molecular
mechanism of toxicant-induced disruption of mtDNA homeostasis will be determined by measuring the
expression levels of components of the autophagy/mitophagy and degradosome types of machinery in
undifferentiated and differentiated cells. This work is significant because it will elucidate the mechanism of
toxicant-mediated mtDNA degradation and thus lay the foundation for future pollutant studies. In the long-term,
this work may form the framework for strategies to treat or prevent toxicant-induced mitochondrial damage.
