VEGA, Rebecca/L; EPEL, David; Stanford University: ABC Transporters and Apoptosis: The First and Last Resorts in Early Development?
Embryos encounter various stressors in their environments. Metazoans have evolved several stress defense mechanisms to avoid, repair, and eliminate cellular damage. One such mechanism is the multi-drug transporter system which effluxes moderately hydrophobic contaminants from cells as a first line of defense. Over expression of these transporters causes multi-drug resistance (MDR), while disruption of this pathway leads to chemical accumulation within cells and subsequently defects in embryos. The p-glycoprotein (P-gp) and multi-drug resistance associated protein (MRP) are two commonly studied ABC transporter family members responsible for MDR. Numerous persistent pollutants (e.g. organochlorine pesticides and PAHs) inhibit the efflux of P-gp substrates in vitro (Bain et. al, 1997). Currently, little is known about how these inhibitors may interfere with normal transport of other low-dose contaminants in vivo. Our previous work shows that, in a manner analogous to lower vertebrates, embryos of the sea urchin, Strongylocentrotus purpuratus, display stage-dependent apoptosis when chemically challenged. That is, urchin embryos will delete cells only around the hatching stage, never before, and rarely afterwards. However, unlike vertebrates, urchin embryos maintain low susceptibility to apoptosis (<50% of embryos) even in the presence of high concentrations of genotoxic chemicals (Vega and Epel, 2004). Recently, we found that this resistance to apoptosis is abolished by combined exposure to genotoxins and inhibitors of multi-drug transporters (in press, Mutation Research). For example, embryos are resistant to low doses of the genotoxin etoposide, as judged by the number of DNA abnormalities, apoptotic cells, and embryo survivorship. However, the lethal dose (LD50) of etoposide, can be reduced from 25uM to 5uM in the presence of the broad spectrum transport inhibitors verapamil [0.375uM] or reversin 205 [0.45uM], and from 25uM to 2.5uM in the presence of the P-pg specific inhibitor PSC833 [0.50uM]. Additionally, when 1uM etoposide (no observable effect alone) is combined with 1uM reversin or 0.75uM verapamil, DNA alterations like chromosome bridges and mitotic catastrophe double in frequency while apoptosis increases 4-fold to 3-fold (p < .001). These data suggest that drug efflux suppresses chemically induced apoptosis which may be maladaptive to larval development. It also suggests that pollutants which inhibit multi-drug transporters may be able to chemo-sensitize embryos to seemingly benign concentrations of genotoxins in the environment. Our continued studies will examine the effects of combined low dose anthropogenic pollution on transporter function and embryo health.
