sensing of disruptions of normal cell function and development due to either physical or chemical agents of the environment (the molecular-stress pathways) or the cell’s own internal imbalance or errors in its synthetic activities (the checkpoint pathways). These pathways are widely present in single-celled eukaryotes (e.g., yeast) and even prokaryotic cells, as well as in animals. Several molecular-stress and checkpoint pathways are listed in Table 6-7 and illustrated in Appendix C.
Molecular-stress pathways are activated when the cell suffers some chemical alteration, such as damage to DNA (e.g., by X-ray, UV, or alkylating agents) or denaturation of proteins (e.g., by hyperosmotic conditions, oxidation, heat, or alcohol). The cell’s signaled response is one of repair and homeostatic counteraction. In the case of the cytosolic unfolded protein pathway (previously called the “heat shock response”), chaperone proteins, such as Hsp90, help to refold denatured proteins, restoring their activity. These same chaperones play a folding role in the normal synthesis and deployment of intrinsically unstable proteins, such as cell-surface receptors. Recent experiments have suggested that if Hsp90 is partially disabled by mutation or overloaded by stress, variant proteins in some members of a population might be unable to fold correctly, resulting in developmental defects (Rutherford and Lindquist 1998). Another example would be the multidrug transport proteins (P-glycoprotein of mammals) that are induced in the presence of high drug levels and serve to export a wide variety of drugs from the cell.
In checkpoint pathways, the cell’s response is one of delaying certain synthetic processes until other processes are complete. These controls are important in coordinating the timing and extent of cellular processes, such as ensuring the completion of DNA synthesis before mitosis begins or ensuring the attachment of chromosomes to the spindle before anaphase begins. An example of relevance to developmental toxicology is that when colchicine (a Vinca alkaloid) inhibits microtubule formation in a mitotic cell, the cell is prevented from initiating anaphase because of a checkpoint control pathway, which assesses the attachment of kinetochores to spindle microtubules (Rudner and Murray 1996). While chromosomes remain unattached, anaphase is not initiated. When the inhibitor is removed, the cells assemble a spindle and proceed with anaphase. Mutant cells have been isolated that lack components of the control (such as the MAD2 protein), and these cells initiate anaphase in the presence of colchicine, without a spindle. They suffer extensive aneuploidy.
Checkpoint and molecular-stress pathways work together. For example, damaged DNA inside the cell triggers various stress pathways, leading to DNA repair. During the repair, the checkpoint pathways delay DNA synthesis or mitosis until repair is complete. In the context of this committee’s evaluation, there are two relevant points about those widely distributed pathways:
They offer possibilties for the detection and analysis of developmental toxicants, because they indicate the cell’s state of stress in the presence of toxi-