Hardly a car on the road today has its odometer clocked above 100,000 miles. Planned obsolescence sees to it that a worn-out or outmoded vehicle is replaced with a new automobile. An owner who keeps his car looking and performing literally as good as new may be regarded as something of an oddball.
Maintaining a vehicle in 100 percent mint condition takes time and money. It involves replacing every component just as it shows signs of wear, and protecting against the slightest traces of rust by frequent paint touchups.
As usual, Mother Nature got into the obsolescence prevention game first. From the lowliest worm to the loftiest human, every cell in every living body has its DNA, when damaged, neatly repaired and fully restored. It takes many highly specialized molecular mechanics and maintenance experts to do this never-ending job. As Thomas Jefferson put it, eternal vigilance is the price of liberty. But when it comes to aging, it’s the price of life itself.
Today’s issue of Science, dated April 19, 2002, provides a close-up guided tour of a cell’s molecular maintenance staff. Its title: “DNA repair pathway stimulated by the forkhead transcription factor FOXO3a through the GADD protein.” Its senior author is molecular neurobiologist Michael Greenberg at Harvard Medical School in Boston.
“The overall findings of our paper are several,” Greenberg told BioWorld Today. “One is a new function for this forkhead transcription factor called FOXO3a, which is implicated in the Akt pathway. What we show here is a role for this factor in DNA damage, repair and cell-cycle control. We think that how this factor is conserved through evolution gives some insight in light of findings in the nematode worm, Caenorhabditis elegans, reported elsewhere. A similar protein, DAF-16, plays a similar important role in stress responses and in aging. So we think this data might be giving clues to the role that forkhead could play in mammalian aging.”
As for the findings’ novelty, Greenberg pointed out, “Nobody has known that FOXO3a would be involved in DNA repair mechanisms in the control of cell cyclin. I think this pathway, which isn’t well defined yet because it’s likely that this transcription factor at least based on the C. elegans work is responding to DNA stress. That may affect its movement within the cell. We think it will be true in mammalian cells as well. In that sense, it can respond to stress and ultimately be a handle on understanding organismal aging in mammals.”
Greenberg made the point, “This factor is essentially a common element that affects C. elegans and Homo sapiens alike. It’s highly conserved in evolution. That’s what really comes out in the Science paper. It’s known to be involved in stress responses in the worm. We think the fact that now this DNA repair mechanism has been implicated in the aging phenomenon, and the FOXO factor in C. elegans, that the commonality will ultimately be shown to be true for this factor in mammals. Although the experiments we’ve done haven’t yet shown its connection to aging, it does indicate that it’s playing a role in the DNA damage response.”
Presenting Forkhead Protein Family
Greenberg introduced forkhead as “a family of proteins that contain the conserved forkhead domain, which is the initial finding on this family. Its molecular structure may have led someone to name it as a winged helix. It’s a standard protein encoded by DNA, which in the cell nucleus binds to a group of genes controlling the expression of other genes. One of these genes encodes a protein called GADD45, which forkhead induces to promote the repair of damaged DNA. FOXO3a is the forkhead family transcription factor,” he pointed out. “It contains a forkhead binding domain that allows it to bind to the DNA.
“In response to environmental insults in a given cell,” Greenberg explained, “DNA can be impaired. For the cell to function properly, it has to be repaired, because it’s constantly being transcribed into RNA and proteins. So if it’s damaged, certain proteins that the cell needs won’t be made.
“Some of the mechanisms that have evolved to repair DNA are operating all the time; others are induced under stressful conditions. So for example, a cell proliferating rapidly and dividing may not apply all of its energy toward repairs being made prior to proliferation. But in certain phases of the cell cycle, it may take its time and energy to repair DNA. That’s what’s happening in this case, and the forkhead protein appears to have an ability to cause cells to delay within a phase of the cell cycle after DNA has been synthesized and replicated. Now it can repair the DNA at leisure, and it’s going to more likely need that respite if the cell has been exposed to its restful situation, where you might have induced DNA damage.
“Akt is a serine-threonine protein kinase, which is activated by growth factors binding to their receptors on cells,” Greenberg explained. “We showed a number of years ago that Akt plays a key role in promoting cell survival. If we block Akt function, cells die.
“If there’s a sufficient amount of stress under those conditions, FOXO may also play a role in DNA damage repair. If there’s too much stress the environment is so harsh that repair won’t be satisfactory FOXO may be harnessed to allow the cell to undergo apoptosis.”
Chip Technology Tracks Forkhead Genes
“Using gene microarray,” Greenberg recounted, “we developed a form of forkhead, or FOXO3a, that we could control. We fused this protein to a portion of the estrogen receptor, allowing us to control the movement of forkhead in and out of the nucleus. Now all we had to do was add a ligand, tamoxifen, which is like estrogen. It gets into the cell in this experiment, rat fibroblasts and binds to this protein, which moves to the nucleus and turns on genes.
“We were surprised,” Greenberg observed, “that some of the genes were involved in DNA repair. That wasn’t what we expected. We expected to find genes that were involved in apoptosis, or some cell-cycle control. We found some that were involved in DNA repair,” he concluded, “so we asked whether forkhead itself was playing a role in that process. It was.”