Basically, a growth factor is a protein for signaling between cells. Growth factors typically bind to specific receptors on a cell's surface, in order to promote cell survival, growth, or proliferation.
The following recent news item now gives me an excuse to make good on my promise:
Hormone May Hold Key To Helping Elderly Men Live Longer (5/27/08)
Elderly men with higher activity of the hormone IGF-1--or insulin-growth factor 1--appear to have greater life expectancy and reduced cardiovascular risk, according to a new study.
IGF-1 is a hormone similar in molecular structure to insulin. It is released from the liver and plays an important role in childhood growth and continues to have anabolic effects in adults. ...
Subjects with the lowest IGF-1 function had a significantly higher mortality rate than subjects with the highest IGF-1 bioactivity. These results were especially significant in individuals who have a high risk to die from cardiovascular complications.
So, does that mean we need to find ways to increase our body's IGF-1 production in order to extend lifespan? Well, not necessarily. It's more complicated than that, as we'll see shortly.
Any hint of longevity enhancement, of course, is something worth paying attention to, but in the case of IGF-1, there's a lot more to the story. It's actually kind of a big deal for several additional reasons.
To begin with, the full name of the hormone is insulin-like growth factor. It is so-named because, as the news item mentions, its molecular structure is similar to that of insulin.
But that's just the beginning of the similarity. Both IGF-1 and insulin affect metabolism. In fact, IGF-1 can bind to the same receptor that insulin does, although a lot less strongly. That, and the not coincidental structural similarity to insulin suggest that perhaps, sometime far back in evolution, the same gene may have coded for some ancestor of both insulin and IGF-1.
If you take into account a striking fact about the IGF-1 receptor, this hypothesis of a common origin for insulin and IGF-1 becomes even more intriguing. The fact is that the (gene for the) IGF-1 receptor is a homologue of the daf-2 gene of the nematode Caenorhabditis elegans (as is the gene for the insulin receptor also). In fact, DAF-2 (the protein product of daf-2) is the only insulin-like receptor in nematodes, so biologists now regard daf-2 as the ancestor of the mammalian receptors for IGF-1 and insulin.
I first mentioned this relationship back here, and went into more detail here, in connection with understanding the effect of sirtuin proteins on aging and longevity of C. elegans.
But the "coincidences" don't stop there. The important function of a receptor is the effect it has, when activated, upon signaling downstream inside the cell. All of the receptors we're talking about here are of the sort called tyrosine kinase receptors. Let's unbundle that term. Tyrosine is one of the 20 amino acids that make up proteins. A kinase is a type of protein enzyme whose function is to attach phosphate groups to specific kinds of amino acids in other proteins. This process is called phosphorylation. When another protein of the right sort is phosphorylated, it becomes able to act as a tyrosine kinase itself, and go on to affect yet other proteins.
This whole process is called signal transduction. The process begins (in the case here) with a receptor tyrosine kinase, which is a cell surface receptor protein that is also a tyrosine kinase – for example DAF-2, and the receptors for IGF-1 and insulin. There may be a number of intermediate steps, but the eventual result is the phosphorylation of a transcription factor, which enters the cell nucleus and facilitates the transcription of certain genes in order to produce new proteins.
In C. elegans, DAF-16 is the transcription factor that is activated by signaling mediated by DAF-2. We discussed DAF-16 in the aforementioned posts here and here. DAF-16 belongs to a family of transcription factors called forkhead box proteins. We have discussed these before too, or rather the subclass called FoxO transcription factors.
We're getting pretty far into the technical weeds here, so if you want more details on this stuff, refer to the earlier posts.
To make the long story short, the effects of the external signaling hormones like insulin and IGF-1 ultimately result from proteins coded for by the genes expressed because of the appropriate transcription factors that were activated by the signaling cascade. There are probably many proteins involved, and sorting them all out, figuring out how they collectively affect longevity, is very much an ongoing project.
The story is interesting to understand because longevity is one of its main themes. In addition to the news item already mentioned, there's more recent news with the same theme. Here are summaries of some of these research announcements:
- When It Comes To Living Longer, It's Better To Go Hungry Than Go Running, Mouse Study Suggests (5/14/08)
- It is once again verified that a low-calorie diet can extend the lifespan of rodents. This benefit is beyond what can be achieved with a higher-calorie diet offset by exercise. However, rats that consumed the most calories, and has less longevity, also had the highest levels of IGF-1. Rats that consumed the fewest calories had the best longevity and the lowest levels of IGF-1. Exercise could only partially counteract the higher IGF-1 levels and reduced longevity of rats on a high-calorie diet. In this study, IGF-1 levels were inversely correlated with longevity. This is a "live-fast, die-young" scenario, which is especially typical of rodents, but not necessarily of humans.
More on this study: here
- Shorter Women May Have Very Long Lives: Gene Mutation Found (3/4/08)
- This study focused attention on the (adult) daugheters of especially long-lived Ashkenazi Jews. A control group consisted of daughters of the same age as the others, but whose families had no history of unusual longevity. The finding was that female children of long-lived individuals (aged 95-110) were on average 2.5 cm shorter than female controls. It was also found that both the centenarians and their daughters were much more likely than the controls to have mutations in the genes for their IGF-1 receptors. However, the daughters also had blood plasma levels of IGF-1 that were 35% higher than the levels in the control group. The interpretation is that the higher IGF-1 levels were due to an attempt to compensate for disruption of IGF-1 signaling due to irregularities of the receptor proteins. This would be consistent with a number of animal studies in which reduced IGF-1 signaling correlates with increased longevity.
More on this study: here, here, here
Interestingly enough, IGF-1 had already been recognized to have an effect on body size – in mice and dogs. The dog research is described here:
One gene between tiny dogs and giant ones? (10/13/06)
Nate Sutter, a geneticist at the National Human Genome Research Institute in Bethesda, Maryland, wanted to know the reason why big dogs, such as Irish wolfhounds, can grow up to 50 times larger than other members of their own species, such as chihuahuas. So he started out looking at large and small dogs of one breed — the Portuguese water dog. ...
The team found that one of the few differences in these Portuguese water dogs occurred in a gene called 'insulin-like growth factor 1', or Igf-1 .
This is one of many genes already known to influence the size of mice: when Igf-1 is knocked out, the animals grow up to be mini-mice.
(The article is subscription-only, but you can find another reference to it here.)
The researchers went on to do further analysis of the IGF-1 gene in many different dog breeds of all sizes, and also in foxes and wolves. They found that almost all of the small breeds had the same variant of the IGF-1 gene as the small Portuguese water dogs had, while almost none of the large breeds had that variant. The team concluded that the IGF-1 variant in small breeds is responsible for the difference because it reduces production of the growth factor.
This should also explain what dog people have always known – that small breed dogs generally live longer than large ones.
Here's a later report of the same research:
What Makes Little Dogs Small? Researchers Identify Gene Involved In Dog Size (4/5/07)
In their study, researchers explored the genetic basis for size variation among dogs by comparing the DNA of various small dog breeds, including Chihuahuas, Toy Fox Terriers and Pomeranians, to an array of larger dog breeds, including Irish Wolfhounds, Saint Bernards and Great Danes. Their investigation found that variation in one gene - IGF-1, which codes for a protein hormone called insulin-like growth factor 1 - is very strongly associated with small stature across all dog breeds studied.
Scientists Explore Queen Bee Longevity (5/8/07) – press release describing research on various factors, including IGF-1 signaling, in queen bee longevity
Mechanisms of lifespan regulation by IGF-I (2/25/08) – blog post that considers some of the paradoxical effects of IGF-1 that may be beneficial in some ways but also shorten lifespan
Not so fast, daf-2: IGF-I is all kinds of good for you (1/23/08) – another blog post on the paradoxical effects of IGF-1
IGF-1 attenuates cardiac aging (11/15/06) – blog post about research on cardioprotective properties of IGF-1
It’s not easy being wee: Does IGF-1 deficiency slow down the brain? (8/30/06) – one more blog post on paradoxical effects of IGF-1
A Single IGF1 Allele Is a Major Determinant of Small Size in Dogs – 4/6/07 research article in Science (sub. rqd.)
Tags: IGF-1, calorie restriction, longevity, aging
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