The primary objective of this investigation was to determine plasma IGF-1 levels in the adult dog, including a wide range of breeds and weights. A few previous studies analyzed IGF-1 levels in some breeds, but were primarily targeting other investigational goals. A priori data, therefore, including plasma IGF-1 measurements across numerous breeds and mixed-breed dogs, have not previously been available. This paucity of data led the investigators to evaluate IGF-1 levels on breeds of widely variable size and expected longevity because the clarity of these levels may provide valuable insight into the mechanisms of aging in a larger mammal. Studying these differences across dog breeds is especially interesting because the smaller breeds tend to live longer than the larger breeds, which is exactly the opposite of the general life span trend across mammalian populations (i.e., the mouse lives a much shorter life than an elephant). The GH/IGF-1 axis has been proven important in life span extension for worms, flies, and mice, but its effects in larger mammals remain undetermined.
Early IGF-1 analyses completed by Eigenmann et al. (1988
) indicated that adult poodles had high correlation of circulating IGF-1 levels and adult body size. The correlation held between dogs of similar breed origin and different adult body size: the toy (N
4), miniature (N
4), and standard poodle (N
4). Given the population structure of purebred dogs, it is possible that these breeds, derived from very similar or identical predecessors, are not representative of the wide range of variability within the species. This variability is extreme in adult body size, with the smallest breed averaging 5 lb at adult weight and the largest breed weighing approximately 225 lb as an adult. Additionally, the domestic dog has the widest life span variation among mammals between its independent breed groups. The species is also intriguing in that with so much species variation, all members of the species are capable of mating regardless of highly variable physical characteristics. By utilizing these unique parameters of the species, analyses can advantageously investigate the natural occurrence of extreme variation within genetically isolated populations which are represented in canines by breed group structure and strictly maintained by national canine organizations. One or two other previous studies into canine IGF-1 concentrations compared dissimilar breeds, but targeted very young animals. Nap et al. (1994
) determined that toy poodles and great Danes between the ages of 7 and 27 weeks maintain similar plasma IGF-1 levels. A later study confirmed this result by demonstrating that beagles (N
6) and great Danes (N
6) at the ages of 6 and 24 weeks have no significant differences in plasma IGF-1 levels regardless of size and gender in the pups (Favier et al. 2001
). Further analysis led these investigators to conclude that early circulating GH levels rather than circulating IGF-1 levels were significantly associated with the dogs’ final adult size. The results reported here significantly expand upon data pertaining to correlations in adult body weight and size rather than early life growth rate or size. With increased sample size, the breeds analyzed are expanded to include breeds that are not closely genetically related in origin. This sample set includes animals ranging from just a few months of age through 204 months, or 17 years, and ranging in size at adulthood from 5 to 160 lb.
Regression analysis of IGF-1 levels is presented as a function of age and weight for four categories of animals, each inclusive of all the breeds and ages. For regression analysis comparing IGF-1 with age, the four groups (intact males, neutered males, intact females, and spayed females) are similar in that all groups have a noticeable, yet not significant, negative correlation of serum IGF-1 levels with age. These changes have been noted in other species as well, but an interesting point of the diminishing IGF-1 levels with age, uniquely facilitated by canine analysis, is the apparent drastic difference in levels between the intact females and the spayed females. Intact females decrease their serum IGF-1 levels at a rate of 55 ng/ml (±0.16) per month of life, while the spayed females lose IGF-1 at a level of 0.19 ng/ml (±0.18) per month of life. The dramatic decrease demonstrated by the intact females may be due to hormonal effects. Clearly, there is a difference in the biological maintenance of IGF-1 levels between the intact and altered females, and the dog provides a unique opportunity to more closely investigate hormonal effects on the GH/IGF-1 axis. Waters et al. (2009
) have noted differences between ovariectomized dogs and dogs with 4 years or more of ovarian exposure, reporting that those maintaining greater ovarian exposure live longer. Therefore, more intense analyses of these effects are currently underway.
Another interesting difference in the data exists between the intact males and the neutered males. There is a significant difference in the serum IGF-1 levels between these two groups, with the neutered males losing IGF-1 at a level of 2.98 ng/ml per month of life while the intact males decrease their serum IGF-1 levels at a much more modest rate, 1.11 ng/ml per month of life. Being the first report of such a difference in males, longitudinal data on the dogs contributing to this investigation will be vital for continued data analyses as the relationship between IGF-1 and life span has not previously been analyzed with consideration to spay/neuter situations. This parameter for the study population may offer insight unavailable in other organisms.
The regression analysis of IGF-1 as a function of weight revealed a significant relationship for all groups of animals except the spayed females. For both intact males and females, an increase in overall body weight was significantly associated with higher levels of IGF-1. While the trend remained the same for the spayed females, the correlation was not significant. The data, therefore, verify the previous work by Eigenmann et al. (1988
) indicating that adult dogs have high correlation of circulating IGF-1 levels with adult body size as opposed to early body size. Apparently, the early life body size and IGF-1 levels were significant for association to GH, but do not correlate well with IGF-1 parameters. It will be interesting upon follow-up of these animals, therefore, to see if the final longitudinal parameters continue to correlate with adult body size and IGF-1 values as indicated in this study.
The adult body size appears to be the key factor for association with IGF-1 levels. This parameter sheds light on the studies by both Favier et al. (2001
) and Nap et al. (1994
) which indicated no correlation between IGF-1 and early life size up to 27 weeks of age. When these results are compared to previously published life span analyses (Greer et al. 2007
; Eigenmann et al. 1988
), the correlation between body size and longevity corresponds to those demonstrated here of larger body weight being in positive correlation to increased IGF-1 levels or diminished longevity. Although this study correlates IGF-1 levels with size in dogs without direct measurements of individual animals’ life span, the findings could be indicative that the rate of growth is actually an important key factor for subsequent life span. Clearly, growth records indicate that the larger breeds experience a significantly more rapid and lengthier period of rapid growth in early life. An indication may exist then that an organism’s extremely accelerated growth trajectory is mechanistically linked to final longevity. This idea, as suggested by Samaras et al. (2003
), notes that the initial birth weight of small dogs is not significantly different than that of large or giant dogs. These parameters were reiterated with Favier and colleague’s publication (2001
), potentially suggesting that a higher cell turnover during rapid growth is required to obtain adult body size. Rapid body growth also presumably lends to more rapid cellular senescence and, therefore, ultimately organismal aging. It seems possible that a rapid and prolonged cell turnover in several tissues increases the risk of cell mutations that are not repaired or eliminated, also resulting in more rapid cellular senescence and thus contributing to those diseases producing a shortened life span (Campisi 2008
). There is evidence for these life span-related events in rapidly growing animals (Coile 2005
; Galis et al. 2007
; Samaras 2009
; Deeb and Wolf 1994
; Miller et al. 2002
; Snibson et al. 1999
Although the exact relationship between IGF-1 and life span may be indirect and proceed through additional biochemical pathways, the correlations between body size, serum IGF-1, and longevity have been demonstrated in shorter lived, much smaller mammals. Mice deficient in IGF-1 are not only smaller, but live longer and have significantly reduced prevalence of age-related physical complications such as cataracts, atherosclerosis, and cancer (Bartke et al. 2003
; Deeb and Wolf 1994
; Menezes Oliveira et al. 2006
; Shechter et al. 2007
; Shevah and Laron 2007
). In fact, when longitudinal studies (as are ongoing in the dog) are completed in the mice, the most apparent correlations with longevity are overall body size and IGF-1 levels. The genetically manipulated mice, including those with mutations in the GH/IGF-axis, Snell and Ames dwarfs, are significantly smaller than their normal littermates and benefit from significantly increased life span (Brown-Borg et al. 1996
). GHRKO mice, and those with a defect in the GH-releasing hormone receptor (GHRHR) signaling pathway, demonstrate similar results. In human studies, low IGF-1 levels have been associated with lack of pituitary and cardiovascular disease (Colao et al. 2008
), and men with higher levels of circulating IGF-1 have significantly increased risk of developing prostate cancer (Cheng et al. 2006
). Longitudinal studies have not been completed in humans because of the length of time involved, similar to longitudinal studies in the dogs; however, the current parameters for indicating increased or decreased longevity as proven in the mouse are present in the dogs analyzed. Furthermore, the indications for human longevity such as a lack of cardiovascular disease are further correlated with diminished IGF-1 levels, which may prove to be associated to longevity in the dog. IGF-1 and its receptor alleles have been linked both directly and indirectly to body size and skeletal structure (Eigenmann et al. 1984
; Sutter et al. 2007
), as well as to life span in several mammals, including dogs (Suh et al. 2008
; Taguchi and White 2008
; Bartke et al. 2003
; Samaras et al. 2003
; Miller et al. 2002
). The longitudinal data currently being collected from animals intermediate of mice and men will certainly prove beneficial in determining definitive correlation of longevity potential and circulating IGF-1. Additionally, these data can be explored for further delineation of sex differences in IGF-1 correlations, while the IGF-1 receptor variants, GH receptor variants, and GHRHR receptor variants are also being explored for polymorphisms and longevity associations in the study animals.