How do Beta-2 Agonists Premote Hypertrophy?
How do Beta-2 Agonists Premote Hypertrophy?
By Karl Hoffmann
It’s been well established that beta 2 agonists promote hypertrophy of skeletal muscle in animals. There is also limited data that beta 2 agonists such as clenbuterol and albuterol are both anabolic and ergogenic in humans. What is the mechanism behind the anabolic effect of such agents? As depicted in the diagram below, beta 2 agonists bind to a cell surface receptor and initiate a sign****g cascade that results ultimately in a metabolic response. One step in the sign****g process involves increasing levels of the so-called second messenger cAMP (cyclic adenosine monophosphate). cAMP in turn activates so-called protein kinases. These enzymes add a phosphate group to various compounds. In some cases the added phosphate group activates the compound; in other cases phosphorylation results in deactivation of a compound.
Skeletal muscle proteins are degraded via several pathways in the body. One important pathway involves ca (2+) dependent proteolysis. This pathway depends on a compound called calpain, which promotes protein breakdown. Calpain in turn is regulated by calpastatin, which inhibits calpain activity.
Studies (1,2,3) have shown that cAMP inhibits calpain, which recall breaks down muscle protein, and activates calpastatin, which further suppresses calpain-induced proteolysis.
So agents like clenbuterol and albuterol which elevate cAMP are not so much anabolic as they are anticatabolic. We see this phenomenon elsewhere in nature where certain anabolic steroids inhibit the catabolic action of glucocorticoids. Insulin is another example. By itself insulin is believed only to prevent protein breakdown. It is only in the presence of abundant plasma amino acids that insulin exerts an anabolic effect.
We mentioned that the Ca (2+) dependent pathway of protein breakdown was only one of several proteolytic pathways in the body. Costelli et.al. (4) have shown that clenbuterol attenuates another major proteolytic pathway, the so-called ATP-ubiquitin-dependent proteolytic pathway.
Stressing the anticatabolic aspects of beta 2 agonists may be an oversimplification however. Awede et.al, (5) administered clenbuterol to rats, and made the following observations and conclusions:
"Clenbuterol induces hypertrophy and a slow-to-fast phenotype change in skeletal muscle, but the sign****g mechanisms remain unclear. We hypothesized that clenbuterol could act via local expression of insulin-like growth factor I (IGF-I). Administration of clenbuterol to 3-mo-old female Wistar rats resulted in a 10 and 13% increase of soleus muscle mass after 3 and 9 days, respectively, reaching 16% after 4 wk. When associated with triiodothyronine, clenbuterol induced a dramatic slow-to-fast phenotype change. In parallel, clenbuterol administration induced in soleus muscle a fivefold increase in IGF-I mRNA levels associated with an eightfold increase in IGF-binding protein (IGFBP)-4 and a fivefold increase of IGFBP-5 mRNA levels on day 3. This increased IGF-I gene expression was associated with an increase in muscle IGF-I content, already detected on day 1 and persisting until day 5 without increase in serum IGF-I concentrations. These data show that muscle hypertrophy induced by clenbuterol is associated with a local increase in muscle IGF-I content. They suggest that clenbuterol-induced muscle hypertrophy could be mediated by local production of IGF-I."
It is currently believed that IGF-1 derived from the liver contributes little to muscular hypertrophy. Rather, it is an isoform of IGF-1 that is produced locally within skeletal muscle that acts to promote hypertrophy. In the study described above, administration of clenbuterol to rats for 3 days induced a significant 10% increase (P < 0.01) of muscle mass (100.8 ± 2.5 mg for Clenb vs. 91.2 ± 1.9 mg for Ctrl). The hypertrophy reached 16% after 4 wk of clenbuterol supplementation.
Concentration of IGF-I peptide (A) in soleus muscles and serum (B) of control and clenbuterol-treated rats. Note that clenbuterol induced a significant increase in locally produced IGF-1, while at the same time causing a small drop in plasma IGF-1 levels. From Awede et.al
The authors were motivated to look for IGF-1 production in skeletal muscle under the influence of clenbuterol by the recent demonstration that clenbuterol could protect cerebral tissue against ischemic damage by inducing local expression of nerve growth factor, basic fibroblast growth factor, and transforming growth factor-1. If clenbuterol is capable of inducing a variety of growth factors, might it not induce the growth factor IGF-1 in skeletal muscle?
Besides IGF-1, IGF-II may contribute to clenbuterol-induced muscle hypertrophy (6). In (6) the authors observed a significant increase in local (produced in the muscle) IGF-II levels under the influence of clenbuterol. Because it is known that IGF-II binds to both the insulin and IGF-I receptor (as well as the IGF-II receptor), it is probable that the downstream consequences of IGF-II binding are similar to those documented for insulin and IGF-I.
Beta 2 agonists may have a place in the treatment of Overtraining Syndrome. Overtraining Syndrome (OTS) is a condition wherein an athlete is training excessively, yet performance deteriorates. OTS has been shown in studies to be associated with high plasma levels of proinflammatory cytokines, such as Tumor Necrosis Factor alpha (TNFalpha) (7). TNFalpha is known to lead to lowered testosterone production by directly inhibiting Leydig Cell steroidogenesis (8). In a model of TNFalpha production where the immune cells known as macrophages and monocytes were stimulated to produce proinflammatory cytokines including TNFalpha by exposing them to lipopolysaccharide (LPS), clenbuterol potently reduced TNFalpha production (9). If the LPS model of inflammation is a valid model for OTS related inflammation, then clenbuterol would be expected to curtail the drop in testosterone associated with overtraining.
The vast bulk of research ascribing anabolic properties to beta 2 agonists such as clenbuterol has been carried out in animals. It is obviously the hope that conditions will be found in which clenbuterol at doses safe for humans, or some other beta 2 agonist will show similar anabolic properties in humans. This would allow for the treatment of a large number of muscle wasting disorders. Perhaps clenbuterol will serve as a model for the development of a new agent that promotes anabolism and inhibits catabolism in humans in the fashion that clenbuterol does in animals, without the side effects that large doses of clenbuterol have in people.
References
Bardsley RG, Allcock SM, Dawson JM, Dumelow NW, Higgins JA, Lasslett YV, Lockley AK, Parr T, Buttery PJ. Effect of beta-agonists on expression of calpain and calpastatin activity in skeletal muscle. Biochimie. 1992 Mar;74(3):267-73Cong M, Goll DE, Antin PB. cAMP responsiveness of the bovine calpastatin gene promoter. Biochim Biophys Acta 1998 Nov 26;1443(1-2):186-92
Navegantes LC, Resano NM, Migliorini RH, Kettelhut IC. Role of adrenoceptors and cAMP on the catecholamine-induced inhibition of proteolysis in rat skeletal muscle. Am J Physiol Endocrinol Metab. 2000 Sep;279(3):E663-8.
Costelli P, Garcia-Martinez C, Llovera M, Carbo N, Lopez-Soriano FJ, Agell N, Tessitore L, Baccino FM, Argiles JM Muscle protein waste in tumor-bearing rats is effectively antagonized by a beta 2-adrenergic agonist (clenbuterol). Role of the ATP-ubiquitin-dependent proteolytic pathway. J Clin Invest. 1995 May;95(5):2367-72.
Awede BL, Thissen JP, Lebacq J. Role of IGF-I and IGFBPs in the changes of mass and phenotype induced in rat soleus muscle by clenbuterol. Am J Physiol Endocrinol Metab. 2002 Jan;282(1):E31-7
Sneddon AA, Delday MI, Steven J, Maltin CA. Elevated IGF-II mRNA and phosphorylation of 4E-BP1 and p70(S6k) in muscle showing clenbuterol-induced anabolism. Am J Physiol Endocrinol Metab. 2001 Oct;281(4):E676-82
Smith LL. Cytokine hypothesis of overtraining: a physiological adaptation to excessive stress? Med Sci Sports Exerc. 2000 Feb;32(2):317-31
Hong CY, Park JH, Ahn RS, Im SY, Choi HS, Soh J, Mellon SH, Lee K. Molecular mechanism of suppression of testicular steroidogenesis by proinflammatory cytokine tumor necrosis factor alpha. Mol Cell Biol. 2004 Apr;24(7):2593-604
Izeboud CA, Monshouwer M, van Miert AS, Witkamp RF. The beta-adrenoceptor agonist clenbuterol is a potent inhibitor of the LPS-induced production of TNF-alpha and IL-6 in vitro and in vivo. Inflamm Res. 1999 Sep;48(9):497-502.
