Understanding IGF-1

Understanding IGF-1

To understand how IGF-1 works you have to understand how muscles grow. The ability of muscle tissue to constantly regenerate in response to activity makes it unique. It’s ability to respond to physical/mechanical stimuli depends greatly on what are called satellite cells. Satellite cells are muscle precursor cells. You might think of them as "pro-muscle" cells. They are cells that reside on and around muscle cells. These cells sit dormant until called upon by growth factors such as IGF-1. Once this happens these cells divide and genetically change into cells that have nuclei identical to those of muscle cells. These new satellite cells with muscle nuclei are critical if not mandatory to muscle growth.

Without the ability to increase the number of nuclei, a muscle cell will not grow larger and its ability to repair itself is limited. The explanation for this is quite simple. The nucleus of the cell is where all of the blue prints for new muscle come from. The larger the muscle, the more nuclei you need to maintain it. In fact there is a "nuclear to volume" ratio that cannot be overridden.
Whenever a muscle grows in response to functional overload there is a positive correlation between the increase in the number of myonuclei and the increase in fiber cross sectional area (CSA). When satellite cells are prohibited from donating new nuclei, overloaded muscle will not grow (Rosenblatt,1992 & 1994; Phelan,1997). So you see, one important key to unnatural muscle growth is the activation of satellite cells by growth factors such as IGF-1.

IGF-1 stimulates both proliferation (an increase in cell number) and differentiation (a conversion to muscle specific nuclei) in an autocrine-paracrine manner, although it induces differentiation to a much greater degree. This is in agreement with the Dual Effector theory. In fact, you can inject a muscle with IGF-1 and it will grow! Studies have shown that , when injected locally, IGF-1 increases satellite cell activity, muscle DNA content, muscle protein content, muscle weight and muscle cross sectional area (Adams,1998).

On the very cutting edge of research scientists are now discovering the signaling pathway by which mechanical stimulation and IGF-1 activity leads to all of the above changes in satellite cells, muscle DNA content, muscle protein content, muscle weight and muscle cross sectional area just outlined above. This research is stemming from studies done to explain cardiac hypertrophy. It involves a muscle enzyme called calcineurin which is a phosphatase enzyme activated by high intracellular calcium ion concentrations (Dunn, 1999).
Note that overloaded muscle is characterized by chronically elevated intracellular calcium ion concentrations. Other recent research has demonstrated that IGF-1 increases intracellular calcium ion concentrations leading to the activation of the signaling pathway, and subsequent muscle fiber hypertrophy (Semsarian, 1999; Musaro, 1999).
I am by no means a geneticist so I hesitated even bringing this new research up. In summary the researchers involved in these studies have explained it this way, IGF-1 as well as activated calcineurin, induces expression of the transcription factor GATA-2, which accumulates in a subset of myocyte nuclei, where it associates with calcineurin and a specific dephosphorylated isoform of the transcription factor nuclear factor of activated T cells or NF-ATc1.
Thus, IGF-1 induces calcineurin-mediated signaling and activation of GATA-2, a marker of skeletal muscle hypertrophy, which cooperates with selected NF-ATc isoforms to activate gene expression programs leading to increased contractile protein synthesis and muscle hypertrophy. Did you get all that?

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