Exactly How Testosterone Increases Muscle Growth (Molecular Mechanisms)

0:00 We'll know that testosterone increases muscle growth and I think a lot of us have just accepted that that is what happens without really probing any deeper into like the exact mechanisms of how testosterone increases muscle growth. But today that's exactly what I wanted to go into. A really deep dive into the exact like molecular mechanisms of how testosterone increases muscle growth. This is going to be quite a complicated video even more than usual. So feel free to slow it down, pause the video, take some notes and maybe Google some stuff. But hopefully I'm going to explain it well enough clearly enough that you don't need to do that and you'll have a good explanation of how this is all works by the end of the video. Testosterone by itself doesn't magically make muscles bigger on its own. It amplifies the existing natural pathways and processes that the body already uses to make muscles bigger. And

0:46 the main mechanisms boil down to a few things. Testosterone binds to androgen receptors. These then move into the nucleus of the cell and they turn genes up or down, certain genes. It turns up genes that build muscle like IGF-1 and some muscle structural proteins and it turns down genes that break muscle down like myostatin and atrogenes. Other than that, testosterone also activates satellite cells causing them to multiply and donate their nuclei to existing muscle fibers. It triggers the mTOR pathway which I've spoken about in another video and it blocks protein breakdown by inhibiting FoxO transcription factors. So that's like most of it in a nutshell, but I'm going to go really deep into each of those things. And the result of all of these together is you have more muscle proteins getting built and less muscle

1:31 proteins breaking down and that means bigger muscles. All of this works together and it's all amplified by resistance training, lifting weights. Before we get really deep into the details though, if you're serious about maximizing your testosterone to enhance this muscle building process through training, sleep, nutrition and lifestyle, I've put together a complete testosterone optimization guide. It is practical, evidence-based and it's easy to understand and apply in your real life. The link to that is in the description. Get it now if you want to apply what you're about to learn in this video. But back to the video. The first part is cellular entry and binding. So testosterone diffuses across the sarcolemma which is the plasma membrane of the muscle fiber at the outer edge of the muscle fiber. Once it's in the cytoplasm of the muscle cell,

2:18 testosterone binds to the ligand binding domain of the androgen receptor which is a nuclear receptor in the steroid hormone receptor family. A ligand is the thing that binds to the receptor. In this case, the ligand is testosterone and the ligand binding domain is the specific part of the receptor that the ligand binds to. This binding causes a conformational change which is basically a shape change and the androgen receptor dissociates from chaperone complexes which it was usually attached to like HSP70, HSP90 and immunophilins. Nuclear localization signals on the androgen receptor become exposed. These are little short positively charged amino acid sequences on the androgen receptor that are like tags that mark the receptor for import into the nucleus by

3:04 importin receptors. And there is site-specific phosphorylation of the androgen receptor which basically like stabilizes it. The androgen receptor dimerizes becoming a homodimer meaning two androgen receptors essentially attach themselves to each other and then this homodimer of two androgen receptors translocates into the nucleus via importins and then it can be further modified. But the androgen receptor is now in the nucleus and it can do its thing to change gene expression. The androgen receptor is also expressed in satellite cells and precursors and the androgen receptor content correlates with hypertrophy responsiveness in resistance training. So basically more androgen receptor means better response to resistance training. All right, so at this point testosterone has bound to the androgen receptor and it has moved into

3:51 the nucleus. Once it's here, it binds to androgen response elements in the promoter or enhancer regions of various target genes. Either directly binding to consensus sequences or tethered or indirect binding via other transcription factors like myocyte enhancer factor 2, serum response factor or T-cell factor 4. The androgen receptor also recruits coactivators to cause chromatin remodeling, recruitment of RNA polymerase 2 to cause transcription of DNA into messenger RNA. So the point of the androgen receptor binding to the DNA and doing all this chromatin remodeling and recruiting RNA polymerase 2 is so that testosterone can change gene expression. This is one of the most important things you need to pay attention to. This is how testosterone

4:37 exerts most of its effects by changing gene expression. It can upregulate the expression of certain genes and downregulate others. Some of the most important upregulated genes or pathways are IGF-1 to start with. There are androgen response elements in the IGF-1 promoter region of the IGF-1 gene. The androgen receptor binds to this and it increases the transcription of IGF-1 EA mRNA. Not necessarily the mechano growth factor isoform but the IGF-1 EA isoform. The androgen receptor also decreases IGF binding protein 4 which is a binding protein that sequesters IGF-1 and kind of reduces its effects and the result of this is that you have more local IGF-1 in the muscle and that acts within the muscle via the IGF-1 receptor to promote

5:24 growth. Watch my video on IGF-1 to see how important that is for growth and we'll come back to IGF-1 in a little bit to talk about mTOR. Another key pathway is polyamine biosynthesis. So the genes for ornithine decarboxylase and S-adenosylmethionine decarboxylase are upregulated via a androgen response element near the ornithine decarboxylase promoter. Anyway, the result of this is increased putrescine, spermidine and spermine and these stabilize the DNA and RNA, promote cell cycle progression and promote the transition from G1 to the S phase of the cell cycle. So what this means is the cell is now committed to dividing and replicating itself. And this is happening in myoblasts or satellite cells. They are dividing and

6:11 they are proliferating, replicating themselves while delaying premature differentiation of those satellite cells into myotubes and eventually myofibers. So you're getting lots and lots of satellite cells through this polyamine biosynthesis pathway and more satellite cells basically means more potential for muscle later on. Another gene pathway is muscle structural and contractile genes. The androgen receptor upregulates myomesin, myotilin and myozenin which are all important for stabilizing the structure of the muscle proteins. Also testosterone upregulates MYLK4 which seems to be one of the mediators of the strength increase that is caused by androgens because when you knock out this MYLK4 in mice, they are weaker even

6:56 when their muscle size stays the same. Testosterone also upregulates certain microRNAs like miR-206, miR-133b and so on and these target post-transcriptional regulation of myogenesis targets. I won't go too deep into that. And lastly, follistatin. This is increased indirectly through other pathways which I'll discuss in a moment with Wnt and beta-catenin signaling. An interesting and quite paradoxical one is myostatin. I made a full video on myostatin fairly recently so you might want to go and watch that. But myostatin is a protein that is like a brake on muscle growth. It prevents your muscles from getting too big. And you might think that since testosterone is so anabolic that it would downregulate the myostatin gene, but it's actually the opposite. Testosterone binds to the

7:42 androgen response element in the myostatin gene and it upregulates transcription of myostatin. So you have more myostatin mRNA as a kind of self-limiting mechanism so that muscle growth doesn't get too out of control. But since testosterone has so many other anabolic effects through other pathways, the overall net effect is still anabolic even though myostatin mRNA is increased. And testosterone also indirectly downregulates myostatin's activity through things like beta-catenin and follistatin. So all of these interacting pathways get so complex times and there's often like several layers of feedback and control making sure that everything is like perfectly optimized. It's amazing how complicated all of this is. So those are some of the genes that are upregulated and there are some that

8:28 are downregulated like atrogenes which are genes that promote muscle atrophy or shrinking. And the outcome of all of these changes is that we have increased messenger RNA for anabolic proteins, more ribosomes being formed, those ribosomes are more able to translate mRNA into muscle proteins, actin and myosin and we get overall more of those muscle proteins. Also, testosterone increases androgen receptor mRNA translation and protein half-life which amplifies sensitivity to androgens. So activating the androgen receptor with testosterone upregulates the number of androgen receptors. So it's like an amplification. If you watched my androgen receptor video from way back, probably like two years ago now, I said one of the ways to upregulate androgen

9:15 receptors was with androgens themselves. So people who take steroids, they not only activate their androgen receptor but they also upregulate the number of them and the receptor resists breakdown for longer. Its half-life is longer. So essentially, all of these changes the gene expression, they promote muscle growth. More muscle proteins, actin and myosin and more of the structural proteins needed to support and stabilize them. Earlier I said that the androgen receptor upregulates production of local IGF-1 in the muscle and one of the major growth pathways for muscle growth is the IGF-1 PI3K AKT mTOR pathway and I'll explain that now. IGF-1 binds the IGF-1 receptor triggering a cascade. PI3K activates AKT which then flips on mTORC1. mTORC1 is the master regulator

10:01 of protein synthesis. I have a full video on mTOR, which you should watch after this one if you haven't already. But, mTORC1 does two main things. It phosphorylates proteins like P70S6K and 4E-BP1, which unlocks the cell's translation machinery so that ribosomes, which are like the biological factories that make proteins, ribosomes can read mRNA faster and make more myofibrillar proteins, actin and myosin, that actually contract and give the muscle its size. So, essentially, the ribosomes become more efficient, more effective at reading their instructions, the mRNA, and making the proteins. And the second thing is that mTORC1 boosts ribosomal biogenesis, so the production of these ribosomal factories themselves. So, essentially, you have more of these

10:48 protein-building factories and more efficient machinery within those factories. So, you're getting just way more muscle proteins. That is what mTOR does. So, the whole process with this cascade is testosterone increases local IGF-1 inside the muscle, IGF-1 activates mTOR, and mTOR increases muscle protein synthesis by basically upgrading the ribosomes. Also, there is AKT, which is downstream of IGF-1. AKT phosphorylates FOXO1/3, excluding it from the nucleus, which leads to decreased transcription of atrogenes. So, normally, FOXO1/3 would transcribe genes that promote muscle atrophy like MuRF1 and MAFbx atrogene 1, which are the enzymes that are responsible for proteasome-mediated degradation of muscle fibers. They are

11:34 atrogenes or genes that promote the atrophy of muscle. So, by reducing atrogenes, IGF-1, via AKT, reduces muscle protein breakdown. IGF-1 reduces atrophy. There's also the Wnt/beta-catenin pathway, which I mentioned earlier a couple times. This is very important for the formation of new muscle cells and inhibiting myostatin. The androgen receptor downregulates axin, which is something that destroys beta-catenin, and it directly interacts with beta-catenin to stabilize it. Stabilized beta-catenin translocates into the nucleus, binds TCF4, and increases transcription of follistatin. Follistatin binds and neutralizes myostatin. Again, watch my video on myostatin, but myostatin is like a brake on muscle growth. Follistatin lifts this brake so that we

12:21 can push forward and grow lots more muscle. So, for muscle growth, follistatin helps us, myostatin hinders us. Now, let's talk about satellite cells. Satellite cells are stem cells between the basal lamina and the sarcolemma. They are the precursor to muscle cells or muscle fibers. Muscle cells and muscle fibers are exactly the same thing. They're just different ways of describing the same thing. These stem cells, these satellite cells, they can eventually turn into muscle cells or muscle fibers. Satellite cells express the androgen receptor, and they are a primary target for sustained hypertrophy. Testosterone stimulates the proliferation of satellite cells, as I mentioned earlier, so they divide and multiply, increasing in number. Testosterone also stimulates the fusion of satellite cells into existing muscle

13:06 fibers, and this is one pathway contributing to hypertrophy. In young men given testosterone, of course, they experience muscle hypertrophy, and this hypertrophy is accompanied by an increased number of satellite cells and proportionate myonuclear addition. Myonuclear addition basically means that satellite cells fuse with existing muscle fibers to increase the number of nuclei in the muscle fiber. The nucleus of a cell contains the DNA. It's like the control center of the cell. So, it contains the blueprints, the DNA, for proteins. So, if you have more nuclei in a muscle fiber, you have more blueprints, essentially, which means those blueprints, that DNA, can be copied and translated into more muscle proteins. So, more myonuclei means more muscle protein synthesis, and

13:52 testosterone can promote that by causing satellite cells to fuse with existing muscle fibers and donate their nuclei. And just to add even more evidence, knocking out the androgen receptor gene or blocking the androgen receptor with antagonists impairs the function and proliferation of satellite cells. So, clearly, androgens, testosterone, is a very important for the function of satellite cells and their role in promoting hypertrophy. But, not all of testosterone's effects are mediated by changing gene expression. Testosterone can act elsewhere in the cell as well, with its effects being displayed in just seconds or minutes. These non-genomic actions, as we call them, are potentially initiated by a classical androgen receptor, but it's at the membrane instead of like in the cytoplasm. It could be an entirely

14:37 distinct membrane androgen receptor. It could be via G protein-coupled receptors or through an SHBG receptor complex. Some proposed pathways for the non-genomic rapid actions are on screen now. I won't go into those. One reason that we know these non-genomic actions are occurring is because you can use antagonists to block the androgen receptor, but you still see some of these androgenic effects occurring. So, clearly, there are some androgen signaling pathways separate from the classical androgen receptor. Anyway, the functional effects of non-genomic actions in muscle are enhanced myoblast proliferation or differentiation, phosphorylation of transcription factors and coactivators, which then amplify later genomic actions. So, the stuff that I've spoken about for most of this video, that gets amplified by the

15:24 non-genomic effects of androgens. There is an acute increase in twitch or tetanic force facilitated by improved calcium handling and sarcoplasmic reticulum release, and then an increase in potential contractility, and maybe some contribution to hypertrophy signaling through like rapid mTOR or ribosomal activation. So, that is basically all you need to know about how testosterone can increase muscle growth. You could go into even more detail, some some other pathways, but I think I've covered like most of the basics. So, here's a quick summary of this video to refresh your mind and just give you the key points to take away. Testosterone enters muscle cells and binds to androgen receptors, and then these move into the nucleus to control gene expression. The testosterone-androgen receptor complex turns up growth genes

16:10 like IGF-1, muscle structural proteins, and polyamines that drive cell division. It turns down breakdown genes like atrogenes. It activates satellite cells, which are muscle stem cells that multiply and fuse into muscle fibers, adding more nuclei to support muscle growth. It turns on mTOR via IGF-1, and it blocks protein breakdown by shutting down FOXO transcription factors. And then it has some non-genomic effects that occur within minutes, like improving calcium handling and improving contractile force very rapidly. And the overall effect of testosterone on muscles is basically to increase muscle protein synthesis, reduce muscle protein breakdown, and the result of that is muscle growth, especially when you combine it with resistance training. So, I hope you learned something new from this video. Testosterone definitely has

16:56 some very powerful effects on hypertrophy, especially amplifying the effects of resistance training. So, if you want to get the most out of your lifting, having a good level of testosterone, good hormonal environment, will certainly help with that. And again, the perfect next step after this video is my testosterone optimization guide. I cover basically everything you need to know to increase your testosterone naturally, sleep, diet, exercise, supplements, sunlight and circadian health, even things like psychology, competition, and behaviors. All of the best-supported interventions that are going to make the biggest difference in your testosterone levels. So, if you enjoy the way I explain things and you want to take your testosterone to the next level, my testosterone guide is the perfect solution for you. That is linked in the description. Thank you for watching, and I will see you in the next video.