1. The cellular locus of anabolism has been relocated. Sakai, Uno, Uezumi, Ohkawa & Imai (Ehime University & Kyushu University), PNAS 2024;121(39):e2407768121, demonstrated AR expression in PDGFRα⁺ mesenchymal progenitors and showed that targeted AR ablation in these cells (achieving a 90% decrease in AR protein in mutant mice) reduces limb muscle mass in mature adult (6-month) male mice — but not young or aged males — via reduced Igf1 expression, reconciling a decade of conflicting myofiber-AR data.
2. Myofiber AR controls quality, not mass. Ghaibour et al. (2023, J Cachexia Sarcopenia Muscle 14:1707–1720; IGBMC Strasbourg) generated the first muscle AR cistrome and showed myofiber AR directly governs glycolysis, fatty-acid handling, polyamine biosynthesis, and ROS scavenging.
3. Membrane/non-genomic androgen signaling now has molecular identity. ZIP9 (SLC39A9) and GPRC6A function as bona fide membrane androgen receptors coupling to G proteins, ERK1/2, and mTORC1.
4. Satellite cells are not required for testosterone hypertrophy, yet androgen-induced myonuclei are retained, providing a substrate for "muscle memory."
The landmark advance is the first genome-wide AR cistrome in skeletal muscle. Ghaibour et al. (2023, Journal of Cachexia, Sarcopenia and Muscle 14:1707–1720; senior authors Delphine Duteil and Daniel Metzger) performed AR ChIP-seq (4,691 AR peaks at FDR <0.1) and H3K4me2 ChIP-seq (47,225 peaks) in limb muscle of 11-week-old wild-type mice, integrated with transcriptomics of ARskm−/y muscle (2,138 differentially expressed genes). HOMER motif analysis confirmed canonical ARE enrichment. The key conceptual result: myofiber AR directly controls transcription factors and genes for glycolysis, fatty-acid processing, polyamine biosynthesis, and ROS scavenging, plus cytoskeletal/sarcomeric components. Functionally, disrupting the androgen/AR axis impaired glycolytic activity and accelerated type-2 diabetes in male (not female) mice; DHT increased glycolysis in C2C12 myotubes by 30%, whereas flutamide had the opposite effect, and hexokinase activity was decreased by 20% in quadriceps of aged AR-deficient mice. AR-deficient fibers showed increased lysine/BCAA catabolism (~30%), decreased polyamine biosynthesis, disrupted glutamate transamination, doubled ammonia, and ~30% higher H₂O₂. This recasts myofiber AR as a metabolic-contractile coordinator rather than a mass driver.
This complements the earlier MacLean group ARΔZF2/ARKO microarray work (96 differentially regulated genes including polyamine-pathway genes) and the candidate-gene literature. The polyamine axis is a notable convergent theme: Odc1 (ornithine decarboxylase, rate-limiting in polyamine synthesis) is a direct AR target (Lee & MacLean, AJP-Endocrinol Metab 2011), down >60% in muscle-specific ARKO mice, and is regulated by age/sex tracking testosterone. The same AR→ODC1-enhancer axis was independently mapped in prostate cancer (medRxiv 2024; supraphysiological androgen driving de novo polyamine synthesis from arginine through AR binding upstream of ODC1), suggesting a conserved AR-anabolic metabolic module across tissues.
Three membrane-proximal mechanisms now have molecular identity:
The most important recent reversal: satellite cells are not required for testosterone-induced hypertrophy. Englund, Peck, Murach, McCarthy, Peterson & Dupont-Versteegden (2019, AJP-Cell Physiology 317(4):C719–C724; PMID 31314585) used Pax7-DTA satellite-cell-depleted mice and concluded that "testosterone-induced muscle fiber hypertrophy does not require an increase in satellite cell abundance or myonuclear accretion." This challenges the older Sinha-Hikim/Bhasin human data associating testosterone hypertrophy with satellite-cell and myonuclear increases, and aligns with the myonuclear-domain-flexibility view (Murach, Dungan, McCarthy, Peterson) that fast-twitch type-2 fibers can hypertrophy substantially without proportional myonuclear accretion (no fixed "myonuclear domain ceiling" has been identified).
Despite this, androgen-induced myonuclei are retained ("muscle memory"). Egner, Bruusgaard, Eftestøl & Gundersen (2013, J Physiol): 14 days of testosterone propionate in female mice produced a 66% increase in myonuclei and 77% increase in fiber CSA; three weeks after withdrawal fiber size normalized but myonuclei were retained, and on subsequent overload the previously-exposed muscle grew ~30% in 6 days while controls failed to grow — a phenomenon spanning ~10–15% of murine lifespan that has influenced anti-doping ban durations. The 2024 human study Cumming et al. (J Physiol 602:4171–4193; doi:10.1113/JP285675) provided the strongest human evidence for myonuclear permanence: in 12 untrained men/women across 10 weeks unilateral elbow-flexor training, 16 weeks detraining, and 10 weeks retraining, myonuclei rose in type 1 (13 ± 17%) and type 2 (33 ± 23%) fibres during training and, after detraining, remained 33% higher in the previously-trained vs control arm in type-2 fibres, with three persistent DEGs (EGR1, MYL5, COL1A1). However, the functional retraining advantage was equivocal: retrained type-2 fCSA was larger than control (P = 0.035) but the delta change was not different between arms. This remains genuinely contested: Rahmati, McCarthy & Malakoutinia (2022, J Cachexia Sarcopenia Muscle meta-analysis) concluded myonuclei ARE lost during atrophy (≥30% atrophy) and aging, favoring epigenetic over myonuclear-permanence mechanisms of memory.
This is the area of fastest paradigm change. The 2024 PNAS paper by Sakai et al. (e2407768121; Ehime/Kyushu Universities) is pivotal: AR is expressed in PDGFRα⁺ mesenchymal progenitors; AR ablation there (90% AR-protein reduction) reduced limb muscle mass in mature adult (6-month, not young/aged) male mice and caused dramatic perineal (levator ani/bulbocavernosus) hypotrophy regardless of age, via dysregulated Igf1 and cell-cycle transcripts. Because Myod-Cre-driven myofiber AR deletion does NOT reduce Igf1 while castration does, the inference is that non-myofiber (mesenchymal-progenitor) AR mediates the androgen→local IGF-1→mass axis. This dovetails with Dubois/Claessens' satellite-cell AR knockout (satARKO, FASEB J 2014) showing limb-muscle AR mainly affects fiber-type distribution and force (7% grip-strength reduction; soleus fiber-type shift), while the perineal levator ani is the markedly mass-sensitive AR compartment (−52% weight), and with the discovery that myostatin is a direct AR target gene (6-fold up in satARKO; androgen-induced hypertrophy augmented in Mstn-knockouts) — i.e., androgens induce their own brake. The broader FAP field (Uezumi, Rossi, and others) has meanwhile resolved PDGFRα⁺ FAP heterogeneity by snRNA-seq (e.g., Fitzgerald 2023 MME⁺/GPC3⁺/CD55⁺ human FAP subsets), providing the cellular substrate on which mesenchymal AR acts.
The updated model integrates: (a) AR-driven local Igf1 from mesenchymal progenitors → PI3K/Akt/mTORC1 → protein synthesis and FoxO suppression (reduced atrogin-1/MAFbx, MuRF1); (b) suppression of the myostatin/activin–SMAD2/3 axis, with androgens both directly transactivating Mstn and modulating follistatin/TGF-β (Singh/Bhasin AR/β-catenin–follistatin crosstalk, Endocrinology 2009); (c) anti-catabolic FoxO3a inhibition (White/Carson). Testosterone produces insulin-like rapid Akt/ERK/mTOR activation, transient GSK3β inhibition, and GLUT4 translocation in human myotubes in an AR-dependent manner (J Endocrinol Invest 2017). The polyamine/ODC1 pathway supports myoblast proliferation (spermidine increases C2C12 number). The relative contribution is now seen as dual: androgens raise synthesis (mTORC1, ribosomal capacity) and reduce degradation (FoxO/atrogene suppression), with the mass component routed predominantly through paracrine IGF-1 rather than cell-autonomous myofiber AR.
SARM research has illuminated AR anabolism while delivering a cautionary clinical message. Proposed selectivity mechanisms: (1) SARMs are not 5α-reductase substrates, so they lack the prostate/reproductive-tissue amplification that DHT provides — much apparent "selectivity" may reflect absent steroidal metabolism rather than engineered tissue targeting; (2) differential coregulator recruitment — Narayanan, Coss, Dalton et al. (Mol Endocrinol 2008) showed the aryl-propionamide SARM S-22 (which became enobosarm) increased levator ani mass while decreasing prostate size in rats, and that S-22 and DHT "mediated their actions through distinct pathways," altering "the recruitment of AR and its cofactors to the PSA enhancer in a ligand-dependent fashion" plus distinct kinase phosphorylation. SRC-1 (NCOA1) robustly increased SARM- but not DHT-induced AR transactivation in C2C12 muscle cells while affecting both equally in PC-3 prostate cells (Dalton group); (3) ligand-specific AR conformations altering surface coregulator binding and post-translational modifications (PIAS1 implicated for the SARM TSAA-291). A relevant nuance for muscle: the classic "tissue-specific AR coactivator" FHL2 (Müller et al., EMBO J 2000) functions in heart and prostate but was explicitly not detectable in skeletal muscle in the original work; in muscle FHL2 instead acts on β-catenin/myoblast differentiation and autophagy — so the muscle-specific AR coactivator repertoire (p160 SRCs SRC-1/2/3, with GRIP1/SRC-2 suppressing and SRC-1A/SRC-3 coactivating MyoD; Duteil et al. Cell Metab 2010 SRC-1/TIF2 metabolic roles) remains incompletely mapped.
A 2025 Frontiers in Endocrinology critical appraisal argues these mechanisms remain unproven as drivers of clinically meaningful selectivity, cautioning that "coregulator research followed observations of selective activity in bioassays rather than that it preceded or guided the rational design of SARMs." Clinically, enobosarm reliably adds lean mass: in the Phase 2 cancer-cachexia trial (Dobs et al., Lancet Oncol 2013;14:335–345; n=159 safety/100 evaluable) median LBM rose +1.5 kg at 1 mg (p=0.0012) and +1.0 kg at 3 mg (p=0.046) vs no change on placebo, and the Phase 2 healthy-elderly trial (Dalton et al., J Cachexia Sarcopenia Muscle 2011; n=120) showed dose-dependent LBM gains (3 mg p<0.001) and improved physical function (p=0.013). However, the Phase 3 POWER 1/POWER 2 trials in NSCLC cachexia (3 mg/day, ~325 patients each, results announced August 2013) failed the physical-function co-primary endpoint: stair-climb-power responder p-values were 0.315 (POWER 1) and 0.289 (POWER 2), while LBM responder p-values were 0.036 and 0.113 — a direct demonstration that AR-driven mass gain and functional gain are dissociable (echoing anamorelin's ROMANA trials). Enobosarm later showed activity in AR⁺/ER⁺ metastatic breast cancer (Palmieri et al., Lancet Oncol 2024;25:317–325; n=136; 24-week clinical benefit rate 32% at 9 mg, 29% at 18 mg, rising to 52% when AR staining ≥40%). The synthetic compound YK-11 reportedly binds AR while increasing follistatin/inhibiting myostatin, linking the SARM and TGF-β axes (largely non-peer-reviewed sourcing).
snRNA-seq has transformed muscle biology (Petrany & Millay, Nat Commun 2020; Kim/Franke "MyoExplorer" 2020; Dos Santos 2020; Murach PoWeR exercise studies), revealing myonuclear heterogeneity unrelated to fiber type — NMJ, MTJ (Col22a1⁺), fiber-type-specific, aging-associated (Nr4a3⁺/Enah⁺), and steroid-signaling-enriched subpopulations (e.g., Bcl2⁺ nuclei expressing the steroid-sensing Nr2f1 and oxysterol-binding Osbpl3). However, no published snRNA-seq or spatial-transcriptomics study has yet directly mapped the androgen-treated muscle transcriptome at cell-type resolution — this is the clearest current frontier. The 2024 PNAS mesenchymal-AR work used targeted lineage/conditional approaches rather than an unbiased single-cell atlas of androgen response.
Multiple 2022–2025 studies refine the classical male-centric model. Male muscle expresses higher AR than female, and AR/GR phosphorylation patterns differ by sex (Steroids 2018). The HSAAR transgenic-rat overexpression model (Endocrinology 2024;165(3):bqae012) showed muscle AR overexpression increases male limb-muscle mass (tibialis anterior, EDL) and reduces gonadal fat in both sexes, but argued pubertal sex differences in body composition are driven more by ligand than by changes in muscle AR protein. Alexander et al. (2024, J Physiol) found bioavailable testosterone and AR activation, but not total testosterone, associate with muscle mass and strength in females — an important refinement of the female model. An in-vitro 46XY vs 46XX human primary myoblast study (Int J Mol Sci 2023) found intrinsic sex-chromosome dimorphism in steroidogenic enzymes (higher 5α-R2 and 17β-HSD in 46XY; higher aromatase/CYP19 in 46XX) independent of pubertal hormone exposure. The ACTN3 R577X genotype influences muscle AR levels and AR-responsive polyamine genes (bioRxiv 2024), with XX individuals showing 65–72% lower muscle AR than RR in GTEx.