Cells treated with testosterone in this way are used historically to study the acute effect of testosterone. For a ‘high testosterone condition’, exogenous testosterone (Sigma, St. Louis, MO, USA) was added to the medium at a final concentration of 5 nM. This traditional protocol is referred to as a ‘low testosterone condition’. This level of testosterone is lower than what has been reported for circulating testosterone in castrated males (8). Commercial FBS contains approximately 0.3 nM testosterone (data provided by vendors, and also confirmed by HPLC-MS analysis).
The qPCR results showed that the mRNA expression levels of ANP, β-MHC and MMP-9 in the myocardial tissue of the OVX + E + T group increased in comparison to the sham-operated, OVX and OVX+E group accompanied by a significant reduction in the TIMP-1 (Fig. 2B, C, D and E). (A, B and C) T induced the mRNA expression levels of mTOR, S6K1 and 4EBP1 in the myocardial tissue. Testosterone aggrandized the expression level of mTOR signaling pathway protein and mRNA in OVX SHR myocardial tissue.
MTOR is localized on the lysosome in the basal state and the mTOR-LAMP2 complex is translocated to the cell periphery under resistance exercise, which provide close proximity to the capillaries. However, the kinase for TSC phosphorylation was unclear in this study since mechanical stimulation was previously shown to activate mTOR in PI3K/Akt-independent manner (Hornberger et al., 2004; O'Neil et al., 2009). The Hornberger group found that mTOR and TSC2 were highly enriched in the lysosome of the muscle in the resting state (Jacobs et al., 2013). It has been established that mTORC1 translocates to the lysosome through regulation of Ragulator-Rag in amino acid signaling (Sancak et al., 2008, 2010).
Although many studies have explored the role of testosterone in regulating vascular tension (Costa et al. 2018, Reckelhoff 2019), few studies have investigated the molecular mechanism of LVH in patients with testosterone-induced postmenopausal hypertension. Our team’s pre-clinical studies also confirmed that testosterone levels in postmenopausal hypertensive women are elevated, and testosoterone plays a potential role in postmenopausal hypertension and HMOD (Li et al. 2019, Jianshu et al. 2021). Animal studies have shown that the serum testosterone of elderly female spontaneously hypertensive rats (SHR) is increased by four times compared with younger rats (Fortepiani et al. 2003). Jiroutek et al. reported an increase in serum total testosterone levels in postmenopausal women after 10 years of follow-up (Jiroutek et al. 1998). Moreover, the hyperphosphorylation of mTOR might lead to resistance to anabolic stimuli in aged muscle.
Hence, further investigation into whether PA produced by DGK ζ during muscle stretch binds to the FRB domain or activates mTOR through an FRB-independent mechanism is warranted. Mechanical stimulation does not induce PLD activity under 3H myristic acid labeling that preferentially labels PC, and FIPI, a PLD inhibitor, did not inhibit mechanical stimulation. However, PA level continued to remain high after the elevated PLD activity returned to basal level 15 min after mechanical stretch using 3H arachidonic acid labeling, suggesting that other enzymes produce PA under mechanical stretch. In addition, treatment with 1-butanol, a PLD inhibitor, inhibited the increase in mTOR activity, supporting the role of PLD in mechanical stretch (Hornberger et al., 2006). PA directly binds to the FKBP12-rapamycin binding (FRB) domain in competition with rapamycin, and activates mTOR (Fang et al., 2001). However, a recent report showed that IGF-I and its receptor IGFR were not important to the induction of hypertrophy and the activation of Akt/mTOR in mechanical loading (Spangenburg et al., 2008).
Effect of mTOR inhibition by rapamycin on AR protein level and AR activity. Rapamycin increased AR expression at 0.03 and 1 nM testosterone (Figure 4B). The inhibition of mTOR activity by rapamycin was not dependent on testosterone concentration. Cells were exposed to 0.03, 1 or 5 nM testosterone, and the activity of mTOR was inhibited by treatment with rapamycin for 24 h. AR knockdown increased the expression of TSC1, but only in a low testosterone condition. Exposure to 5 nM testosterone is used routinely to evaluate the responsiveness of prostate cancer cells to testosterone. The above experiment demonstrated that as little as 0.03 nM testosterone is sufficient to positively regulate mTOR activity, suggesting that mTOR is a high priority mediator of AR signaling.
Effect of glucose deprivation on induction of cell death in high testosterone- or low testosterone-acclimated cells. The low testosterone-acclimated cells were much more resistant to apoptotic death than the high testosterone-acclimated cells (Figure 7). As noted above, the induction of AR protein by inhibition of mTOR activity is only operative in a low testosterone condition. Thus, in the face of an energy crisis when mTOR activity is greatly compromised, AR function is needed to keep cells in a survival mode. The data suggest that the growth inhibitory effect of bicalutamide may be reversible, and cells are able to recover from growth arrest over time. First, the effects of glucose deprivation, bicalutamide, or the combination treatment on cell growth were evaluated in a low testosterone condition. LNCaP cells were treated with 10 nM rapamycin in the presence of 0.03, 1 or 5 nM DHT.
Thus, the attempt to change IGF-I levels in aged muscle for sarcopenia warrants further investigations These results suggested that both mTORC1 and myostatin-Smad2 signaling negatively regulate each other. MTOR regulation by myostatin has sophisticated the molecular mechanism of myostatin signaling. One of the potential IGFR-independent mTOR regulators in skeletal muscle is phosphatidic acid (PA). Maintenance of skeletal muscle mass is regulated by the balance between anabolic and catabolic processes. Recently, a novel polypeptide encoded by the long non-coding RNA (lncRNA) LINC00961 was shown to regulate mTOR activation and muscle regeneration (Matsumoto et al., 2017), implying crosstalk between mTOR and non-coding RNAs in skeletal muscle. Recent studies suggest an additional role of mTOR in skeletal muscle related to the regulation of non-coding RNAs.