In contrast to testosterone, DHT did not prevent the induction of AR by rapamycin (Figure 4C). Another experiment was carried out to determine whether dihydrotestosterone (DHT) has the same effect on rapamycin induction of AR as testosterone. The increase of PSA and KLK2 at 0.03 and 1 nM testosterone paralleled the increase of AR expression. The data suggest that AR expression is up-regulated when mTOR is depressed, but this loop was operative only in a low testosterone condition. However, at 5 nM testosterone, the induction of AR by rapamycin was no longer evident. Up to this point, the results indicated that AR positively regulates mTOR activity in both low and high testosterone conditions.
Akt phosphorylates TSC1/2, which inhibits the GTPase-activating protein (GAP) activity of TSC1/2 toward small G protein Rheb. Although IRS-1 activates the Ras-Raf-MEK-ERK pathway, the role of this pathway in skeletal muscle is not clear (Rommel et al., 1999). The deficiency of mTOR and raptor in muscle induces defects in mitochondrial metabolism and a decrease in mitochondrial gene expression (Bentzinger et al., 2008; Risson et al., 2009).
There was no difference in SBP and DBP levels between the medium-dose group and the low-dose group at the end of day 21 after intervention. The levels of IVST, LVPWT, LVM, E/A, and E/e’ in the OVX + E + T group were greater than those in the OVX + E group (Fig. 2A and Table 2). There were statistically significant differences in blood pressure at various time points after testosterone intervention, and the effect of testosterone on blood pressure remained stable (Fig. 1A and B). The testosterone level in OVX + E + T group was higher than that in other groups (Table 1). Before drug intervention, the baseline level of blood pressure levels was same in each group.
In addition, both the association of mTOR with eIF3F and S6K1 activity are increased in fed conditions after exercise, which provides an explanation for the enhanced muscle protein synthesis (Song et al., 2017). These results implied that an unknown upstream mediator beyond IGFR might regulate Akt/mTOR signaling in skeletal muscle hypertrophy. Understanding of the role of mTOR in muscle growth and hypertrophy has progressed recently from the evidence of several loss-of-function animal models, although it is relatively well-understood as being similar to the mechanism of the function of mTOR in cell growth regulation.
Cell death induced by glucose deprivation was assessed in both high testosterone- and low testosterone-acclimated cells after three days. In the scrambled siRNA control cells, the activity of AR was enhanced by adding testosterone to the culture. The protein kinase mammalian target of rapamycin (mTOR) is a crucial signal transducer for cell growth and survival (2). Finally, it is clear that rapamycin reversed the effect of myocardial hypertrophy under specific conditions and further screened the optimal dose. Our findings aim to elucidate the cellular basis for increased relative tendency of hypertension and left ventricular hypertrophy in postmenopausal women and serve to help open up new research pathways. Considering that the treatment of myocardial hypertrophy by rapamycin is not proportional to the degree of high blood pressure overload damage, we used echocardiography to evaluate LVEF and LVFS as measures of cardiac function. In addition, the protective effect of mTORC1 inhibitor on testosterone-induced myocardial injury in rats may be in the range of 1.5–2 mg/kg.
On the other hand, recent papers report contradicting results in IGF-I levels and activity of Akt/mTOR/p70S6K1 in aged muscles. Hence, glucocorticoids may regulate mTOR by modulating the level of both BCAT2 and myostatin to regulate catabolism in skeletal muscle. Glucocorticoids also elicit muscle atrophy via controlling transcription of myostatin, an inhibitory regulator of muscle growth, which we discussed in the previous section. Notably, the circulating levels of glucocorticoids are increased under many pathological conditions which are accompanied by muscle atrophy such as cachexia, starvation, sepsis, metabolic acidosis, and severe insulinopenia (Braun and Marks, 2015). Glucocorticoids are some of the most fundamental regulators of energy homeostasis and adjust the metabolism of carbohydrates, fat, and protein in skeletal muscle (Munck et al., 1984). Hence, myostatin may regulate protein synthesis in both an mTOR-dependent and an mTOR-independent manner; it controls the translation through Akt/mTORC1/p70S6K1/S6 signaling and, at the same time, it directly acts on unknown regulators of translation.