The role of beige adipocyte formation in energy regulation and metabolism has been extensively reported. The key features of browning include increased mtDNA copy numbers, upregulation of marker genes specific to mitochondrial biogenesis, and enhanced expression of genes related to browning. As shown in
Figure 2A, the relative mtDNA copy number was significantly reduced in the SAT of the 34 M cattle (14.0-fold,
p < 0.01). Generally, there is a significant increase in the number of copies of mtDNA during the first year of an animal’s life, which exhibits a robust positive correlation with lipogenesis [
16,
17]. Concurrently, the content of mtDNA reflects, to some extent, the physiological state of the organism, its metabolic intensity, and its energy demand. The results indicated that the basal metabolic rate of SAT in 10 M cattle may be higher than that of SAT in 34 M cattle, which increased the mitochondrial activity and copy number of mtDNA. Thus, mtDNA copy numbers decline with advancing age. This result is consistent with those of Barazzoni et al.’s study [
18]. However, relative mRNA expression levels of critical regulators of the mitochondrial biogenesis program, including NRF1, NRF2, and TFAM, were markedly increased in the SAT of 34 M cattle (5.2-, 2.8-, and 3.4-fold, respectively,
p < 0.01;
Figure 2B). Hou et al. [
19] demonstrated that mitochondrial biogenesis is increased during the browning fat process. Ultimately, aged cattle stimulate browning by enhancing mitochondrial biogenesis.
Figure 2C shows the relative expression levels of brown/beige adipocyte-related adaptive thermogenesis genes and UCP2 in the SAT of 34 M cattle compared with 10 M cattle. The qPCR results indicated that the relative expression levels of mRNAs for UCP1, UCP2, PRDM16, Cox8b, Cidea, and DIO2 were significantly increased in the 34 M group, except for PGC1α (5.8-, 2.9-, 1.8-, 1.7-, 8.8-, and 2.7-fold, respectively,
p < 0.05). In particular, high UCP1 expression in mitochondria was the most striking feature of brown and activated beige adipocytes. UCP1 is essential for maintaining energy homeostasis and is closely related to energy metabolism. Similarly, Cidea has been extensively reported to be relevant in regulating energy homeostasis and lipid metabolism [
20]. PRDM16 is a critical transcriptional regulator of brown and beige fat, primarily involved in upregulating BAT/beige adipocyte function and heat production [
21]. In contrast, the expression of PGC1α was downregulated in the 34 M group (2.2-fold,
p < 0.05). This downregulation may be attributed to various repressors that target PGC1α, inhibiting beige adipocyte development [
17]. Furthermore, the level of PGC1α, which was directly proportional to the amount of mtDNA, exhibited a negative correlation with aging [
22,
23]. In the current study, the observed changes in relative gene expression align with findings from previous studies, providing further support for the rationale behind the experimental group design. Therefore, the expression of beige adipocyte markers was further observed. As shown in
Figure 2D, the relative mRNA expression levels of CD137, Tmem26, Tbx1, and Cited1 were markedly upregulated in the SAT of 34 M cattle (8.9-, 2.0-, 1.6-, and 2.1-fold, respectively,
p < 0.05). The expression of CD137, Tmem26, Tbx1, and Cited1 would distinguish beige adipocytes from brown and white adipocytes [
24]. Overall, the abundance of beige adipocytes may increase in aged cattle compared to calves, potentially contributing to the observed yellowing of SAT in aged cattle. The study’s findings contradicted the observations made in mice [
25] and humans [
26]. Although species and tissue differences can partially explain variations in beige adipocytes with aging, the primary factors contributing to these disparities are likely attributed to the cold climate and year-round free movement of Mongolian cattle. Li et al. [
5] showed that inducing subcutaneous WAT browning through cold exposure benefits heat production and helps regulate body temperature in cattle. In the inguinal WAT of long-term exercise-aged mice, there was an increase in the expression of genes related to mitochondrial biogenesis, thermogenesis, and beige adipocytes [
27]. In addition, the browning of white adipocytes and the activation of brown adipocytes enhance energy metabolism [
28]. Thus, the formation of beige adipocytes is a significant physiological adaptation for naturally grazed Mongolian cattle to cope with the cold climate in the Xilingol grassland, which could improve meat quality.
Figure 2E shows the correlation analysis of UCP2 and brown/beige adipocyte-related adaptive thermogenesis genes. The strongest correlation was observed between UCP2 and DIO2 (r = 0.999,
p < 0.01), implying the potential roles of UCP2 in energy metabolism during Mongolian cattle growth. Cidea (r = 0.995,
p < 0.01) and UCP1 (r = 0.992,
p < 0.01) were closely followed. The results were similar to those reported by Shigematsu et al. [
29], who reported correlations between UCP2 expression levels and certain brown or beige adipocyte-related genes. Thus, the increased relative expression levels of UCP2 may be related to the formation of beige adipocytes, although the precise mechanism remains unclear.