The effects of heat stress on the heat-tolerant Jersey cattle breed (Bos taurus) have been investigated by Smith et al. [32]. In this study, an animal model of heat stress was successfully induced by an average temperature of 30.3°C and an average humidity of 68% from July to August in Chongqing (29.4316°N, 106.9123°E). It was confirmed that the Jersey dairy cattle initiated panting for evaporative heat dissipation. The RR was significantly affected by THI: it was 51 bpm at THI77, 76 bpm at THI82, and 121 bpm at THI88. In addition, the gas exchange parameters of pH and pCO2 in the blood of Jersey dairy cattle were significantly affected: they were 7.41 and 41 mmHg at THI77, 7.45 and 37.5 mmHg at THI82, and 7.49 and 33 mmHg at THI88, respectively. Furthermore, the findings were consistent with the following standard categories: THI < 72, no stress; 72 < THI < 79, vasodilation and increased RR; and 80 < THI < 90, increased RR or even panting [2].
In this study, transcriptome sequencing analysis screened 212 DEGs and 1370 heat-related genes in the lymphocytes of Jersey dairy cattle naturally exposed to hot environmental conditions of THI77, THI82 and THI88. Ultimately, 16 heat-related DEGs were screened from the 212 DEGs and 1370 heat-related genes, including transforming growth factor beta 2 (TGFB2), ITBG3, VEGF, CP027091.1, PTGS2, NR4A2, EDN1, DUSP6, IL1A, CXCL8, ICAMI, MYL9, DnaJ heat shock protein family (Hsp40) member B13(DNAJB13), PRDM1, SAMSN1 and BHLHE40. The functional annotation results revealed that the 16 heat-related DEGs were associated with the response to hypoxia, oxidative stress, the immune response, inflammation and endocrine disorder. Likewise, six genes were annotated to the immune system, including VEGF, IL1A, ITBG3, PTGS2, MYL9 and CXCL8; five genes were annotated to the endocrine system, including PTGS2, NR4A2, MYL9, EDN1, and ITBG3; and eleven genes were annotated to signal transduction, including DUP6, ICAM1, CP027091.1, IL1A, ITBG3, PTGS2, VEGF, MYL9, END1, TGFB2 and CXCL8.
In a previous study, hypoxia was found to affect cytokine gene expression, inflammation and immunosuppression in immune cells in a manner that involved VEGF, PTGS2, EDN1, CXCL8, IL1A and MYL9. Hypoxia has also been found to affect the expression of the gene encoding EDN1 in U87 glioma cells [11] and the genes encoding PTGS2 and VEGFA in breast cancer and melanoma cells [9]. Overproduction of PTGS2 has also been observed in lung cancer for immunosuppression [10]. Furthermore, HIF-1 is activated in response to hypoxia to modulate the expression of genes such as VEGF and COX-2 via cytokines and chemokines. For example, endothelin-2 (EDN2) has been investigated in macrophages and cancer cells [9]. An investigation has also proven that MYL9 is a ligand activated by CD69 on leukocytes that is strongly detected inside blood vessels in inflamed lungs [18]. In addition, the IL1A gene plays a role in vascular regulation and hematopoiesis induced by hypoxia, but factor 1α (HIF-1α) induces the cytokine CXCL8, which activates heat shock transcription via AKT/mTOR/STAT3 pathways [15–17]. Moreover, ITBG3 (αVβ3), a type of integrin that consists of two components, integrin alpha V and integrin beta 3 (CD61), is a receptor for phagocytosis on macrophages. ITBG3 is physically and functionally associated with important therapeutic targets [33]. The findings in this study show that ITBG3, PTGS2, VEGF and MYL9 are hub genes that are possible biological markers of the immune response in dairy cattle under heat stress.
Oxidative stress is provoked in dairy cattle as a physiological response to heat stress [19]. Several studies have investigated whether exposure of dairy cattle to heat stress can lead to enhanced ROS production, oxidative stress and cytotoxicity [20]. Oxidative stress can interfere with metabolic and physiological pathways by damaging biological macromolecules [21]. Moreover, heat stress and oxidative stress induce very similar gene expression patterns in dairy cattle; for example, they induce heat-shock proteins and antioxidant enzymes [22]. The results of this study show that the gene expression of VEGF and PTGS2 in lymphocytes significantly increases in order to defend against oxidative stress in Jersey dairy cattle when the THI increases. Furthermore, the results show that the PTGS2 gene plays important roles in oxidative stress, oxidation-reduction processes, cellular oxidant detoxification, peroxidase activity, prostaglandin-endoperoxide synthase activity, and dioxygenase activity. It is possible that VEGF and PTGS2 expression in the VEGF signaling pathway promotes the production of antioxidants and enzymes.
Moreover, the results of this study show that the main signaling pathways are the AGE-RAGE pathway, the VEGF pathway, cytokine-cytokine receptor interactions, and TNF and HIF-1 pathways. These pathways mediate hypoxia responses and immunosuppression and are enriched for the VEGF and PTGS2 genes as well. Lee et al. [34] found that VEGF plays vital roles in hypoxia responses by controlling the expression of numerous hypoxia-responsive genes functioning in diverse processes of oxygen delivery. In addition, an investigation has shown that VEGF not only regulates oxygen supply and growth but also acts as a mediator of inflammatory cytokines [35].
Moreover, Ferrara et al. [36] have proven that in low-oxygen environments, VEGF can bind to the VEGF receptor on the endothelial cell membrane, causing receptor autophosphorylation. Therefore, it is possible that heat stress may cause immunosuppression, oxidative stress and endocrine disorder by significantly increasing VEGF and PTGS2 gene expression when the THI increases. In addition, heat may induce the VEGF signaling pathway to increase PTGS2 release. Camacho et al. [37] found that PTGS2 increases PGI₂ release in human vascular cells. During exposure to inflammatory stimuli, blood vessels are also induced to contract by PGI₂. Given that this study revealed that both the VEGF and PTGS2 genes are upregulated by THIs of 77, 82 and 88, it is possible that blood vessels in the lung exhibit changes in resistance during panting in dairy cattle.