Glucose promotes cell growth and casein synthesis via ATF4/Nrf2-Sestrin2- AMPK-mTORC1 pathway in dairy cow mammary epithelial cells

In the dairy industry, glucose (Glu) is used as bioactive substance to increase milk yield. However, the molecular regulation underneath needs further clarification. Here, the regulation and its molecular mechanism of Glu on cell growth and casein synthesis of dairy cow mammary epithelial cells (DCMECs) were investigated. When Glu was added from DCMECs, both cell growth, β-casein expression and the mechanistic target of rapamycin complex 1 (mTORC1) pathway were increased. Overexpression and silencing of mTOR revealed that Glu promoted cell growth and β-casein expression through the mTORC1 pathway. When Glu was added from DCMECs, both Adenosine 5'-monophosphate-activated protein kinase α (AMPKα) and Sestrin2 (SESN2) expression were decreased. Overexpression and silencing of AMPKα or SESN2 uncovered that AMPKα suppressed cell growth and β-casein synthesis through inhibiting mTORC1 pathway, and SESN2 suppressed cell growth and β-casein synthesis through activating AMPK pathway. When Glu was depleted from DCMECs, both activating transcription factor 4 (ATF4) and nuclear factor (erythroid-derived 2)-like 2 (Nrf2) expression were increased. Overexpression or silencing of ATF4 or Nrf2 demonstrated that Glu depletion promoted SESN2 expression through ATF4 and Nrf2. Together, these results indicate that in DCMECs, Glu promoted cell growth and casein synthesis via ATF4/Nrf2-SESN2-AMPK-mTORC1 pathway.

Effects of L-arginine and arginine-arginine dipeptide on amino acids uptake and αS1-casein synthesis in bovine mammary epithelial cells

Arginine (Arg), as an important functional amino acids (AA), is essential for milk protein synthesis in lactating ruminants. Arg shares transporters with cationic and neutral AA in mammary epithelial cells. Therefore, competitive inhibition might exist among these AA in uptake by mammary epithelial cells. In this study, cultured bovine mammary epithelial cells (BMEC) were used as the model to investigate whether the availability of L-Arg (0.7, 1.4, 2.8, 5.6, and 11.2 mM) affects the uptake of other AA and if this related to αS1-casein synthesis, and whether Arginine-Arginine (Arg-Arg) substituting part of free L-Arg can alleviate competitive inhibition among Arg and other AA, so as to promote αS1-casein synthesis. Our results showed that 2.8 mM L-Arg generated the greatest positive effects on αS1-casein synthesis and the activation of mammalian target of rapamycin (mTOR) signaling pathway (P < 0.01). With L-Arg supply increasing from 0.7 to 11.2 mM, the net-uptake of other AA (except Glu and Ala) decreased linearly and quadratically (Plinear < 0.01; Pquadratic < 0.01). Compared with 2.8 mM, the net-uptake of essential amino acids (EAA) and total amino acids (TAA) were lower at 11.2 mM L-Arg group, while greater at 1.4 mM L-Arg group (P < 0.01). Arg-Arg dipeptide replacing 10% free L-Arg increased αS1-casein synthesis (P < 0.05), net-uptake of EAA and TAA, as well as phosphorylation level of mTOR and p70 ribosomal protein S6 kinase (P70S6K) and mRNA expression of oligopeptide transporter 2 (PepT2; P < 0.01). These observations suggested that the increased αS1-casein synthesis by 10% Arg-Arg dipeptide might be related to the increase of AA availability and the activation of mTOR signaling pathway in BMEC.

Sestrin2 Negatively Regulates Casein Synthesis through the SH3BP4-mTORC1 Pathway in Response to AA Depletion or Supplementation in Cow Mammary Epithelial Cells

Sestrin2 (SESN2) negatively regulates the mammalian target of rapamycin complex 1 (mTORC1) pathway and casein synthesis in response to amino acid (AA) depletion in cow mammary epithelial cells (CMECs); however, the underlying mechanism is unclear. In the current study, the regulation of SESN2 on AA-mediated β-casein (CSN2) synthesis in CMECs and its mechanism were investigated. Overexpression and silencing of SESN2 demonstrated that SESN2 negatively regulated AA-mediated expression of CSN2 and mTORC1 pathway. Co-immunoprecipitation analysis showed that SESN2 interacted with SH3 domain-binding protein 4 (SH3BP4). Overexpression and silencing of SH3BP4 demonstrated that SH3BP4 negatively regulated AA-mediated expression of CSN2 and mTORC1 pathway and that SESN2 negatively regulated expression of CSN2 and mTORC1 pathway through the SH3BP4 in the presence and absence of AA. The absence or presence of AA demonstrated that AA negatively regulated expression and nuclear localization of activating transcription factor 4 (ATF4). Overexpression and silencing of ATF4 demonstrated that AA negatively regulated SESN2 expression through ATF4. Together, these results indicate that SESN2 negatively regulates the mTORC1 pathway and subsequent CSN2 synthesis through the SH3BP4 in response to AA absence or presence in CMECs.

Protein 14-3-3ε Regulates Cell Proliferation and Casein Synthesis via PI3K-mTOR Pathway in Dairy Cow Mammary Epithelial Cells

Cell proliferation and casein synthesis of dairy cow mammary epithelial cells (DCMECs) are regulated by many factors. This research aimed to investigate the effect of 14-3-3ε on cell proliferation and casein synthesis in DCMECs and to reveal the underlying mechanism. Overexpressing or inhibiting 14-3-3ε demonstrated that cell proliferation; casein synthesis; expression of mTOR, p-mTOR, S6K1, and p-S6K1; and lysosomal localization of mTOR were all up-regulated by 14-3-3ε overexpressing and down-regulated by 14-3-3ε inhibiting. In addition, inhibiting mTOR demonstrated that the up-regulation of cell proliferation and casein synthesis in response to 14-3-3ε overexpressing was removed by inhibiting mTOR. Furthermore, the regulatory mechanism of 14-3-3ε was analyzed by coimmunoprecipitation, and we found that 14-3-3ε could interact with PI3K and activate mTORC1 pathway via PI3K. In addition, DCMECs were treated with insulin and prolactin, and the result showed that the cell proliferation and the expression of CSN2 and 14-3-3ε were all up-regulated by these hormones. In conclusion, the current research showed that 14-3-3ε is an important positive regulatory factor for cell proliferation and casein synthesis in DCMECs, as it up-regulates cell proliferation and casein synthesis via activating PI3K-mTOR pathway.