Methionine (Met) plays a critical regulatory role in milk production, however, the molecular mechanism of action of Met is largely unknown. This study therefore aimed to investigate the influence of Met on milk synthesis in and proliferation of bovine mammary epithelial cells (BMECs) and explore the underlying mechanism. BMECs cultured in fetal bovine serum (FBS) free Dulbecco’s modified eagle’s medium (DMEM)/F-12 medium were treated with Met (0, 0.3, 0.6, 0.9, and 1.2 mM). Results showed that Met (0.6 mM) significantly increased milk protein and fat synthesis and cell proliferation. Met stimulation also increased mTOR phosphorylation and protein expression of SREBP-1c and Cyclin D1. Gene function study approaches further revealed that SNAT2 is a key regulator of these signaling pathways. PI3K inhibition experiments demonstrated that SNAT2 stimulates these pathways through regulating PI3K activity, and SNAT2 inhibition experiments further revealed that SNAT2 is required for Met to activate PI3K. Furthermore, immunofluorescence observation detected that Met stimulates SNAT2 cytoplasmic expression. Collectively, these findings demonstrate that Met positively regulates milk protein and fat synthesis and cell proliferation via the SNAT2-PI3K signaling pathway in BMECs.

The development and use of urban land is a major source of carbon emissions. How to reduce carbon emissions in the process of urban land use without harming the economy has become an extremely important issue. This paper integrating carbon emissions into the urban land use efficiency evaluation index system, measures low-carbon urban land use efficiency using a non-radial directional distance function and analyses its spatial and temporal evolution and its influencing factors using a combination of a kernel density estimation method and a Tobit model. The study found that: (1) China’s low-carbon urban land use efficiency shows a fluctuating development and tends to converge; (2) there is much room for reducing land input and carbon emissions in China, and in 2016 alone, land input and carbon emissions in the sample could be reduced by 10.38% and 5.31%, respectively; (3) at the national level, land finance, economic level and population density have a positive impact on low-carbon urban land use efficiency, while the traffic level has negative effects, and these effects show regional heterogeneity. Accordingly, the paper proposes corresponding policy recommendations.

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