Ubiquitination and De-Ubiquitination in the Synthesis of Cow Milk Fat: Reality and Prospects

Abstract

Ubiquitination modifications permit the degradation of labelled target proteins with the assistance of proteasomes and lysosomes, which is the main protein degradation pathway in eukaryotic cells. Polyubiquitination modifications of proteins can also affect their functions. De-ubiquitinating enzymes reverse the process of ubiquitination via cleavage of the ubiquitin molecule, which is known as a de-ubiquitination. It was demonstrated that ubiquitination and de-ubiquitination play key regulatory roles in fatty acid transport, de novo synthesis, and desaturation in dairy mammary epithelial cells. In addition, natural plant extracts, such as stigmasterol, promote milk fat synthesis in epithelial cells via the ubiquitination pathway. This paper reviews the current research on ubiquitination and de-ubiquitination in dairy milk fat production, with a view to providing a reference for subsequent research on milk fat and exploring new directions for the improvement of milk quality.

Keywords: de-ubiquitination; fatty acids; milk fat; proteasome; ubiquitination.

Figure 1

Schematic representation of how ubiquitin is linked to substrates, with differently coloured spheres representing ubiquitin molecules linked by different sites. (A) Both Met1 and the seven Lys residues in ubiquitin can form specific chain bonds with different conformations. (B) Substrates can be modified by mono-, multi-mono-, or polyubiquitin. Polyubiquitin includes homo- and hetero-chains. Heterodimeric chains include both homo- and heterodimeric chains, and heterodimeric chains have branched forms in addition to straight chains. In addition, ubiquitin is affected by other post-translational modifications, such as phosphorylation and hydroxylation.

Figure 2

A functional model of ubiquitin-mediated degradation of substrate proteins and de-ubiquitinating enzymes. The ubiquitin molecule undergoes labelling of the substrate protein by ubiquitin-activating enzyme E1, ubiquitin-conjugating enzyme E2, and ubiquitin–lyase E3, followed by degradation of the substrate protein via the 26S proteasome or lysosome. De-ubiquitinating enzymes (DUBs) can inhibit the action of ubiquitin ligases to antagonise the ubiquitinated degradation of substrates, as well as edit ubiquitin chains by cleaving ubiquitin, facilitating the recovery of ubiquitin molecules. In addition, de-ubiquitinating enzymes are regulated by various post-translational modifications.

Figure 3

A model of the milk lipid synthesis process in the mammary epithelial cells of dairy cows. LCFA is translocated into the cell via CD36 and forms LCFA-CoA in the presence of ACSL. SCFAs can be taken up directly by the cell and form FA-CoA catalysed by ACSS, ACACA, and FASN. These acyl-coenzyme A molecules are translocated into the endoplasmic reticulum via FABPs and undergo desaturation and triglyceride formation.

Figure 4

A model of ubiquitination- and de-ubiquitination-regulating components related to milk fat synthesis. In the use of long-chain lipids, ubiquitination regulates the translocation of long-chain fatty acids through the lysosomal pathway and influences the intracellular transport of long-chain fatty acids through ubiquitination-like SUMO chemistry. During fatty acid de novo synthesis, ubiquitination affects fatty acid chain elongation and translocation in cells by regulating SREBP abundance, while de-ubiquitinating enzymes antagonise ubiquitination-mediated degradation of target proteins. In fatty acid desaturation and lipid droplet generation, ubiquitination affects unsaturated fatty acids and TAG formation by influencing PPARy and ADFP.

Figure 5

A model for the promotion of milk fat synthesis in dairy cow mammary cells through the ubiquitination pathway by soysterols and myristic acid. Soysterols promote CD36 and ADFP expression by increasing intracellular ubiquitination levels. Myristic acid promotes fatty-acid-synthesis-related gene expression by antagonising the MARCH14-mediated ubiquitination degradation of ORP5. In addition, myristic acid activated the mTOR signalling pathway to assist milk synthesis.