Table 1.
Major components of lipolysis pathways in adipose tissue (AT) of dairy cows
| Protein(s) | Encoding gene(s) | Activation stimulus | Function(s) | Transition period dynamics |
|---|---|---|---|---|
| Catecholamines (adrenaline and noradrenaline) | Metabolic stress [125] | - Bind to βAR on the adipocyte cell surface and initiate the lipolytic cascade [33] | - Plasma adrenaline and noradrenaline: - Increase during the dry period and immediately before calving - Decrease slightly after calving until 60 d in lactation [62] |
|
| Adipocyte β-adrenergic receptors (βAR), types: β1R, β2R, and β3R. |
B1AR
B2AR B3AR |
Catecholamine binding | - Activate adenylyl cyclases that convert ATP to cAMP, and induce PKA activation that, in turn, phosphorylates PLIN1 initiating the lipolytic cascade - βAR are increased in abdominal adipose and during increased rate of lipolysis [33] |
- βAR increase during first month after parturition and are positively associated with milk production [27] - B2AR decreases immediately after parturition and increases after peak lactation [27] |
| Perilipin-1 | PLIN1 | PKA | - Protects the lipid droplet from hydrolytic activity of HSL | - Decreases during first 3 wks following parturition compared to other periods of the lactation cycle [21] |
| α/β hydrolase domain-containing protein 5 (ABHD5/CGI-58) | ABHD5 | Phosphorylation of PLIN1 | - Increases the activity of ATGL and provides increased DAG substrate for PKA-activated HSL | - Decreased gene expression after calving - Protein abundance remains unchanged through the transition period [21] |
| Adipose tissue triglyceride lipase (ATGL) | PNPLA2 | ABHD5/CGI-58 | - Hydrolysis of TAG c | - Decreased gene expression during the last week of gestation and first week of lactation compared to the dry period and mid lactation - Lower protein abundance around parturition compared to the dry period [21, 22] |
| Hormone sensitive lipase (HSL) | LIPE | Activation of PKA | - Hydrolyzes DAG b, releasing free fatty acid and MAG a
- Activity varies depending on anatomical location of AT |
- Decreased gene expression during the first 3 wks following parturition compared to the dry period [23] - Protein content unchanged through the transition period [21, 123] |
| Fatty acid-binding protein (aP2/FABP4) | FABP4 | Association with HSL | - Transports fatty acids to the plasma membrane | - Increased gene expression during induced milk fat depression [126] |
| Monoglyceride lipase (MGL) | MGll | DAG b hydrolysis by HSL | - Hydrolyzes MAG a, releasing glycerol | - Decreased gene expression during the first 3 wks of lactation [27] - Protein content currently unknown in cattle |
a MAG: Monoglycerol
b DAG: Diglycerol
c TAG: Triglycerol