Hi, I'm confused about the metabolic affects of insulin. Insulin increases triglyceride synthesis but then it also activates lipoprotein lipase (which breaks down triglyceride); this seems counterproductive. Can someone please help me understand the mechanism behind this?
In order for triglycerides to make it inside an adipocyte, it must be first broken down into free fatty acids. This is done by insulin activating endothelial lipoprotein lipase (endothelial LPL) which breaks down triglycerides. Once the fatty acids are freed from the triglycerides, they enter the cell and are reconstituted with a G3P-derived backbone; the G3P comes from glucose entering the adipocyte through the GLUT4 transporter (which is also upregulated by insulin). To prevent these newly reconstituted triglycerides from being broken down inside the cell, insulin also inhibits Hormone Sensitive Lipase.
On the flip side, other types of LPL that you see on skeletal muscle are activated by glucagon and epinephrine, the opposite of insulin. It makes sense because skeletal muscle primarily relies on glucose, so after feeding on something that activates insulin (e.g. a carb-rich meal), there's no reason for muscle to utilize both glucose and fatty acids at the same time in significant amounts.
In summary, insulin helps break down triglycerides into free fatty acids outside of the adipocyte so they can enter the cell and then be re-formed inside back into triglycerides.
