If I zap all the catecholamine receptors in your fat cells, and then starve you, you wont lose ANY body fat. It doesnt matter about calories in calories out.
You see, most of the fat in fat cells is locked away inside the central lipid droplet, like car's in a car park. They dont just "flow" naturally outside when needed anymore than car's roll out of car park's if all the roads outside are empty.
For car's to get out of car park's, you need people to drive them out, on purpose. Deliberately. In the same way, you get fatty acids out of the central lipid droplet so they can be oxidised mainly by catecholamine binding.
Perilipin, the main protein that surrounds lipid droplets, is activated by catecholamine binding, it then lets fats out of the central lipid droplet so they can be broken down by hormone sensitive lipase and other enzymes.
Perilipin is hyperphosphorylated by PKA following β-adrenergic receptor activation. Phosphorylated perilipin changes conformation, exposing the stored lipids to hormone-sensitive lipase-mediated lipolysis. Although PKA also phosphorylates hormone-sensitive lipase, which can increase its activity, the more than 50-fold increase in fat mobilization (triggered by epinephrine) is primarily due to perilipin phosphorylation