Beyond the Classic Endocrine Glands
The student should be able to:
Understand that while hormones are synthesised in classic endocrine
glands, biologically active steroids can also be synthesised in non-
endocrine tissues from circulating precursors
Explain why fat can be considered as an endocrine organ. Identify the
hormones secreted by the adipocytes and their role in insulin
resistance and metabolic syndrome
Identify the hormones from fat and the gut that control appetite
Describe the circadian control of melatonin and cortisol secretion from
the pineal gland and adrenal gland respectively, and the significance of
the biological clock
Outline the endocrine functions of the kidney - vitamin D,
erythropoietin, the renin, angiotensis aldostereone system and of the
heart - atrial natriuretic peptide
Understand that tissues (including fat) can synthesise active hormones
and this may be important in relation to puberty, infertility and hormone-
dependent cancers eg. breast cancer
Appreciate that tumours can also synthesise and secrete active
hormones
Adipocytes store energy as triglycerides and release energy as free fatty acids. There
are two key enzymes:
Lipoprotein lipase found on the outside of fat cells and hormone sensitive lipase on
the inside.
LPL can be stimulated by insulin and lead to the uptake of fatty acids in the
circulation for storage. Circulating lipids can be in the form of FFA in which case they
would be bound to serum proteins. But mostly they are part of lipoproteins, so
chylomicrons which contain newly digested ones or VLDLs from the liver. These are
sources of TAGs.
LPL is a lipase so actually hydrolyses the TAG into FFA and glycerol and taken up,
these are then re-synthesised into TAG and stored.
This similar thing can occur when there is too much glucose, more than can be put
into glycogenesis, the glucose is taken up by adipocytes and synthesised into glycerol
or acetyl-CoA -> TAG.
In order to release energy, the stored TAG can be broken down (undergo hydrolysis)
and FFA and glycerol released into the circulation, for energy use (so this process is
energy releasing). It is hormone sensitive lipase that allows this to happen. It is called
hormone sensitive because its activity can be stimulated by hormones such as
adrenaline and cortisol.
In medical terms it is more the energy storage that is the problem because it is
associated with the epidemic of obesity.
, Fat as an endocrine organ—
Hormones released by adipose tissue include
-Leptin
-Adiponectin
-Resistin
These are released into the circulation and act at distant targets, so by definition they
are hormones.
Leptin is released by fat cells and acts on receptors on the hypothalamus where it
triggers pathways that are associated with satiety (fullness). It triggers pathways
that say we don’t need more fat, we can start burning energy.
So it is a type of negative feedback, as adipose stores increase they release leptin
which triggers metabolic pathways that turn down lipogenesis and turn down
appetite behaviour.
So the initial hope was to give leptin to obese people, but it turned out only a small
minority had an actual mutation with leptin. For most obese people it was more
complex and instead associated with very high leptin levels and leptin resistance.
Conversion of sex hormones
-Weak androgens produced by the adrenal glands don’t have much biological
function, however they can be converted in various tissues, especially in adipose
tissue. Notably they can convert weak androgens to strong androgens and androgens
to oestrogens.
Cytokines are local signaling molecules that are paracrine. However some of these
molecules can enter into the circulation (blurring lines between paracrine and
hormone)
Adiponectin and Resistin are other hormones released from adipose tissue.
Adiponectin seems to be a hormone that potentiates insulin, seems to be needed to
maintain insulin sensitivity and can increase sensitivity. We know in obesity that
The student should be able to:
Understand that while hormones are synthesised in classic endocrine
glands, biologically active steroids can also be synthesised in non-
endocrine tissues from circulating precursors
Explain why fat can be considered as an endocrine organ. Identify the
hormones secreted by the adipocytes and their role in insulin
resistance and metabolic syndrome
Identify the hormones from fat and the gut that control appetite
Describe the circadian control of melatonin and cortisol secretion from
the pineal gland and adrenal gland respectively, and the significance of
the biological clock
Outline the endocrine functions of the kidney - vitamin D,
erythropoietin, the renin, angiotensis aldostereone system and of the
heart - atrial natriuretic peptide
Understand that tissues (including fat) can synthesise active hormones
and this may be important in relation to puberty, infertility and hormone-
dependent cancers eg. breast cancer
Appreciate that tumours can also synthesise and secrete active
hormones
Adipocytes store energy as triglycerides and release energy as free fatty acids. There
are two key enzymes:
Lipoprotein lipase found on the outside of fat cells and hormone sensitive lipase on
the inside.
LPL can be stimulated by insulin and lead to the uptake of fatty acids in the
circulation for storage. Circulating lipids can be in the form of FFA in which case they
would be bound to serum proteins. But mostly they are part of lipoproteins, so
chylomicrons which contain newly digested ones or VLDLs from the liver. These are
sources of TAGs.
LPL is a lipase so actually hydrolyses the TAG into FFA and glycerol and taken up,
these are then re-synthesised into TAG and stored.
This similar thing can occur when there is too much glucose, more than can be put
into glycogenesis, the glucose is taken up by adipocytes and synthesised into glycerol
or acetyl-CoA -> TAG.
In order to release energy, the stored TAG can be broken down (undergo hydrolysis)
and FFA and glycerol released into the circulation, for energy use (so this process is
energy releasing). It is hormone sensitive lipase that allows this to happen. It is called
hormone sensitive because its activity can be stimulated by hormones such as
adrenaline and cortisol.
In medical terms it is more the energy storage that is the problem because it is
associated with the epidemic of obesity.
, Fat as an endocrine organ—
Hormones released by adipose tissue include
-Leptin
-Adiponectin
-Resistin
These are released into the circulation and act at distant targets, so by definition they
are hormones.
Leptin is released by fat cells and acts on receptors on the hypothalamus where it
triggers pathways that are associated with satiety (fullness). It triggers pathways
that say we don’t need more fat, we can start burning energy.
So it is a type of negative feedback, as adipose stores increase they release leptin
which triggers metabolic pathways that turn down lipogenesis and turn down
appetite behaviour.
So the initial hope was to give leptin to obese people, but it turned out only a small
minority had an actual mutation with leptin. For most obese people it was more
complex and instead associated with very high leptin levels and leptin resistance.
Conversion of sex hormones
-Weak androgens produced by the adrenal glands don’t have much biological
function, however they can be converted in various tissues, especially in adipose
tissue. Notably they can convert weak androgens to strong androgens and androgens
to oestrogens.
Cytokines are local signaling molecules that are paracrine. However some of these
molecules can enter into the circulation (blurring lines between paracrine and
hormone)
Adiponectin and Resistin are other hormones released from adipose tissue.
Adiponectin seems to be a hormone that potentiates insulin, seems to be needed to
maintain insulin sensitivity and can increase sensitivity. We know in obesity that
