So now let's take a closer look at how hormones activate genes. Since they cannot go directly
into the cell because they're not lipid-based, amino acid-based hormones, except for our
thyroid hormone, are going to exert their effects through something called second
messengers. There are 2 main types of second messenger systems that we find in our cells.
The 1st second messenger is going to be cyclic AMP. The 2nd is the PIP₂ calcium second
messenger system. If we take a closer look at the cyclic AMP signaling mechanism, we find that
first our messenger is going to bind to the receptor on the target cell. So the messenger in
this case is the hormone. Binding of this hormone to the receptor is going to activate a
G protein since this receptor is usually something called a G protein-coupled receptor. Once
the G protein is active, it's going to activate or inhibit other enzymes known as the adenylate
cyclase. The adenylate cyclase is then going to convert ATP in the cell to the 2nd messenger
which is cyclic AMP. Cyclic AMP can then go on to activate other proteins known as protein
kinases. These are enzymes that are going to phosphorylate or add a phosphate to other
proteins. This usually happens in a signal transduction pathway or kind of like a relay where
each message is going to be passed from one molecule to the next until we get to our actual
target activity. The phosphorylated proteins are then going to be activated or inactivated.
The cyclic AMP is then rapidly degraded by another enzyme that's pretty much the opposite
of a protein kinase known as a phosphodiesterease. This is going to stop that signal
transduction pathway cascade. Cascades have the ability to have huge amplification effects
and so the second messenger system allows for 1 hormone signal to cause a very big effect in
our target cell. The 2nd second messenger system that we find in our cells is the PIP₂ calcium
signaling mechanism. In this mechanism, a hormone is going to bind to a G-coupled protein
receptor in the membrane of the target cell and this is going to activate a G protein that is
going to then activate a different effector enzyme known as phospholipase C. This activated
phospholipase C is then going to split the membrane protein PIP₂ into 2 different second
messengers. The 1st is diacylglycerol or DAG which is going to activate protein kinases which
can subsequently phosphorylate proteins. The 2nd is inositol trisphosphate or IP₃. This is
going to cause calcium to be released from intracellular stored sites inside of the cell.
The calcium is then going to act as another second messenger or a 2nd second messenger.
The calcium alters the enzyme activity and channels or it binds to regulatory proteins such
as calmodulin. Once calcium is bound to calmodulin, this is going to activate enzymes which
will then amplify the cellular response. So now that we have discussed how second messengers
work, let's take a closer look at how genes are activated by these proteins. Lipid-soluble
steroid hormones and thyroid hormone can actually diffuse directly into the target cell and
bind to receptors inside of the cell or your intracellular receptors. From here, they make a
receptor hormone complex that enters the nucleus and then binds to a specific region of the
DNA inside the nucleus. From there, this initiates DNA transcription to produce our mRNAs.
Once the mRNA is produced, it is then translated into specific proteins in the cytoplasm of that
cell. Proteins that are synthesized have various functions including things like inducing
metabolic activity or creating structures or being exported or secreted from the cell.