REGULATION OF DRUG TRANSPORTER ACTIVITY - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 518

normal duties. Key players involved in the regulation of transporters are hormones,
protein kinases, nuclear receptors, scaffolding proteins, and disease conditions. These
players may affect transporter activity at multiple levels, including (1) when and how
often a gene encoding a given transporter is transcribed (transcriptional control),
(2) how the primary RNA transcript is spliced or processed (RNA processing con-
trol), (3) which mRNA in the cytoplasm is translated by ribosomes (translational
control), (4) which mRNA is destabilized in the cytoplasm (mRNA degradation con-
trol), and (5) how a transporter is modified and assembled after it has been made
(posttranslational control). Regulation of transporter activity at the gene level usu-
ally occurs within hours and days and is therefore classified as long-term or chronic
regulation . Long-term regulation usually occurs when the body undergoes massive
change, such as during the development or occurrence of disease. Regulation at the
posttranslational level usually occurs within minutes or hours and is therefore classi-
fied as short-term or acute regulation . Short-term regulation usually occurs when the
body has to deal with rapidly changing amounts of substances as a consequence of
variable intake of drugs, fluids, or meals as well as metabolic activity. In this chapter
we describe the mechanisms for posttranslational regulation of drug transporters.

17.2. GLYCOSYLATION

Glycosylation is the most common and diverse form of posttranslational modification
for newly synthesized proteins. It is a process in which sugars are added covalently
to proteins. When sugars are added to the NH 2 group on the side chain of an as-
paragine (Asn) residue of the protein, the process is called N-linked glycosylation .
When sugars are added to the OH group of serine (Ser) or threonine (Thr) side chains
of the proteins, the process is called O-linked glycosylation . N-linked glycosylation
occurs primarily in the endoplasmic reticulum (ER) (Figure 17.1). 1 Briefly, a dolichol
pyrophosphate precursor (Glc 3 Man 9 GlcNAc 2 ) is at first transferred to an Asn side
chain of Asn- X -Ser/Thr consensus sequence (X can be any amino acid except pro-
line) for N-linked oligosaccharides in a nascent polypeptide in the ER. Processing
is initiated by the removal of the three terminal glucose residues and at least one
mannose residue in the endoplasmic reticulum, followed by transportation to Golgi
apparatus, where mannose residues are further trimmed, and N -acetylglucosamine,
galactose, and sialic acid residues are added sequentially. The newly synthesized
glycoproteins then exit the Golgi and are transported to their final destination.
O-linked glycosylation mainly happens in the Golgi and at a later stage during pro-
tein processing. The enzyme responsible for this process is called UDP- N -acetyl-
D-galactosamine:polypeptide N -acetylgalactosaminyltransferase. It starts with the
attachment of N -acetylgalactosamine to the hydroxyl groups of serine or threonine
and is followed by the addition of other carbohydrates, such as galactose and sialic
acid. Glycosylation has been demonstrated to play critical roles in the regulation of
membrane targeting, 2 , 3 protein folding, 4 , 5

the maintenance of protein stability (re-

sistance to proteolysis), 6 , 7 and providing recognition structures for interaction with

diverse external ligands. 8 , 9