INSIG, SCAP and HMG-CoA Reductase
Sterols and oxysterols regulate cholesterol synthesis by binding to SCAP
or INSIG, and via SREBP-2 modulate the expression of the enzymes of the
cholesterol biosynthesis pathway (Brown, Radhakrishnan & Goldstein,
2018). At times of cholesterol excess, cholesterol in the endoplasmic
reticulum binds to SCAP, which is already in complex with SREBPs, and
causes a conformational change in SCAP which results in binding of the
SCAP-SREBP complex to the endoplasmic reticulum - anchor protein INSIG.
The consequence of this event is retention of SREBP-2 in the endoplasmic
reticulum and prevention of its transport by SCAP to the Golgi for
processing to its active form as a transcription factor for the enzymes
of the cholesterol biosynthesis pathway and for the LDLR (Goldstein,
DeBose-Boyd & Brown, 2006; Sun, Seemann, Goldstein & Brown, 2007).
Oxysterols also have a role in regulating cholesterol biosynthesis
(Kandutsch, Chen & Heiniger, 1978), but in most cases by providing
“fine tuning” of this regulation (Gill, Chow & Brown, 2008). The
brain is an organ with high oxysterol concentrations (24S-HC 10 – 20
ng/mg cf. cholesterol 10 – 20 µg/mg) and it may be in the CNS that the
involvement of oxysterols in regulation cholesterol biosynthesis is most
significant (Saeed et al., 2014). Another case when oxysterols may be
especially important in regulating cholesterol biosynthesis is in
response to infection, both bacterial and virus (Bauman, Bitmansour,
McDonald, Thompson, Liang & Russell, 2009; Blanc et al., 2013). Rather
than bind to SCAP, side-chain oxysterols i.e. 22R-HC, 24S-HC, 25-HC,
(25R)26-HC and 24S,25-EC, bind to INSIG and tether the SCAP-SREBP-2
complex in the endoplasmic reticulum (Radhakrishnan, Ikeda, Kwon, Brown
& Goldstein, 2007). Elegant experiments performed by Radhakrishnan et
al exploiting [3H]25-HC and site-directed
mutagenesis confirmed that side-chain oxysterols bind to INSIG via a
mechanism involving transmembrane helices 3 and 4, which are also
involved in INSIG binding to SCAP (Radhakrishnan, Ikeda, Kwon, Brown &
Goldstein, 2007). Radhakrishnan et al reported that 19-HC, 4,4-dimethyl
cholesterol and lanosterol did not bind to either INSIG or SCAP and did
not inhibit the processing of SREBP-2 (Radhakrishnan, Ikeda, Kwon, Brown
& Goldstein, 2007).
Conversely, lanosterol, the first sterol in the cholesterol biosynthesis
pathway (Figure 2), stimulates the formation of an INSIG-HMG-CoA
reductase (HMGCR) complex (Song, Javitt & DeBose-Boyd, 2005) leading to
the ubiquitination and degradation of HMGCR, the rate-limiting enzyme of
the cholesterol biosynthesis pathway. Cholesterol has no effect on
INSIG-HMGCR formation, however 25-HC and other side-chain oxysterols,
e.g. (25R)26-HC, 25H,7O-C and 26-hydroxylanosterol (26-HL, also called
27-hydroxylanosterol) and can induce the formation INSIG-HMGCR complex
and reductase ubiquitination (Song, Javitt & DeBose-Boyd, 2005). In an
early, and largely forgotten study, Axelson et al showed that
(25R)26-HC, (25R)26-hydroxycholest-4-en-3-one ((25R)26-HCO),
7α,(25R)26-dihydroxycholest-4-en-3-one (7α,(25R)26-diHCO),
7α,12α,26-trihydroxycholest-4-en-3-one (7α,12α,26-triHCO) were potent
suppressors of HMGCR (Axelson, Larsson, Zhang, Shoda & Sjovall, 1995).
The first three of these compounds have been found in brain (Griffiths
et al., 2019a; Meljon et al., 2019). However, Axelson et al’s findings
were before the involvement of INSIG in HMGCR degradation was uncovered
and a mechanism of HMGCR suppression by oxysterols was not described by
these authors (Axelson, Larsson, Zhang, Shoda & Sjovall, 1995).
Radhakrishnan et al suggested that it is the oxysterol bound form of
INSIG can form a complex with HMGCR, just as it can with SCAP, while
lanosterol acts by binding to HMGCR, much like cholesterol binds to SCAP
(Radhakrishnan, Ikeda, Kwon, Brown & Goldstein, 2007). Lanosterol is a
4,4-dimethyl sterol and a very recent study has confirmed its ability to
stimulate HMGCR degradation and its inability to inhibit SREBP-2
cleavage (Chen et al., 2019; Coates & Brown, 2019). Surprisingly, other
4,4-dimethyl sterols in the cholesterol biosynthesis pathway promoted
both HMGCR degradation and inhibited SREBP-2 cleavage (Chen et al.,
2019).