3.1 CD36 ubiquitination
Protein ubiquitination is the coupling of protein and ubiquitin,
mediated by ubiquitin ligase (E3)(Popovic et al., 2014). CD36 is the
target of E3 ligase Parkin(Abumrad and Moore, 2011) (Figure 1).
Differing from the degradation effect of ubiquitination,
monoubiquitylation of CD36 by Parkin enhances its stability and increase
its plasma membrane level. In the presence of parkin(Kim et al., 2011),
LCFA-induces polyubiquitination and degradation of CD36 is significantly
down-regulated, probably because one of the ubiquitination sites (Lys469
and Lys472) of LCFA is also the target of Parkin. A recent study
confirmed that USP14 is a deubiquitinating enzyme that mediates CD36
deubiquitylation on macrophages, USP14 cleaves ubiquitin chains from
ubiquitinated CD36 proteins, thus avoiding the fate of CD36 being
transported into the proteasome for degradation(Zhang et al.,
2020)(Figure 1). However, whether Parkin and USP14 respectively act as
E3 ligase and deubiquitinating enzyme in cardiomyocytes remains unclear.
LCFA, a ligand of CD36, significantly enhances the ubiquitination of
CD36(Tran et al., 2011) and promotes its degradation after a long-term
interaction(Smith et al., 2008) (Figure 1). This is a negative feedback
for the cellular uptake of fatty acids. Recent studies have shown that
ubiquitinated CD36 in myocytes stabilizes the structure of insulin
receptor substrates 1(Sun et al., 2018), and thus maintains insulin
signaling. Giving that insulin reduces the ubiquitination of it(Smith et
al., 2008) (Figure 1), CD36 may be involved in self-regulation of
insulin signaling pathway. Moreover, CD36 ubiquitination in macrophages
inhibits the formation of atherosclerosis by decreasing fatty acids
uptake(Srikanthan et al., 2014). But the role of ubiquitinated CD36 in
heart have not yet been elucidated.
3.2 CD36
glycosylation
The glycosylation of CD36 is N-linked at asparagine residues (Asn)
mediated by glycosyltransferase(Hoosdally et al., 2009; McDonald et al.,
2018). There are 10 glycosylation modification sites of CD36 in human,
located in the extracellular segment of CD36(Vinals et al., 2003).
Apart from increasing the molecular weight of CD36, glycosylation could
also stabilize the tertiary folding of polypeptides and is therefore
essential for forming the spatial structure of CD36(Mitra et al., 2006).
And it affects the folding of CD36 theoretically thus influences the
correct translocation of it to the cell membrane. It has been proved
that carboxyl-terminal sites Asn247, Asn321, and Asn417 are
indispensable for CD36 trafficking(Hoosdally et al., 2009). And
mutations in Asn108 and Asn173 sites result in the abnormal distribution
of CD36 on the COS m6 cells membrane(Vinals et al., 2003). Mutations in
Asn 102 of CD36 has been found in spontaneously hypertensive rats
(SHR)(Lauzier et al., 2011). Since Asn102 is located in the fatty acid
binding pocket, mutations at this glycosylation site may have a greater
possibility to affect fatty acid docking in the CD36 pocket then affect
fatty acid transport. CD36 protein levels in SHR are significantly
downregulated and fatty acid intake is reduced, which may be related to
the glycosylation mutation of Asn102. However, the role of Asn102 in SHR
has not yet been confirmed by experiments. Further researches are
required to clarify the role of Asn102 and other glycosylation sites in
cardiovascular diseases.
3.3 CD36
phosphorylation
CD36 has two phosphorylation sites, Thr92 and Ser237, phosphorylated by
PKC(Chu and Silverstein, 2012) and PKA(Hatmi et al., 1996),
respectively(Figure 1). CD36 in small intestinal epithelial cells is
also dephosphorylated by intestinal alkaline phosphatase (IAP)(Lynes et
al., 2011) (Figure 1). In platelets, phosphorylation of Thr92 mediated
the binding of CD36 and thrombospondin(Asch et al., 1993).
Phosphorylation of CD36 at Thr92 is also necessary for the adhesion of
plasmodium falciparum-infected erythrocytes to human dermal
microvascular endothelial cells under flow condition(Ho et al., 2005).
CD36 phosphorylated at Ser237 downregulates the fatty acid uptake rate
of platelets and enterocytes(Guthmann et al., 2002; Lynes et al., 2011).
3.4 CD36
palmitoylation
Most members of the DHHC(Asp–His–His–Cys)family of proteins have
palmitoyl transferase activity, and are confirmed to be the main PATs(De
and Sadhukhan, 2018). DHHC4/5 has been proved to be the palmitoylase of
CD36 in adipocytes (Figure 1). The absence of either DHHC4 or DHHC5
prevents the palmitoylation and the insertion of CD36 on the adipose
membrane, thereby destroying the CD36-dependent fatty acid uptake(Wang
et al., 2019). Besides, SelK(Meiler et al., 2013), which belongs to
neither PATs nor PPT, is also required for palmitoylation of CD36 in
macrophages, suggesting that other proteins may also be involved in the
palmitoylation of CD36.
CD36 has four palmitoylation sites Cys 3, Cys7, Cys464, and Cys466
(Figure 1). And the palmitoylation is strengthened by insulin
stimulation. Combined mutations of these four palmitoylation sites
hinders CD36 translocate to the cell membrane for fatty acid uptake even
in the presence of insulin and AMPK(van Oort et al., 2014). With the
inhibition of CD36 palmitoylation by ceruloplasmin(Thorne et al., 2010),
a kind of palmitoylation specific inhibitor, the processing of CD36 at
the endoplasmic reticulum and transport through the secretory pathway
extends from 90 minutes to 4 hours in melanoma cells, indicating that
palmitoylation is essential for the transport and translocation of CD36.
Increased palmitoylation of CD36 is found in liver steatosis and
fibrosis(Zhao et al., 2018) .The decrease of CD36 palmitoylation
down-regulates the uptake of fatty acids and helps to balance the fatty
acid metabolism in the liver cells, thus eliminates the liver steatosis
and fibrosis(Zhao et al., 2018). However, no research has revealed the
effects of the palmitoylation of CD36 in cardiovascular system.