Discussion
Belonging to the fatty acid binding protein gene family, fabp1mainly expressed in liver and intestine with critical functions
in
intracellular transport and fatty acid uptake (Wang et al., 2015). In
intestine,
Fabp1 binds to endoplasmic reticulum, participates in budding of the
pre-chylomicron, and transfers the pre-chylomicron transport vesicle to
Golgi (Cifarelli & Abumrad, 2011). Revolving in the fatty acid
dependent activation of PPAR, Fabp1 also plays an important role in
regulation of lipid metabolism and cellular
signaling
pathways (Georgiadi & Kersten, 2012). In addition, Fabp1 is proved to
be related to obesity (Atshaves et al., 2010), and overexpression offabp1 may cause diseases, such as nonalcoholic fatty liver
disease (NAFLD), that are relevant to excessive accumulation of fat (Pi
et al., 2019). In conclusion, the tandem triplication of fabp1gene in the striped catfish genome
may make a great difference on fatty acid trafficking and storage in
this fish species.
PPAR signaling pathway is an important regulatory mechanism offabp1 . With different
PPAR-selectivity, various promoter regulatory elements (PPRE) may lead
to differential control of duplicated fabp1 genes (Laprairie et
al., 2016; Taylor & Raes, 2004). Compared to the fact that PPARβ/δ is
ubiquitously expressed, the expression of PPARα and PPARγ varies in
different tissues. PPARα is highly expressed in the liver (Kim et al.,
2015); PPARγ is divided into two isoforms (PPARγ1 and PPARγ2), in which
PPARγ1 is expressed in many tissues, whereas PPARγ2 is exclusively
expressed in adipose tissue (Desvergne & Wahli, 1999). In addition,
PPARα and PPARγ were proved to selectively bind to the PPREs in the
liver and the adipose tissue, respectively (Shimizu et al., 2004). A
recent study (Venkatachalam et al., 2017) reports that retention ofiLBP genes in the zebrafish genome may be caused by
subfunctionalization of PPREs in
the iLBP promoters. In our present study, we detected
PPAR-selectivity for the PPREs offabp1-1 , -2 and -3 in the striped catfish genome by
silico analyses, and all putative PPREs were predicted to be
PPARγ-selective, suggesting similar regulation of the three fabp1isotypes based on the putative PPAR regulatory mechanism. In addition,
subfunctionalization of PPRE was not detected in the promoters of
various fabp1 isotypes, implying that there may be some other
factors for the retention of fabp1 genes in the striped catfish
genome.
According to the duplication-degeneration-complementation (DDC) model,
there are three possible consequences of
duplicated
genes. First, with the accumulation of mutations, one or more copies of
the duplicated genes will be lost or become
non-functional (Force et al.,
1999). Second, some of the duplicates may acquire a novel function
(Lynch & Conery, 2000). Third, the subdivision of functions between the
duplicated genes will occur (Taylor & Raes, 2004). From our RNA-Seq
data (Table 3), the mRNA levels offabp1-1and -3 were detectable
while fabp1-2 was not. It is possible that fabp1-2 lost
the original function, although this proposal is still waiting for more
investigations. Although both fabp1-1 and -3 were
predicted to be PPARγ-selective, the R126T mutation of Fabp1-3 may
potentially affect the fatty acid binding capacity of the protein,
suggest that subfunctionalization may be differential betweenfabp1-1 and -3 .
With great economic significance,
striped catfish has become an
important aquaculture species for worldwide consumption. In the process
of breeding and trading, the high-fat trait of striped catfish has both
advantages and disadvantages. On one hand, the good ability of fat
storage may enhance adaptability of striped catfish to a harsh condition
with insufficient food sources, and these
striped catfishes with a high fat
content are good resources for fish-oil extraction (De Silva & Soto,
2009). On the other hand, during the preparation of fish products for
current consumer markets, visceral mass is usually discarded as waste
instead of being utilized, once they are commonly rich in fat (De Silva
et al., 2006). The increasement of fat content in striped catfish
fillet, therefore, may affect its economic value (Islami et al., 2014).
Our detailed investigation on fabp1 genes may support further
molecular breeding to select individuals with low fat content.
Fatty-acid trafficking and storage
in cells are complicated and dynamic, with a combined regulation by
multiple genes. Meanwhiles, the fatty acid storage is also influenced by
environment, especially temperature can affect lipid content and fatty
acid composition of fish oil (Hemung et al., 2010). Since the high-fat
trait of striped catfish is related to various factors, in-depth
researches are necessary.