Regulation of steriodogenic enzyme genes

Steroid
hormones biosynthesis or steroidogenesis regulates an array of developmental and
physiological processes in the life span of an organism. The gonadal
development is preliminary controlled by steroidogenesis followed by the
hormonal regulation by the hypothalamic-pituitary axis. Steroidogenesis is a
complex process that involves a number of enzymatic processes by which
cholesterol is converted to biologically active steroid hormones, the potential
end products being the estrogens and the androgens. The biochemistry of the enzymes
involved in steroidogenesis has been reviewed in detail concerning on human steroidogenesis and its disorders
(1). Steroidogenesis, in other words, can be marked as the beginning
of cholesterol trafficking from the sub cellular stores to the mitochondria,
where by the first enzymatic conversion occurs, i.e., pregnenolone is formed by
the action of cholesterol side chain cleavage cytochrome P450 (P450scc). Steroid
hormone synthesis there by requires a number of essential enzymes namely, steroidogenic
acute regulatory protein (StAR), cholesterol side chain cleavage cytochrome
P450 (P450scc), cytochrome P450 17-hydroxylase/ C17–20 lyase (CYP17), and 3-hydroxysteroid
dehydrogenase/isomerase (3-HSD) and further more. The upstream in the steroid
hormone biosynthesis is taken care of by Steroidogenic Acute Regulatory protein
(StAR), its function being the translocation of cholesterol to the inner
mitochondrial membrane. This review summarizes the pertinent literature focussing
on the enzymatic regulation of androgen biosynthesis with special reference to
teleostean models.  

In teleosts,
two biologically important steroid mediators have been identified with
reference to spermatogenesis and sperm maturation namely, 11-ketotestosterone
(11-KT) and 17?,20?-dihydroxy-4-pregnen-3-one. Biosynthesis of these steroids
and gonadal development is controlled by the expression of steroidogenic enzyme
genes and their related transcription factors. In fishes, testosterone (T) and
11-KT are the principal androgens (2). Several studies in teleosts (Hippoglossus hippoglossus, Cyprinus carpio) depicts high levels of 11-KT during spermatogenesis
as compared to T implicating former’s potential role in spermatogenesis
progression (3, 4) and inducing male sex phenotype, secondary sexual
characteristics and female-to-male sex-reversal (5). This prompted
researchers to speculate whether 11-KT could pilot testis formation from
bipotential gonad during critical period of sex differentiation, similar to
estradiol-17?, whose presence in juvenile fish favours ovarian development (6,
7, 8). However, the role of 11-KT seems to have no role testis
determination so also T (9). Nevertheless, testicular growth,
spermiation and recrudescence are essentially regulated by 11-KT and T in
teleosts. Factors responsible for sperm maturation are largely unknown except
for the identification of activin and inhibin playing critical role in
spermatogenesis using eel and zebrafish models. In lower vertebrates including
catfish, multiple forms of dmrt1, sox9 and wt1 have been identified that play crucial role in
gonadal differentiation wherein dmrt1
influences female to male sex reversal (10, 11). Previous study also
suggested that 11?-hsd promoter is
controlled dominantly by the binding of Sox3 with a moderate influence of Wt1
that regulates testicular development and recrudescence (12). In
spite of these, distinct information pertaining to factors or genes and their
interactions during testicular growth and recrudescence are not clear. Thus,
understanding the regulation of steroidogenic enzyme genes directly or
indirectly involved in T and 11-KT production seems critical to address the
molecular mechanisms of spermatogenesis with special reference to fish
reproduction. 

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