Steroidogenesis inner mitochondrial membrane [4]. The central

Steroidogenesis involves the synthesis of steroid
hormones that are derivatives of cholesterol which are synthesized by various
tissues, most prominently the adrenal
gland and
gonads. These are usually found in chordates and arthropods. Fishes, for example teleosts, produce several types of
bioactive gonadal steroids, including progestogens, estrogens, androgens and various
derivatives of steroids. Steroids are the absolute requirement for development,
maintenance, homeostasis and reproduction. In adult vertebrates, these steroids
are produced at appropriate times in specialized steroid producing cells called
gonads. These cells express a group of steroidogenic enzyme genes whose
products modify its cholesterol and derivatives 1. Steroids direct the development
of germ cells and accessory glands and organs, as well as the modification of
the behaviour, to ensure that sexual reproduction can take place. Although many
steroids are chemically identical in all major vertebrate classes, the role of
these steroids may differ. However, unique steroid hormones have evolved in
some vertebrate classes, especially amongst fishes, to fulfil particular
functions.

 

Sex differentiation is initiated and
controlled by gonadal steroid hormones. These hormones performed
different functions and permanently differentiated into sex organs during
development. Steroid hormones are synthesized in steroidogenic cells of the
ovary testis and brain that are required for normal reproductive function and bodily homeostasis. Steroidogenic endocrine
tissues such as the adrenal and the gonads respond to trophic hormones and
other external stimuli with rapid surge in steroid hormone production 2. The
acute and chronic regulation of steroidogenesis is controlled by trophic
hormones that usually occur on
order of minutes and hours, respectively. Chronic regulation of steroidogenesis
by LH or ACTH occurs at the level of gene transcription 3. The acute response is initiated by the
mobilization and delivery of the substrate for all steroid hormone biosynthesis. Cholesterol, from the outer to
the inner mitochondrial membrane, where, it is metabolized to the pregnenolone
by the cytochrome P450 cholesterol side chain cleavage enzyme (p450scc). The Steroidogenic acute regulatory
(StAR) protein is the one which regulates the true rate-limiting step in steroid biosynthesis, i.e. the
delivery of cholesterol from the outer to the inner mitochondrial membrane 4. The central role of StAR was proven by two
observations by robust steroid hormone synthesis followed co-transfection of
StAR and the cholesterol side-chain cleavage system into nonsteroidogenic COS-1
cells 4, 5. Second, patients with StAR mutations have
congenital lipoid adrenal hyperplasia, in which all adrenal and gonadal
steroidogenesis was disrupted 5, 6. The expression of this protein is
predominantly regulated by cAMP-dependent mechanism in the adrenal and gonads.

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