Steroidhormones biosynthesis or steroidogenesis regulates an array of developmental andphysiological processes in the life span of an organism. The gonadaldevelopment is preliminary controlled by steroidogenesis followed by thehormonal regulation by the hypothalamic-pituitary axis. Steroidogenesis is acomplex process that involves a number of enzymatic processes by whichcholesterol is converted to biologically active steroid hormones, the potentialend products being the estrogens and the androgens. The biochemistry of the enzymesinvolved in steroidogenesis has been reviewed in detail concerning on human steroidogenesis and its disorders(1).
Steroidogenesis, in other words, can be marked as the beginningof cholesterol trafficking from the sub cellular stores to the mitochondria,where by the first enzymatic conversion occurs, i.e., pregnenolone is formed bythe action of cholesterol side chain cleavage cytochrome P450 (P450scc). Steroidhormone synthesis there by requires a number of essential enzymes namely, steroidogenicacute regulatory protein (StAR), cholesterol side chain cleavage cytochromeP450 (P450scc), cytochrome P450 17-hydroxylase/ C17–20 lyase (CYP17), and 3-hydroxysteroiddehydrogenase/isomerase (3-HSD) and further more. The upstream in the steroidhormone biosynthesis is taken care of by Steroidogenic Acute Regulatory protein(StAR), its function being the translocation of cholesterol to the innermitochondrial membrane. This review summarizes the pertinent literature focussingon the enzymatic regulation of androgen biosynthesis with special reference toteleostean models. In teleosts,two biologically important steroid mediators have been identified withreference to spermatogenesis and sperm maturation namely, 11-ketotestosterone(11-KT) and 17?,20?-dihydroxy-4-pregnen-3-one.
Biosynthesis of these steroidsand gonadal development is controlled by the expression of steroidogenic enzymegenes and their related transcription factors. In fishes, testosterone (T) and11-KT are the principal androgens (2). Several studies in teleosts (Hippoglossus hippoglossus, Cyprinus carpio) depicts high levels of 11-KT during spermatogenesisas compared to T implicating former’s potential role in spermatogenesisprogression (3, 4) and inducing male sex phenotype, secondary sexualcharacteristics and female-to-male sex-reversal (5). This promptedresearchers to speculate whether 11-KT could pilot testis formation frombipotential gonad during critical period of sex differentiation, similar toestradiol-17?, whose presence in juvenile fish favours ovarian development (6,7, 8). However, the role of 11-KT seems to have no role testisdetermination so also T (9). Nevertheless, testicular growth,spermiation and recrudescence are essentially regulated by 11-KT and T inteleosts. Factors responsible for sperm maturation are largely unknown exceptfor the identification of activin and inhibin playing critical role inspermatogenesis using eel and zebrafish models.
In lower vertebrates includingcatfish, multiple forms of dmrt1, sox9 and wt1 have been identified that play crucial role ingonadal differentiation wherein dmrt1influences female to male sex reversal (10, 11). Previous study alsosuggested that 11?-hsd promoter iscontrolled dominantly by the binding of Sox3 with a moderate influence of Wt1that regulates testicular development and recrudescence (12). Inspite of these, distinct information pertaining to factors or genes and theirinteractions during testicular growth and recrudescence are not clear. Thus,understanding the regulation of steroidogenic enzyme genes directly orindirectly involved in T and 11-KT production seems critical to address themolecular mechanisms of spermatogenesis with special reference to fishreproduction.