Insulin-like first discovered in media of seminiferous tubule

Insulin-like growth
factors (IGFs)

 

The IGFs, IGFs receptors, and IGF
binding protein (IGFBPs) play important roles in the regulation of growth and
development. IGFs are single-chain polypeptides and has two major forms, IGF-I and
IGF-II. Both has 62% sequence homology. The IGF-I receptor and the IGF-II
receptor are the two known receptors that binds IGF specifically. Also, IGFs
can bind insulin receptor low affinity. IGF-I and IGF/insulin receptors
contains a highly conserved tyrosine kinase catalytic domain in their
cytoplasmic domain whereas the IGF-II receptor may involve GTP-binding protein
activation. There are at least six IGF-binding proteins (IGFBPs) that modulate
the biological actions of the IGFs.( Cohick et al.,1993, Jones et al., 1995,
LeRoith et al., 1995)

 

IGF-I in testes was first discovered
in media of seminiferous tubule cultures from adult rats, sertoli cells cultured
from immature rats and in acid extracts of rat testes (Ritzen, 1983, D’Ercole et al.,1984, Handelsmanet al 1985). IGF-I is also
produced from cultured Leydig cells from immature rats (Cailleau et al., 1990).IGF immunoreactivity varies with age and
development, all cells in the growing testis show IGF-I-like immunoreactivity
only Up to 2 weeks after birth,thereafter, the frequency declines whereas a
rapid increase in the number of spermatogenic cells showing IGF-I-like
immunoreactivity is seen during puberty.In the human testis, IGF-I is localized
in Sertoli cells whereas less localised  in primary spermatocytes (Vannelli et al.,
1988 ).Proliferation of rat and pig prepubertal sertoli cells  are stimulated by IGF-I and IGF-II (Borlandet al.,1984). Under in
vitro conditions, IGF-I and IGF-II stimulate spermatogonial DNA synthesis,
have a maintaining effect on premeiotic DNA synthesis in the rat (Teerds et al., 1993).
IGF-I induces the differentiation of mouse type
A spermatogonia, stimulates testosterone
production(Tajima et al.,1995). When an intraperitoneal injection of recombinant
IGF-I was given to immature Snell dwarf mice for 7 days resulted in significant
increase of testicular LH receptors and an acute steroidogenic response to hCG
similar to that induced by recombinant hGH treatment was observed suggesting
that may be the effects of GH on the testis mediated by IGF-I (Chatelain et al., 1991). IGF-II gene
is essential only for normal placental and fetal growth.(456). The IGF-I null mutation has impact on the development and physiology
of the reproductive systems of both sexes (414).  Infertility of IGF-I null males is
predominantly  due to inadequate serum
testosterone levels which is not insufficient for perinatal androgenization.

 

 

Transforming growth
factor-b
(TGF?)

 

TGF? a polypeptide growth factor, has
two disulfide-linked monomers which are synthesized as large precursors. Out of three receptors, types I and II are involved in
signaling, whereas the type III receptor  regulates access of the TGF? to the signaling
receptors.  Expression of TGF? effects cell
proliferation, differentiation, and organization. Inhibitory or stimulatory
effect depends on the  on the conditions(467). In culture, both Sertoli cells and peritubular
cells isolated from 20-day-old rats was found to synthesize and secrete TGF? .(468). Sertoli cells produce TGF?1, peritubular cells
produce both TGF?1 and TGF?2.When the population of germinal cells at various
stages of development  were mixed ,it was
not found to contain the TGF?. TGF?3 mRNA has also been detected in murine
adult testis (469).Multiple forms of TGF? (TGF?1,
TGF?2, and TGF?3) was found in whole testis, isolated peritubular
cells and Sertoli cells from prepubertal, midpubertal, and late pubertal rat
testes . (470). TGF?1 has  role in reproductive function both during
embryogenesis and in the adult. TGF?1 modulate the  migration of PGC to the genital ridges in vivo (477). TGF?  Inhibits  Leydig cell steroidogenesis. It was found that
in immature pig and rat Leydig cells, TGF? is responsible for reduction in  the number of hCG receptors. It has a
biphasic effect on hCG-stimulated testosterone production i.e stimulatory at
low concentrations and inhibitory at high concentrations.480). Sertoli cells isolated from immature porcine
testes under the influence of TGF?1 stimulates lactate production and glucose
uptake (475).TGF?1 also  inhibits the TGF?-induced PMC DNA synthesis at
each stage of pubertal development (470).  Inspite the above data, preliminary results
obtained from the generation of mice homozygotes where TGF?1 allele was
disrupted showed no gross developmental abnormalities until about 20 days after
birth. Testicular abnormality in these knockout animals was mild
to moderate inflammation of the serosa of the testis (486,
487).

 

TGF?/EGF

EGF
and TGF? belong to the EGF family of growth factors. EGF is a 53-aa polypeptide
growth with approximately 40% sequence homology with TGF?. EGF and TGF?
prohormone have hydrophobic transmembrane domains near to the COOH termini. EGF
and TGF? bind to a single EGF receptor (EGFR). EGFR has intrinsic tyrosine kinase
activity (492, 493). Reports are available that TGF?
and EGF are produced within the testis. mRNA isolated from peritubular cells
and Sertoli cells isolated from 20- day-old rats species hybridizes with a
human TGF? cDNA probe during

Northern
blot analysis. Immunoblotting with a TGF? antiserum has confirmed the production
of TGF? by both Sertoli cells and peritubular cells (494).
Expression analysis in
whole testis and isolated cell types has confirmed that TGF? gene is
predominant during early testis development and decreased during puberty. (495). TGF? has been localized
in leydig cells. Specific binding sites are present in  clonal strain of Leydig tumor cells (500),  interstitial
cell preparations from adult rats (501), immature
porcine Leydig cells (502) and  in murine

Leydig
cells (503). Specific EGF binding was also reported
in isolated interstitial cell preparations from both intact and Leydig
cell-depleted adult rat testis. This suggested that EGF binds to Leydig cells
and to an interstitial cell fraction, may be 
the precursor of the mature Leydig cell (504).  In rats, the EGFR expression  was higher during  the early pubertal stages with  low expression level  in Sertoli cells, pachytene spermatocytes,
and round spermatids (495).Effect of  TGF? and EGF on Leydig cells, Sertoli
cells varies depending on time of exposure and animal species.EGF induces a significant
decline in the number of LH receptors with a corresponding reduced ability of
hCG to stimulate steroidogenesis was observed in the MA-10 tumor cell line (500,511). Due to EGF, enhancement in the gonadotropin
action on testosterone formation was seen in porcine immature Leydig cells .
This enhancement is caused by an increase in the availability of cholesterol
substrate and in the activity of 3b-HSD (504).

In
humans, steroidogenesis was stimulated by EGF in  isolated Leydig cells.450).
In culture of  immature rat Leydig cells  TGF? interacts with Luteinizing hormone to
promote DNA synthesis, stimulates peritubular cell
proliferation, migration, and colony formation(441) (494).
No effect was observed on Sertoli cell growth and transferrin production (495). Recent data have suggested TGF?  involvement 
in the control of net protease activity of the seminiferous

tubule.
When of porcine cultured Sertoli cells were treated with EGF increases lactate
production, glucose transport, and lactate dehydrogenase activity was reported.
(528). Inhibition of FSH stimulated germ cell
differentiation of adult mice testicular fragments by EGF may be by  blocking the proliferation of type A
spermatogonia (529). According to the reports by
Tsutsumi et al submandibular gland EGF may control testicular function
in an endocrine fashion. They reported that 
removal of the salivary glands of the mouse resulted  in a decrease of circulating EGF to an
undetectable level with a concomitant impairment of spermatogenesis.(530) When sexually
mature male mice was sialoadenectomiced 
resulted in decline in sperm count, motility, and fertility which was reversed
by EGF administration (532). Reports of  Tokida et al. (533) suggests that
sialoadenectomy does not decrease plasma EGF or
cause infertility in male mice whereas Russell et al.
(534) suggested that although
sialoadenectomy decreased certain functional
testicular parameters, the effects are negligible.

.

Platelet-derived growth
factor (PDGF)

Platelet-derived
growth factor (PDGF) was end result for the search for serum factors that
stimulate the proliferation of arterial smooth muscle cells (Ross et al., 1974). PDGF is involved in
cellular responses such as proliferation, survival, migration, the deposition
of extracellular matrix (ECM), tissue remodeling factors, embryonic testis
development to PDGFs are essential during mouse development. (Renée et al., 2003)

(Levéen et al., 1994; Soriano,

1994; Boström et al., 1996; Soriano, 1997; Fruttiger et al.,

1999; Karlsson et al., 1999; Gnessi et al., 2000;
Karlsson et al.,

2000

PDGF
regulates vascular, hematopoietic development, neural/oligodendrocyte
development, neural crest cell development, somitogenesis, skeletal patterning,
organogenesis, cytoskeletal rearrangements (Antoniades.,1991,
Claesson-Welsh L.,1994), stimulation of chemotaxis and growth in human and
experimental animals (Basciani
et al., 2010). The PDGF signalling pathway, in mouse and humans, has
four ligands, PDGF A-D and two receptors, PDGFR? and PDGFR?, in Drosophila
melanogaster has three PDGF/VEGF-like ligands and a single receptor and
teleosts   have three PDGF receptor (Cobreros et al.,2008, Williams et al.,2002,  Renée
et al., 2003). The
PDGF family and related receptors are conserved throughout the animal kingdom,
suggesting their crucial roles in developmental and physiological processes  (Basciani et al., 2010).  Pdgfra-null embryos have enlarged testis
cords. Wild-type embryonic testes treated with PDGFR inhibitors  shows  pattern similar to null embryos.  These results suggests that PDGF signaling is
important for the development of mouse testis (Uzumcu
et al., 2002; Brennan et al., 2003). During embryonic testis
development, Pdgfb ,Pdgfa and Pdgfra are expressed in scattered
cells, coelomic surface and the mesonephros/gonad boundary has expression
of  Pdgfc and Pdgfra((Brennan
et al., 2003)Evidences from cell culture based studies have indicated  that during later in embryonic testis
development   PDGF signaling may be
involved in the development or function of perimyoid cells (PMCs). During PMC
differentiation, Pdgfa expression is restricted to testis cords and all
interstitial cells expresses  Pdgfrs(Uzumcu et al., 2002;
Brennan et al., 2003).  PMCs
are interstitial cells which in tight association with  testis cords and  Sertoli cells, secrete
components of the basement membrane that surrounds testis cords (Hadley et al., 1985; Skinner et al., 1985). PDGFA
 is essential for adult Leydig cell
development, interstitial cell proliferation and completion of spermatogenesis (Gnessi et al., 2000).
In mouse, PDGF signaling is important for testis vasculature development  (Brennan et al., 2003). Coelomic vessel branching and
endothelial migration require PDGFRa signalling (Brennan et al., 2003).

 

Fibroblast
growth factor (FGF)

FGF
are members of polypeptide growth factors that comprise of at least seven members.
Members of FGFs family

are acidic FGF
(aFGF or FGF-1), basic FGF (bFGF or FGF-2), int-2 (FGF-3), kaposi sarcoma FGF
(K-FGF or FGF-4), FGF-5, FGF-6, and keratinocyte growth factor (KGF or FGF-7) (Burgess et a.,1989).

 FGFs are well-established for their role in
stimulation of cell division, tissue patterning organogenesis during
embryogenesis, neuronal growth and angiogenesis (544, 545).

 

 Receptors for FGFs are  single-chain transmembrane glycosylated proteins
having characteristics of the tyrosine

kinase
superfamily with two or three extracellular immunoglobulin-like domains. Four
distinct FGF receptor

genes
have been cloned.  (Partanen  et al., 1991, Werner et al., 1993,
Klagsburn et al., 1991). FGFs also binds  to cell surface proteoglycans but with lower
affinity(Schlessinger  et al., 1991). Results
from  bFGFs  localization in the 18-day-old rat fetus
have revealed that thy are completely absent in sex cords  but strongly present in basement membranes  (Gonzales  et al.,1991).
bFGF expression is prominent during early prepubertal testicular development
and

decreases
as sexual maturity is attained (Mullaney et al., 1992). Reports from cultured neonatal
rat testicular cells and cultured immature rat Leydig cells FGF inhibits
LH-stimulated testosterone production by reducing 17ahydroxylase

and
D5–3b-HSD activity (Fauser
et al., 1988, Muronoet al., 1990). But results from  cultured immature porcine Leydig cells  indicate enhancement in hCG-stimulated
testosterone formation (Sordoillet
et al, 1992) and aromatase activity (Raeside et al., 1988). bFGF stimulates
proliferation of  immature pig
Sertoli cells which is not observed in pubertal mice testes (de Lapeyriere., 1990).
When rat PMC cells treated with bFGF induce dose- dependent increase in
production of  plasminogen activator
inhibitor I Suggesting involvement of bFGF in the regulation of protease activity
of the seminiferous tubule (Le
Magueresse-Battistoni et al.,1996).FGF 9 coordinate with Wnt4 to
regulate mammalian sex determination. In XY sex reversal, due to loss of  FGF9 male gonad cannot maintain Sox 9
expression and downstream male pathways. Whereas partial testis development was
reported loss of Wnt4 in XX gonads (Yuna et al., 2006)