Somatotropin: Effects on Ovarian Function in Swine and Transgenic Mice

Identification of the Problem

	Somatotropin (ST) or growth hormone is a major regulator of animal 
growth and metabolism.  Advances in molecular biotechnology have made it 
feasible to produce recombinant ST in quantities needed for prolonged 
in vivo treatment, and to manipulate ST levels by gene transfer.  
Bovine ST (bST) is currently being used in the dairy industry to increase 
feed conversion to milk production.  Studies have found that porcine ST 
(pST) will increase economically important traits in swine such as feed 
efficiency and lean:fat ratios.  Pending FDA approval, pST will be used in 
commercial swine operations as a tool to increase the efficiency of swine 
production.  In addition to the growth and metabolic responses, ST appears 
to play an integral role in reproduction(1-4).  Our long term goals are to 
understand the actions of ST on reproductive functions in swine, and other 
domestic species.  In this proposal, our primary objective is to further 
elucidate the role of ST as a molecular modulator of sterol metabolism and 
ovulation propensity in the ovaries of swine, and a bST-transgenic mouse 
model.
	Pork-production is a $5 billion per year industry in the United 
States; 5.7 million hogs and pigs are produced in Illinois alone.  We have 
selected a parameter of reproductive function which has considerable 
economical importance to the swine industry, namely ovulation rate.  
Specifically, this research is designed to define cellular events that 
determine follicle recruitment for ovulation versus follicle atresia in 
laboratory and domestic species.  Follicular atresia is mediated by 
apoptosis or programmed cell death.  If apoptosis could be decreased, 
follicular survival would increase, thus increasing the propensity for 
ovulation.  Increased ovulation rate in swine (if coupled with increased 
rates of embryo survival) makes possible an increase in average litter size.
An average increase of one pig per litter nationwide would allow us to 
produce the currently produced 110 million market pigs with fewer sows 
than presently required for this level of production.  Therefore, the 
research in this study will improve efficiency of pork production, a pork 
producer priority. 

Justification

	Somatotropin, acting directly or via stimulation of insulin-like 
growth factor I (IGF-I) production, influences reproductive 
functions(1,5,9-12).  The mechanisms of ST (or IGF-I) action on 
reproductive organs are poorly understood.  Known or suspected involvement 
of both circulating and locally produced IGF-I, and the existence of 
multiple forms of IGF-I binding proteins, which are differentially 
regulated and can exert divergent effects on IGF-I action, suggest an 
enormous degree of complexity.
	Dr. Bartke's group has used ST-transgenic mice to study the 
effects of this hormone on reproduction(1).  ST-transgenic mice are 
exposed to very high amounts of circulating ST throughout their entire 
postnatal life yielding information on the potential consequences of 
maximal ST stimulation.  Maximal responses provide a good starting point 
for mechanistic studies.  The costs and logistics of producing comparable 
levels and duration of ST exposure in domestic animals are prohibitive.  
The consistency and the magnitude of this effect provide what we believe 
is an excellent model system for identifying the cellular and molecular 
mechanisms responsible for increased ovulation rate. 
	It is well documented that ST can stimulate follicular growth and 
concentration of IGF-I in the follicular fluid(2,3).  In isolated granulosa 
cells from pigs, ST was shown to stimulate IGF-I and steroid hormone 
production(2).   IGF-I has been found to enhance steroidogenic responses to 
follicle stimulating hormone (FSH)(2,6,7).   Dr. Winters has extensively 
studied the effects of these hormonal modulators on the expression of genes 
mportant in ovarian steroid production, primarily the cytochrome P450 
cholesterol side chain cleavage enzyme (P450scc), and low density 
lipoprotein  receptor (LDLR)(7).  P450scc is the rate-limiting 
steroidogenesis enzyme in the ovary, and LDLR is necessary for the uptake 
of cholesterol, the primary steroid hormone precursor in the ovary.   
	Treatment with ST increased ovulation in gilts(3) and infertile 
women(13), but not in heifers or ewes.  We have observed increased 
ovulation rate (estimated from the numbers of corpora lutea and 
implantation sites) in transgenic mice from several lines overexpressing 
bST or hST(4).  Increases in the numbers of follicles in ST-treated heifers 
and in the ovulation rate of ST-injected gilts(3) imply that ST can reduce 
follicular atresia.  We have already reported that the percentages of 
atretic pre-antral and antral follicles are significantly reduced in 
ST-transgenic mice (1,4).  There is evidence that ovarian cells are lost 
by apoptosis (programmed cell death) in several species, and that 
follicular atresia involves apoptosis of granulosa cells(8,23).  We are 
not aware of any studies of the effects of ST on apoptotic cell death in 
the ovary.  However, endogenous IGF-I was recently reported to act as a 
"survival factor" for rat ovarian follicles(8).  We predict that apoptosis 
will be inhibited by ST (directly or via IGF-I), thus increasing the 
propensity for ovulation.

References Cited

1.	Bartke, A., Cecim, M., Tang, K., Steger, R.W., Chandrashekar, V. 
	and Turyn, D.:  Neuroendocrine and reproductive consequences of 
	overexpression of growth hormone in transgenic mice.  
	Proc Soc Exp Biol Med, 206:345-359, 1994.

2.	Hsu, C.J. and Hammond, J.M.:  Concomitant effects of growth hormone 
	on secretion of insulin-like growth factor-I and progesterone by 
	cultured porcine granulosa cells.  
	Endocrinology, 121:1343-1348, 1987.

3.	Kirkwood, R.N., Thacker, P.A., Gooneratne, A.D., Guedo, B.L. 
	and Laarveld, B.:  The influence of exogenous growth hormone on 
	ovulation rates in gilts.  
	Canadian J. Anim. Sci., 68:1097-1103, 1988.

4.	Cecim, M., Kerr, J. and Bartke, A.: Effects of bovine growth 
	hormone (bGH) transgene expression or bGH treatment on reproductive 
	functions in female mice.  Bio. Reprod., 52:1144-1148, 1995.

5.	Hausler, C.L., Hodson Jr. H. H, Kuo, D.C., Kenney, T.J., Rauwolf, 
	V. A. and Strack L.E.:  Induced ovulation and 	conception in 
	lactating sows. J. Anim. Sci., 50:773-____, 1980.  

6.	Veldhuis, J.D. and Rogers, R J.:  Mechanisms subserving the 
	steroidogenic synergism between follicle-stimulating hormone and 
	insulin-like growth hormone factor I (somatomedin C).  
	J. Biol. Chem., 262:7658-7664, 1987.

7.	Winters, T.A. and Veldhuis, J.D.:  IGF-I and FSH amplify the 
	in situ expression of P450scc mRNA in single porcine granulosa 
	cells.  Biol. Reprod. 50(Suppl. 1):113, 1994.

8.	Chun, S-Y, Billig, H., Tilly, J.L. and Furtua, I.:  Gonadotropin 
	suppression of apoptosis in cultured preovulatory follicles: 
	mediatory role of endogenous insulin-like growth factor I.  
	Endocrinology, 135:1845-1853, 1994.

9.	Steger, R.W., Bartke, A., Parkening, T.A., Collins, T., Buonomo, 
	F., Tang, K., Wagner, T.E. and Yun, J.S.:  Effects of heterologous 
	growth hormones on hypothalamic and pituitary function in 
	transgenic mice.  Neuroendocrinology, 53:365-372, 1991.

10.	Mertani, H.C., Waters, M.J., Jambou, R., Gossard, F. and Morel, G.: 
	Growth hormone receptor binding protein in rat 	anterior pituitary. 
	Neuroendocrinology, 59:483-494, 1994.

11.	Bex, F., Bartke, A., Goldman, B.D. and Dalterio, S.:  Prolactin, 
	growth hormone, luteinizing hormone receptors, and seasonal changes 
	in testicular activity in the golden hamster.  
	Endocrinology, 103:2069-2080, 1978.

12.	Flint, D.J. and Gardner, M.:  Evidence that growth hormone 
	stimulates milk synthesis by direct action on the mammary gland 
	and that prolactin exerts effects on milk secretion by maintenance 
	of mammary deoxyribonucleic acid content and tight junction status. 
	Endocrinology, 135:1119-1124, 1994.

13.	Homburg, R., Wesr, C., Torresani, T. and Jacobs, H.S.:  Co-treatment
	with human growth hormone and gonadotropins for induction of 
	ovulation:  A controlled clinical trial.  
	Fertil Steril, 53:254-260, 1990.

14.	Palumbo, A. and Yeh, J.:  In situ localization of apoptosis in the 
	rat ovary during follicular atresia.  
	Biol. Reprod., 51:888-895, 1994.