CR, pregnancy and insulin-like growth factor-I

[Guest guest]

Hi All,

It seems that late pregnancy in guinea pigs is a good model for that in humans,

and

insulin-like growth factor-I is affected similarly in the two species. CR seems

to

involve low levels of insulin-like growth factor-I. How does CR affect the

level of

insulin-like growth factor-I in the pregnant female? The below seems to address

this issue and may suggest that the combination of pregnancy and CR may affect

insulin-like growth factor-I levels differentially from pregnancy alone.

See the pdf-available below.

Grant PA, Kind KL, CT, Sohlstrom A, Owens PC, Owens JA.

Late pregnancy increases hepatic expression of insulin-like growth factor-I in

well

nourished guinea pigs.

Growth Horm IGF Res. 2005 Apr;15(2):165-71.

PMID: 15809021

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve & db=pubmed & dopt=Abstra\

ct & list_uids=15809021 & query_hl=9

1. Introduction

The concentration of IGF-I in maternal blood during human pregnancy increases

progressively with advancing gestation [1], [2], [3] and [4] and correlates with

increasing body weight and size of the fetus or newborn [4], [5] and [6].

Maternal

treatment with IGF-I abolishes the typical negative correlation seen between

fetal

weight and litter size in mice [7] and increases placental and fetal weight in

the

guinea pig [8]. Therefore, it has been proposed that endocrine IGF-I in the

pregnant

mother may influence fetal growth by acting either directly on the placenta or

on

maternal tissues, to regulate the distribution of substrates between maintenance

and

adaptation of the mother and the growth and maintenance of the placenta and

fetus

[8] and [9].

The effect of pregnancy on the concentration of endogenous IGF-I in the

circulation

varies in non-human species with nutrition [9] and [10]. In polytocous species,

such

as the rat and rabbit, circulating levels and/or tissue production of IGF-I in

the

mother decrease in late gestation, which may reflect the need to meet a high

conceptus demand for substrates by reducing competition by the mother. In guinea

pigs, however the concentrations of IGF-I in blood plasma increase throughout

gestation [11], as occurs in women, suggesting a role in adaptation to

pregnancy.

Only under circumstances such as maternal undernutrition is this increase

abolished

in the guinea pig [11], consistent with a need to reduce substrate utilization

when

their availability is scarce. Despite the apparent maternal importance of

appropriate changes in maternal IGF-I abundance in late gestation, the major

tissue

source of endocrine IGF-I and hence the target of various regulatory influences

has

not been established in species such as the human or guinea pig.

In the non-pregnant guinea pig and in mid-pregnancy, both liver and adipose

tissue

have been identified as potential sources of endocrine IGF-I. The aim of this

study

was to characterize hepatic IGF-I gene expression in late pregnancy, the impact

of

undernutrition on this and on its relationship to plasma IGF-I, in the guinea

pig.

It was hypothesized that there would be increased gene expression of IGF-I in

liver

during late gestation, that this would be reduced by maternal undernutrition and

that hepatic IGF-I gene expression would correlate with maternal plasma IGF-I

overall.

.... Nulliparous female guinea pigs (IMVS coloured strain) aged 3–4 months and

weighing 472±56 g (n = 31) were from the Gilles Plains Resource Centre, South

Australia. Guinea pigs were provided a guinea pig/rabbit ration supplemented

with

vitamin E (165 mg/kg). Water containing vitamin C (400 mg/l) was freely

available.

.... Guinea pigs were maintained on one of two different feed regimens for a

total of

88 days. One group was fed ad libitum (n = 13) and the other was provided 30%

less

than ad libitum per unit body weight (n = 18) for 28 days. Eight animals from

each

nutrition group were then mated and became pregnant. One group of pregnant

animals

(n = 8) was continued on ad libitum feeding until day 60 of pregnancy (term is

69

days). The other group of pregnant animals (n = 8) continued feeding at 30% less

than ad libitum for the first 34 days of pregnancy, then at 10% less than ad

libitum

for the next 26 days, to avoid weight loss. Non-pregnant guinea pigs were

similarly

fed (ad libitum throughout, n = 5; feed restricted, n = 10) for 88 days.

.... 3. Results

Pregnancy increased body weight in ad libitum fed and feed restricted guinea

pigs

(p<0.001) (Table 1). Liver weight in absolute (p<0.001) and relative terms was

lower

(p<0.01) in feed restricted pregnant and non-pregnant guinea pigs (Table 1).

Pregnancy decreased liver weight in relative terms (p<0.05).

Table 1. Effect of pregnancy and nutrition on maternal hepatic IGF-I gene

expression

--------------------------------------------

--------Non-pregnant---Pregnant---ANOVA

----Ad libitum Restricted---Ad libitum Restricted---Feed Pregnancy

--------------------------------------------

Body weight (g) 745±28a,c 506±20a,d 957.4±22b,c 656.8±22b,d p<0.001 p<0.001

Liver (g) 35.1±1.4e 21.7±0.9e 31.2±1.1f 22.4±1.1f p<0.001 ns

(% of Body wt) 4.7±0.1g,i 4.3±0.1g,j 4.4±0.1h,i 4.1±0.1h,j p<0.01 p<0.05

Total IGF-ImRNA/â-actin (based on liver wt) 102.0±29.0k,m 35.4±7.1k,n 507±105l,m

69.7±7.3l,n p<0.001 p<0.001

(Based on liver wt as % body wt) 0.14±0.03o,r 0.07± 0.01o,s 0.53±0.1q,r

0.1±0.01q,s

p<0.001 p<0.005

--------------------------------------------

Expressed as mean±SE. Body weight includes weight of the uterus, fetal and

placental

tissues. Effect of treatment was assessed by two-way ANOVA with pregnancy status

and

nutritional regimen as between factors. All pairwise multiple comparison

procedures

were performed by the Student–Newman–Keuls method. Similar superscript letters

denote significant differences.

Fetal and placental weights are reported for those guinea pigs in which the

relative

abundance of IGF-I mRNA in maternal liver was measured. Feed restriction halved

the

total fetal weight (ad libitum fed 198.3±12.5 g, feed restricted 87.8±12.3 g,

p<0.001), while litter size was unaffected (ad libitum fed 2.9±0.3 g, feed

restricted 2.1±0.3 g). Feed restriction reduced mean fetal weight by 40% (ad

libitum

69.70±1.9 g, feed restricted 41.7±3.4 g, p<0.001), mean placental weight by 26%

(ad

libitum 4.5±0.18 g, feed restricted 3.3±0.2 g, p<0.01), and mean fetal to

placental

weight ratio by 17% (ad libitum 15.5±0.5, feed restricted 12.8±0.6, p<0.005).

Feed restriction did not alter hepatic expression of IGF-I mRNA in non-pregnant

animals (Fig. 1(a)). Pregnancy increased the relative abundance of IGF-I mRNA in

livers of ad libitum fed guinea pigs (p<0.001) (Fig. 1(a)), but did not in those

of

feed restricted guinea pigs. The relative abundance of IGF-I mRNA in livers from

pregnant ad libitum fed guinea pigs was four times that observed in any other

treatment (p<0.001). Pregnancy increased the total abundance of liver IGF-I mRNA

(relative abundance multiplied by liver weight or fractional liver weight)

(p<0.001)

(Table 1), while feed restriction decreased these (p<0.001) in pregnant or

non-pregnant guinea pigs (Table 1).

Feed restriction did not alter plasma IGF-I concentration in non-pregnant female

guinea pigs (Fig. 1(). Pregnancy increased (p<0.001) the concentrations of

IGF-I

protein in plasma in ad libitum fed, but not in feed restricted, guinea pigs

(Fig.

1(). Plasma IGF-I concentration in pregnant ad libitum fed animals was more

than

2.5 times that observed in any other treatment (p<0.001).

The was no significant relationship between plasma IGF-I and the relative

abundance

of IGF-I mRNA in liver in all restricted pregnant and nonpregnant guinea pigs

combined. However the concentration of IGF-I protein in blood was correlated

positively with IGF-I mRNA in liver in all ad libitum fed pregnant and

nonpregnant

guinea pigs (r2 = 0.74, p = 0.001) and in all animals combined (Fig. 2).

Maternal

plasma IGF-I concentration was positively correlated with the number of fetuses

per

dam (r2 = 0.4, p<0.02), total fetal weight (r2 = 0.72, p<0.001), total placental

weight (r2 = 0.63, p<0.001), average fetal weight (r2 = 0.59, p<0.001) and

fetal to

placental weight ratio (r2 = 0.48, p<0.005).

4. Discussion

This study is the first to demonstrate that hepatic expression of IGF-I is

increased

in late pregnancy in the guinea pig. Hepatic IGF-I mRNA as measured by

quantitative

RT-PCR ELISA late in pregnancy was fourfold higher than in the non-pregnant

state,

in the ad libitum fed guinea pig. However, IGF-I expression in liver of pregnant

guinea pigs analyzed by solution hybridization and an RNase protection assay, a

method which differs from that used in the current study, also appears to be

about

twice as high as in mid-gestation, when referenced to that in non-pregnant ad

libitum fed guinea pigs studied concomitantly [14]. This contrasts with other

species, such as the rat [10], [15], [16], [17] and [18] and cow [19], where

hepatic

abundance of IGF-I mRNA increases in early gestation, then decreases in late

gestation and is no longer elevated compared to the non-pregnant state. These

findings are consistent with liver as a potential source of increased

circulating

IGF-I throughout pregnancy in the guinea pig. There may be additional sources

however, as adipose tissue expresses comparable levels of IGF-I to liver in the

non-pregnant guinea pig and in mid-gestation [14].

The effects of pregnancy and reducing feed intake on the concentration of IGF-I

protein in blood plasma in the guinea pig in late pregnancy are qualitatively

similar to their effects on hepatic expression of IGF-I mRNA. The numerical

value of

the square of the correlation coefficient (r2 = 0.68) indicates that 68% of the

overall variation in IGF-I protein concentration in plasma can be attributed to

variation in the relative abundance of IGF-I mRNA in liver. While the relative

abundance of IGF-I mRNA in liver and the plasma concentration of IGF-I protein

were

strongly related, this relationship was best described as second order.

Consequently, at levels of hepatic IGF-I gene expression above a ratio of about

4

(Fig. 2), and in well fed pregnant guinea pigs in particular, further increases

in

IGF-I expression are no longer accompanied by increases in plasma IGF-I.

Furthermore, comparison of circulating IGF-I in mid-pregnancy in the guinea pig

[14], suggests that plasma IGF-I concentrations in mid (612±173 ng/ml) and late

gestation (730±35 ng/ml) are similar. This suggests that either, increased

turnover

and net breakdown of mRNA or increased IGF-I protein breakdown and clearance

from

plasma occurs, once very high levels are reached, in the pregnant guinea pig in

late

gestation. These could represent mechanisms to limit maternal tissue competition

with the conceptus for substrates, while allowing sufficient increase in IGF-I

anabolic action compared to the non-pregnant state to promote substrate transfer

to

the placenta and fetus. In humans and rats, pregnancy induces or increases

activity

of the IGFBP-3 protease, which reduces the stability of the IGFBP-3 ALS and IGF

complex [20]. Although this does not prevent increased circulating IGF-I levels

in

late pregnancy in women it may increase turnover and limit increases beyond the

levels seen [21]. While plasma IGF-I changes in pregnancy in human and guinea

pigs,

a pregnancy-induced protease for IGFBP-3 has yet to be identified in the guinea

pig

[11], although other mechanisms to increase protein turnover in the circulation

cannot be excluded.

It is also possible that other tissue sources of IGF-I have down-regulated their

production in late pregnancy in the guinea pig. This could be to allow factors

regulating hepatic IGF-I production, to be the predominant influence on

circulating

levels in the mother or alternatively, to limit local actions of IGF-I in such

non-hepatic tissues. Adipose tissue expresses IGF-I at comparable levels to the

liver in the non-pregnant guinea pigs and this is increased together with that

in

liver in mid-pregnancy in the guinea pig [14]. It remains to be determined if

pregnancy elevates adipose tissue IGF-I gene expression into late pregnancy or

declines in the guinea pig. The latter might occur, as IGF-I does have potent

insulin-like metabolic effects on adipocytes and promotes their formation and

proliferation [22], which may therefore need to be limited to spare nutrients

for

the conceptus.

The mechanism by which hepatic expression of IGF-I is elevated in the guinea pig

during late pregnancy compared to the non-pregnant state is unknown. The major

endocrine stimulator of hepatic expression of IGF-I in non-pregnant mammals is

GH

[23], [24] and [25], and the pituitary gland of the non-pregnant guinea pig

produces

GH [26] in a pulsatile manner like other mammals [27]. However, removal of the

pituitary gland does not alter growth nor does treatment of intact or

hypophysectomized guinea pigs with heterologous (bovine pituitary) GH [26]. The

effect of administration of homologous (i.e., guinea pig) GH on growth of guinea

pigs has not been reported however and this remains a candidate factor. Recent

studies strongly implicate placental GH as the major stimulus to increase

endocrine

IGF-I late in human pregnancy [28]. Although the guinea pig liver expresses GH

receptors [29] that bind (ovine) GH with high affinity [30], whether the guinea

pig

placenta also expresses a similar GH-related protein is not known.

Feed restriction abolished the pregnancy-induced increase in hepatic IGF-I mRNA

expression in the guinea pig in late gestation in the current study as in other

species [31]. This may be a direct consequence of reduced maternal nutrient and

insulin levels or result from reduced hepatic GH receptor expression [32] and

[33]

and a GH resistant state [23] and [34]. Consistent with the latter possibility,

undernutrition increases the concentration of growth hormone in blood in the

guinea

pig [35] as in other species [31].

In summary, this is the first demonstration of increased hepatic expression of

IGF-I

mRNA in late pregnancy in any species. Hepatic expression of IGF-I mRNA in late

pregnancy, appears to be double that seen earlier at mid-pregnancy [14] in the

guinea pig. The close relationship between the relative abundance of hepatic

IGF-I

mRNA and circulating levels of IGF-I further suggests the liver is a major

source,

although not necessarily the only source, of endocrine IGF-I in the guinea pig

late

in pregnancy. Whether other tissues, particularly adipose tissue, also

contribute to

plasma IGF-I, as suggested in mid-pregnancy, remains to be determined. The

similarity observed between humans and guinea pigs in their plasma IGF-I

profiles

throughout pregnancy suggests that maternal liver could be a potential source of

endocrine IGF-I in pregnant women. As other evidence suggests elevation of

plasma

IGF-I throughout pregnancy in humans and guinea pigs is a major anabolic factor

for

fetal and placental growth, factors influencing hepatic IGF-I production in the

mother may be important determinants of fetal outcome and require further

elucidation.

Al Pater, PhD; email: old542000@...

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