Phosphoprotein Phosphatase Activity Associated with Estrogen-Induced Protein in Rat Uterus

Abstract

Estrogen-induced protein was purified from rat uteri and assayed for several enzymatic activities involved in the metabolism and action of cyclic nucleo tides. No adenylate and guanylate cyclase (EC 4.6 One of the earliest effects of estrogen on rat uterus is the induction within 1 hr after hormone administration, of the synthesis of a specific protein (IP) (1). IP represents a very minor component of uterine soluble proteins that can only be demonstrated on the basis of increased incorporation of labeled amino acids in this protein after estrogen treatment. IP has been purified by ion exchange chromatography and preparative gel electrophoresis as a polypeptide of 45,000 molecular weight (2).The biological function of this induced protein (IP) is still unknown, although it has been implicated by some authors to play a decisive role in the subsequent increase in RNA and protein synthesis involved in the trophic effects of the hormone on its target tissues (3).Cyclic AMP and cyclic GMP control several processes related to cell growth in fibroblastic and lymphoid cell lines in titro (4-7). It is, therefore, interesting to examine whether the same mediators might be implicated in the growth-promoting action of hormones such as steroids on epithelial tissues. Despite earlier results (8), estrogens do not seem to affect cyclic AMP levels in the uterus (9, 10), but treatment with cyclic AMP induces a response that can mimic estrogen effects (11-13). Indeed, the endometrium contains a cyclic AMPdependent protein kinase (14). Although cyclic AMP does not influence the early synthesis of IP (1), cyclic nucleotides might play a role in subsequent effects of estrogens.We have investigated whether IP has enzymatic activity related to the regulation of uterine cell functions by the intracellular mediators, cyclic AMP and cyclic GMP. In this study, we report that IP exhibits a significant phosphoprotein phosphatase (EC 3.1.3.16; phosphoprotein phosphohydrolase) activity strongly inhibited by Zn++ and unaffected by either cyclic AMP or cyclic GMP. This phosphatase activity shows great substrate specificity, being much more efficient on phosphorylated protamine and histone F1 than on phosvitin. MATERIALS AND METHODSIsolation of Purified IP. IP was prepared following the described general procedure (2) with a slight modification. Immature female rats, 21-24 days old (Wistar), were injected intraperitoneally with 5 ug of 17T-estradiol; 1 hr later, the animals were killed and the excised uteri were incubated for 1 hr with [14C]leucine (100 ,Ci/ml). Uteri from control rats were incubated with' [3H]leucine (500 ,Ci/ml). At the termination of the incubation period, control and treated uteri were rinsed and combined with unincubated uteri from 40 to 50 mature virgin female rats. After homogenization in 1.5 mM EDTA (pH 7.6), the homogenates were centrifuged at 105,000 X g for 30 min, and the combined cytosol fractions were applied to a DEAE-cellulose column (1 X 20 cm). After the column was washed, fractionation was performed by...

Regulation of Macromolecular Synthesis by Oestrogen: A Developmental Approach

“…proposed that it is the 'key intermediary protein' in the stimulation of bulk RNA synthesis by oestradiol because of its rapid appearance hours before the rise in bulk RNA and protein synthesis. Vokaer et al (1974) reported that phosphoprotein phosphatase activity (EC 3.1.3.16) is associated with preparations of induced protein that have gone through several steps of purification. However, we have studied the distribution of induced protein and phosphoprotein phosphatase during fractionation by (NH&S04 precipitation and electrophoresis on cellulose acetate gels and found it possible to separate induced protein and phosphoprotein phosphatase activity (Kaye et al, 1975).…”

Separation of "estrogen-induced" protein from phosphoprotein phosphatase activity of immature rat uterus.

“…However, until a recent report by Vokaer et al (9), no experimental data on the possible function of IP were available. These authors reported that phosphoprotein phosphatase (phosphoprotein phosphohydrolase, EC 3.1.3.16) activity (PPPase) is associated with purified preparations of IP. The proposal of Liu and Greengard (10), that increases in PPPase activity may modulate the response of toad bladder to the steroid hormone aldosterone, lends plausibility to the idea that an increase in PPPase activity may also be involved in the mechanism of estrogen action.…”

“…The standard assay mixture was modified from that of Vokaer et al (9) by the addition of ethylenediaminetetraacetic acid (EDTA) and sodium barbitone. It contained, in a volume of 0.2 ml: 0.1 M Tris-HCl pH 7.2, 0.1 mg of disodium EDTA-2H20, 3.5 ,jg of protamine [32P]phosphate (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20),000 cpm), 20 mM sodium barbitone, together with an amount of PPPase adjusted so that less than 25% of the substrate was degraded.…”