Effect of dietary zinc or pyridoxine deficiency upon estrogen directed gene expression in the rat uterus

In the present study the effect of diets restricted in either zinc or pyridoxine upon estrogen directed gene expression in the mature rat uterus was tested. Sexually mature female rats were maintained on zinc-adequate (40 mg/kg diet) ad libitum or restricted-fed, pyridoxine-deficient, or zinc-deficient ( < 1 mg/kg diet or 3mg/kg diet) ad libitum-fed diets for 35 days. All animals were bilaterally ovariectomized and used for experimentation at 14 days post ovariectomy. On day 35 each rat was injected intraperitoneally with estrogen. They were killed at different times post injection and thymidine kinase (TK, EC 2.7.1.21) or creatine kinase (CK, EC 2.7.3.2) activity was assayed in uteri cytosol fractions. In addition the steady state level of ckb mRNA in uteri cytosol fractions was measured following estrogen administration.

The weight gain of the rats fed the low zinc and low pyridoxine diets was significantly lower than those fed the zinc-adequate diet ad libitum. The consumption of the zinc-deficient diet resulted in a significant decrease in plasma zinc while a pyridoxine deficient diet produced a significant reduction in plasma pyridoxine. Vehicle-injected uterine TK activity was 2-3 pmoles of d-TMP/min/mg protein. The TK activity was significantly increased (p < 0.05) 42 h post estrogen injection on the zinc-adequate diet ad libitum and pair-wt fed rats. This activity was sustained at 48 h post injection prior to declining to control values within 60 h. The maximum (4-fold) increase occurred at 36 h post estrogen injection in pyridoxine-deficient rats which was sustained at 42 & 48 h. The increase in uterine TK activity was 3-fold at 42 hand 48 h post injection. However this increase was not significantly different from the peak value seen in zinc-adequate and pyridoxine-deficient diet fed rats. No measurable effect of estrogen on CK activity was observed on the zinc adequate or zinc-deficient diet fed rats using a coupled enzyme assay. However the time course of ckb mRNA induction on the zinc-adequate pair-wt fed rats following estrogen administration paralleled the time course of estrogen induced protein (IP) synthesis previously observed by Gorski et al. (1970). IP is now known to be the brain type isoenzyme of creatine kinase. An induction of ckb mRNA between 0-3 h post estrogen injection was not observed on the zinc-deficient diet fed rats. However in a subsequent experiment an induction of uterine ckb mRNA 2 h following estrogen administration was observed in zinc-deficient rats. The possible reasons for this discrepancy are discussed.

Zinc ions are known to be required to enable the estrogen receptor complex to bind to DNA and initiate transcription. It has been hypothesized that inadequate provision of dietary zinc may therefore reduce compliance to estrogen directed gene expression by limiting the efficiency of recruitment of zinc ions for stabilization of the zinc finger of the steroid receptor. The results of the present study failed to support this hypothesis at this moderate level of zinc depletion.