Estradiol Activates Methylating Enzyme(s) Involved in the Conversion of Phosphatidylethanolamine to Phosphatidylcholine in Rat Pituitary Membranes*

Abstract

17 beta-Estradiol (E2) affects the sensitivity of pituitary cells to several neurohormones as LHRH, TRH, or dopamine, presumably by modulating receptor coupling mechanisms. We attempted to pinpoint the membrane processes underlying this modulation and studied the effect of E2 on pituitary membrane phospholipid methylation. Anterior pituitary membranes prepared from ovariectomized (ovx) or ovx plus E2-treated rats were assayed for phospholipid methylation. Methylated phospholipids were separated by TLC. Incorporation of [3H]methyl groups into phospholipids increased with membrane concentration and incubation time with S-adenosyl-L-methyl [3H]methionine; it was not Mg2+ dependent and was inhibited in a dose-dependent manner by S-adenosyl-L-homocysteine, methyltransferase inhibitor. pH was found to be critical. Formation of phosphatidyl-monoethanolamine, phosphatidyl-dimethylethanolamine, and phosphatidylcholine was markedly stimulated by treatment with E2. The effect increased progressively when animals were killed 15 h to 5 days after E2 implantation. The response involved a shift in the maximum velocity (Vmax) although there was no change in the available substrate for the methylating enzyme. This change in Vmax probably reflects changes in the amount of the methylating enzyme itself. Administration of 17 alpha-estradiol, an inactive stereoisomer of E2 was ineffective, pointing to a stereospecific interaction. After differential centrifugation of pituitary membranes, the highest specific methyltransferase activity was found in light mitochondrial (L) and microsomal (P) fractions and the lowest in nuclei (N) and the heavy mitochondrial (M) fractions. After sucrose density gradient centrifugation, methylated phospholipids were preferentially recovered from fractions corresponding to the endoplasmic reticulum and/or secretory granules. E2 treatment for 5 days did not modify the subcellular distribution of methyltransferase activity but stimulated it in all fractions; in contrast, it did not modify the activity of the other enzymes measured as fraction markers. Under the same experimental conditions, phospholipid methylation in membranes prepared from cortex, and anterior and mediobasal hypothalamic structures was not affected by the steroid, with the exception of a slight increment of [3H]methyl incorporation into mediobasal hypothalamic membrane phospholipids after 5 days of E2 treatment. These results indicate that E2-induced changes in pituitary responsiveness might be concomitant with selective effects of the steroid on specific membrane enzymatic activities involved in coupling mechanisms.

Genetic variation of folate-mediated one-carbon transfer pathway predicts susceptibility to choline deficiency in humans

“…SAH is a potent inhibitor of phosphatidylethanolamine methyltransferase, which catalyzes the endogenous formation of choline moiety in liver (10). Phosphatidylethanolamine methyltrans- ferase activity is increased by estrogen (37), and this mechanism probably explains why we observed that premenopausal women were relatively resistant to developing signs of organ dysfunction when fed a low-choline diet, compared with men. It is in these premenopausal women that we observed the most significant effect of the MTHFD1 1958A SNP on susceptibility to developing signs of choline deficiency (Table 2).…”

Common genetic polymorphisms affect the human requirement for the nutrient choline

“…PEMT activity is responsible for endogenous biosynthesis of choline moiety (1), and this activity is increased by estrogen treatment (36). We suggest that the promoter region of this gene is likely to have an estrogen response element (ERE).…”

Gonadotropin Regulation, Oestrogens and the Immune System

“…They have been more exten sively studied at the pituitary level. Oestrogens appear to modulate GnRH receptor number [20,21] as well as the coupling efficiency of its receptor on gonadotrophs [22][23][24][25][26][27]. This results particularly from alterations of mem brane transduction processes.…”

“…For instance, oestradiol can influence voltage dependent Ca2+ channels of the L type [24], as well as G proteins and membrane enzymes involved in secretion-coupling mech anisms [22,23,26], Most of such steroid effects involve changes in protein kinase C (PKC) activity and in phosphorylation/dephosphorylation processes [25][26][27], PKC is not directly activated by oestradiol, but the steroid increases the amount of enzyme available, so that stimu lation of PKC following interaction of GnRH with its receptor results in a greater release of LH in cells treated with the steroid [26]. In addition, ovarian steroid induced methyltransfcrasc activity [22,23] represents another bio chemical process leading to increased sensitivity of go nadotrophs. Alterations of membrane phospholipid com position induce changes in the availability of phosphati dylcholine, a substrate of phospholipase A2.…”

“…Changes in membrane microviscosity, for in stance, alter activities of ionic channels, kinetic properties of membrane multienzyme systems, and the topology and activity of cytoskeletal proteins. Furthermore, since the highest specific activity of methyltransferase is recovered from membrane of endoplasmic reticulum and secretory granules [22], one might speculate that steroid-induced phospholipid methylation at these levels also alters mech anisms involved in the packaging, storage and release of gonadotropins. Fig.…”