The transport of amino acids across the plasma membrane is a process of fundamental physiological importance. If this process is modified by estrogens and if estrogen receptors play a role in this regulation, then the alteration of metabolic events will be of significant importance in cancer cells which have high estrogen receptor content because estrogens modify cellular physiology through transactivational effects of their cognate receptors. In the present study, we investigated the role of 17β-estradiol on the regulation of different amino acid transport systems, in particular, Systems A, ASC and y+ in estrogen receptor positive (MCF-7, T47D, H-301) and estrogen receptor negative (MCF-10, SKBR-3, MDA-MB-231) cell lines. The cells were treated with 17β-estradiol (10 nM), ICI 182780 (1 μM), or 17β-estradiol plus ICI 182780. We discovered that 17β-estradiol specifically stimulates System A activity by 2- to 4-fold in estrogen receptor positive cell lines with a maximal stimulation 48 h after estrogen-treatment while no stimulation was observed in estrogen receptor negative cell lines. Estrogen-dependent activation of System A was inhibited by co-treatment with the antiestrogen ICI 182780. Northern blot analysis showed that System A mRNA levels are also increased following estrogen treatment, and this induction of mRNA transcript levels by estrogen can be inhibited by co-treatment with antiestrogen ICI 182780. The increase in System A transport activity following estrogen treatment was abbrogated when estrogen receptor positive cells were stably transfected with human antisense ERα cDNA. Kinetic analysis demonstrated that estrogen-induced stimulation results in a doubling of Vmax with no changes in Km. However, 17β-estradiol did not stimulate the activation of transport systems responsible for the transport of arginine (y+) or threonine (ASC). In summary, we have provided evidence that estrogen receptors play a role in the activation of System A by estrogen. This adaptation may have important physiological and nutritional significance on estrogen dependent growth of breast tumors.

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