Most
breast cancer risk factors are associated with prolonged exposure of the mammary gland to high levels of
estrogens. The actions of
estrogens are predominantly mediated by two receptors,
ERalpha and
ERbeta, which act as
transcription factors binding with high affinity to
estrogen response elements in the promoter region of target genes. However, most target genes do not contain the consensus
estrogen response elements, but rather degenerated palindromic sequences showing one or more mutations and other ER-binding sites such as
AP-1 and SP-1. Using the differential display reverse transcription-polymerase chain reaction technique, our group identified several genes differentially expressed in normal tissue and in ER-positive and ER-negative primary
breast tumors. One of the genes shown to be down-regulated in
breast tumors compared to normal breast tissue was the PHLDA1 (
Pleckstrin homology-like domain, family A, member 1). In the present study, we investigated the potential of PHLDA1 to be regulated by
estrogen via ER in MCF-7
breast cancer cells. The promoter region of PHLDA1 shows an imperfect palindrome, an AP-1- and three SP-1-binding sites potentially regulated by
estrogens. We also assessed the effects of 17beta-estradiol on PHLDA1
mRNA expression in MCF-7
breast cancer cells. MCF-7 cells exposed to 10 nM 17beta-estradiol showed more than 2-fold increased expression of the PHLDA1 transcripts compared to control cells (P = 0.05). The anti-
estrogen ICI 182,780 (1 microM) inhibited PHLDA1
mRNA expression and completely abolished the effect of 10 nM 17beta-estradiol on PHLDA1 expression (P < 0.05), suggesting that PHLDA1 is regulated by
estrogen via ER.