Downstream Signaling Mechanism Underlying MAPK-Induced Generation of the ER-Negative Phenotype
GEORGETOWN UNIV WASHINGTON DC
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Estrogen receptor a ERalpha negative breast tumors often overexpress growth factor receptors, resulting in increased growth factor signaling and hyperactivation of MAPK ERK1 and ERK2. We have previously shown that ERalpha-positive MCF-7 cells engineered to stably overexpress various signaling molecules leading to MAPK hyperactivation lose expression of ERalpha without inducing its transcriptional activation. The downregulation of ERalpha in these cells is transcriptional and is a specific action of MAPK hyperactivation that is reversible by MAPK abrogation. Here, we show that downregulation of ERalpha is not mediated specifically by either ERK-1 or 2. TAM67, a construct preventing AP-1 transcriptional activity, was used to determine that AP-1 activity does not play a role in ER downregulation. AP-1 activity is upregulated in response to MAPK activation, and increased AP-1 activity has been observed in ERalpha negative and hormone independent breast cancers. However, these are the first data indicating mechanistically that despite data correlating increased AP-1 activity with hormone independenceERalpha-negativity, increased AP-1 activity is not responsible for ERalpha downregulation. Use of a dominant negative RSK1 construct indicates that RSK1 activity does not downregulate ERalpha. Transfection of ERK2A19-25, which is dominant negative for nuclear MAPK substrates while allowing activation of cytoplasmic substrates, revealed that a cytoplasmic substrate of MAPK is responsible for the generation of the ERalpha-negative phenotype in these cells. Collectively, these data reveal that the association between increased AP-1 activity and the ERalpha- negative phenotype is correlative, not causative, and that a cytoplasmic MAPK substrate other than RSK1 is responsible for ERalpha downregulation in our cell line models.
- Anatomy and Physiology
- Medicine and Medical Research