RSK2 Maintains Adult Estrogen Homeostasis by Inhibiting ERK1/2-Mediated Degradation of Estrogen Receptor Alpha

Abstract

SUMMARY In response to estrogens, estrogen receptor alpha (ERα), a critical regulator of homeostasis, is degraded through the 26S proteasome. However, despite the continued presence of estrogen before menopause, ERα protein levels are maintained. We discovered that ERK1/2-RSK2 activity oscillates during the estrous cycle. In response to high estrogen levels, ERK1/2 is activated and phosphorylates ERα to drive ERα degradation and estrogen-responsive gene expression. Reduction of estrogen levels results in ERK1/2 deactivation. RSK2 maintains redox homeostasis, which prevents sustained ERK1/2 activation. In juveniles, ERK1/2-RSK2 activity is not required. Mammary gland regeneration demonstrates that ERK1/2-RSK2 regulation of ERα is intrinsic to the epithelium. Reduced RSK2 and enrichment in an estrogen-regulated gene signature occur in individuals taking oral contraceptives. RSK2 loss enhances DNA damage, which may account for the elevated breast cancer risk with the use of exogenous estrogens. These findings implicate RSK2 as a critical component for the preservation of estrogen homeostasis.

Phosphorylation site S122 in estrogen receptor α has a tissue‐dependent role in female mice

“…Thus, it is possible that a normal ERα turnover is essential for the physiological regulation of fat mass, while this might be compensated for in other estrogen-responsive tissues. 39 We have evaluated the consequences of the S122A amino acid substitution, rendering the ERα incapable to be phosphorylated at this site, in various tissues in vivo. The sequence surrounding site S122 in the murine ERα is highly homologous to site S118 in the human ERα and lack of phosphorylation in S118A mutated receptors have been verified in humans.…”

“…31,32 Phosphorylation has also been shown to modify the function of the receptor by affecting its hormone sensitivity, nuclear localization, DNA binding, and protein stability. 31,[33][34][35][36][37][38][39] There are several phosphorylation sites in ERα, and the most well-characterized is S118 (S122 in mouse). 38 This site is found in the activation function 1 (AF-1), the N-terminal transactivation domain of ERα.…”

“…Phosphorylation of ERα has been shown to modulate the interaction of the receptor with other proteins, including coregulatory factors and the transcriptional machinery, thereby affecting the ability of both liganded and unliganded ERα to modulate gene transcription 31,32 . Phosphorylation has also been shown to modify the function of the receptor by affecting its hormone sensitivity, nuclear localization, DNA binding, and protein stability 31,33‐39 . There are several phosphorylation sites in ERα, and the most well‐characterized is S118 (S122 in mouse) 38 .…”

FACS protocol for direct comparison of cell populations isolated from mice

“…We include protocols necessary to batch stage animals within the cycle to proceed directly to FACS, which provides optimal RNA yields for RNA-seq. For complete details on the use and execution of this protocol, please refer to Ludwik et al. (2020) .…”

RSK2 and ERα comrades-in-arms in homeostasis and transformation

“…Therefore, to identify possible novel targets for therapeutic intervention in ER+ breast cancer we investigated and described a mechanism that limits estrogen responsiveness in the adult mammary gland . 1 …”