RSK in tumorigenesis: connections to steroid signaling

Abstract

The Ser/Thr kinase family, RSK, has been implicated in numerous types of hormone-dependent and -independent cancers. However, there has been little consideration of RSKs as downstream mediators of steroid hormone non-genomic effects or of their ability to facilitate steroid receptor-mediated gene expression. Steroid hormone signaling can directly stimulate the MEK/ERK/RSK pathway to regulate cellular proliferation and survival in transformed cells. To date, multiple mechanisms of RSK and steroid hormone receptor-mediated proliferation/survival have been elucidated. For example, RSK enhances proliferation of breast and prostate cancer cells via its ability to control the levels of the estrogen receptor co-activator, cyclin D1. While in lung and other tumors RSK may control apoptosis via estrogen-mediated regulation of mitochondrial integrity. Thus the RSKs could be important anti-cancer therapeutic targets in many different transformed tissues. The recent discovery of RSK-specific inhibitors will advance our current understanding of RSK in transformation and drive these studies into animal and clinical models. In this review we explore the mechanisms associated with RSK in tumorigenesis and their relationship to steroid hormone signaling.

Figure 1. RSKs are downstream mediators of the ERK1/2 pathway that regulate proliferation in a variety of cancer cell lines and are overexpressed or hyperactivated in many human cancers

The RSKs can increase proliferation, inhibit apoptosis and promote the invasive phenotype by increasing migration. The participation of RSK in these three major pathways that promote tumorigenesis argues that this kinase family has potential as therapeutic targets.

Figure 2. RSK activation

The RSK isoforms are activated by the same general mechanism. That mechanism is summarized in this figure using amino acid numbering corresponding to human RSK2. Color coding identifies the kinase with its associated phosphorylation. A) Inactive ERK1/2 binds to the extreme C-terminus of inactive RSK. In some cell types inactive RSK is also phosphorylated at Tyr-707 and Y529 by FGFR3 or SRC. B) In response to mitogen, ERK1/2 phosphorylates RSK on Ser-369 and Thr-577, activating the CTKD. After phosphorylating its target sites, ERK1/2 disassociates. C) The active CTKD autophosphorylates Ser-386. D) PDK1 is recruited to phospho-Ser-369 and then phosphorylates Ser-227 in the NTKD, completing activation of the NTKD. E) Summary of RSK activation steps leading to NTKD-mediated phosphorylation of exogenous substrates.

Figure 3. RSK regulates proliferation and cyclin D1 levels in breast cancer cell lines

A) Cells were treated with vehicle (−) or 20 μM SL0101, and cell viability was measured after 48 hr of treatment. Values are % of the growth observed in vehicle-treated cells. Columns, mean (n=2, in triplicate); bars, SD. *, p=0.005, Student’s t test B) Lysates of the normal human cell lines, MCF-10A and RWPE1, and of the human cancer cell lines, MCF-7, LNCaP and PC-3 were prepared from cells grown in the appropriate media as recommended by ATCC. C) Cells were treated with vehicle (−) or 50 μM SL0101 for 4 hr before lysis. To permit detection of cyclin D1 the total protein loaded differed between cell lines. Equal loading of the lysate within a cell line is shown by the anti-Ran immunoblot.

Figure 4. RSK regulates proliferation and cyclin D1 levels in prostate cancer cell lines

A) RWPE1, LNCaP, and PC3 cells were treated as in Figure 3A. Columns, mean (n = 2 in quadruplicate); bars, SD. *, p = 0.005, Student’s t test. B) RWPE1, LNCaP, and PC3 cells were treated as in Figure 3C.

Figure 5. Major mechanisms proposed to regulate hormone-dependent transformation in some tumors

Estrogens stimulate ERα complex formation with cytoplasmic signaling proteins like MNAR, cas, and c-SRC. This complex activates the ERK1/2 signal transduction pathway, and phosphorylation of RSK. RSK1 signaling inhibits apoptosis via phosphorylation and inactivation of the pro-apoptotic protein Bad. RSK2 can translocate to the nucleus where it regulates nuclear targets that drive proliferation. In the nucleus RSK directly phosphorylates ERα stimulating its transcriptional activity.