In a new approach to developing treatments for cancers of the breast and prostate, researchers are homing in on the activity of a protein that is vital to cell survival, proliferation, and enlargement. When activated, p90 ribosomal S6 kinase contributes to cancer progression, tumors associated with the genetic disease Carney complex, and even cardiac hypertrophy. Researchers have deciphered the specific mechanisms and regions of a regulatory protein to control the activity of this kinase.
In almost half of all prostate cancers, p90 ribosomal S6 kinase (RSK) levels are higher compared with normal prostate tissue, suggesting that increased RSK levels may participate in raising the levels of prostate-specific antigen (PSA) in prostate cancer. Further, by controlling the levels of cyclin D1, an estrogen receptor coactivator, RSK enhances proliferation of breast and prostate cancer cells. In other tumors, including lung, RSK may control programmed cell death via estrogen-mediated regulation of mitochondrial integrity.
Tarun B. Patel, PhD, and colleagues at Chicago's Loyola University Stritch School of Medicine have discovered the specific region of the regulatory protein that binds RSK.
RSK is a serine/threonine kinase activated downstream of both mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K). Four isoforms of RSK (RSK1 through RSK4) are members of a family with two kinase domains: an N-terminal kinase (NTK) domain and a C-terminal kinase (CTK) domain. RSK1 and RSK2 play key roles in regulating several biological functions, including inhibition of apoptosis, activation of the mTOR pathway, and cell proliferation.
Dr. Patel is chair of the molecular pharmacology and therapeutics department at the university. His coauthors are Deepti Chaturvedi, PhD, and Xianlong Gao, PhD.
Rethinking the PKA paradigm
Dr. Patel's group discovered that the NTK domain of RSK1 is necessary for interactions with PKARI alpha, the regulatory subunit of protein kinase A (PKA): The NTK domain of RSK1 competes with a catalytic subunit (PKAc) of PKA for binding to the pseudosubstrate region [amino acids 93-99(93 RRRRGAI 99)] of PKARI alpha (J Biol Chem 285:6970-6979, 2010).
Unlike PKAc, which requires amino acids Arg-95 and Arg-96 in the pseudosubstrate region of PKARI alpha for its interactions, RSK1/PKARI alpha association requires all four arginine residues (Arg-93 through Arg-96).
PKA is critical in maintaining normal body functions including heart rate, contraction of the heart, and hormone release. PKA also is involved in modulating tumor growth and progression. Because RSK and PKA compete for binding with the same regulatory protein, they end up modulating each other's activities.
"Our data suggest that the PKA heterotetramer can be more complex, as a dimer of PKARI alpha regulatory subunits can bind to a catalytic subunit of PKA as well as to a molecule of RSK1," Dr. Patel said. In addition, "since RSK1 competes with the binding of the PKA catalytic subunit to the PKARI-alpha subunits, RSK1 can activate PKA in a cyclic AMP-independent manner" turning the current concept of cAMP-dependent PKA activation on its head. "These are major deviations from the traditional paradigm of how PKA is regulated," he said.
Dr. Patel's group observed that disruption of the interaction of RSK1 with PKARI alpha can activate RSK1 and protect cells from apoptosis in the absence of any growth factor action. The next step will be to identify small-molecule inhibitors that can regulate RSK1 activity, he said.
These findings may have implications for epithelial cancers such as cancers of the breast, prostate, colon, lung, and ovary, in terms of drug development designed to inactivate RSK by not only inhibiting cell proliferation but also opposing molecular mechanisms that drive cancer cell metastasis, said Joyce Slingerland, MD, FRCP(C), director of the Braman Family Breast Cancer Institute at the University of Miami Sylvester Comprehensive Cancer Center.
Dr. Slingerland's team has been involved in research demonstrating that RSK1 may contribute to loss of actin stress fibers and increased cell motility after Ras/MAPK and PI3K activation in human cancers. In particular, her group found that oncogenic MAPK- and PI3K-mediated RSK1 activation may constitute one mechanism underlying the mislocalization of cytoplasmic p27, a key regulator of cell proliferation and differentiation. This can lead to rearrangement of the actomyosin cytoskeleton and increased cellular motility and metastatic potential in human cancers (Proc Natl Acad Sci USA 106:9268-9273, 2009).
The present studies are extremely noteworthy in that they describe a novel mechanism for the reciprocal modulation of two protein kinases, PKA and RSK1, that are central for normal growth control, said Dr. Bertics, who is the Kellett professor and vice chair of biomolecular chemistry at the University of Wisconsin School of Medicine in Madison and a member of the UW Carbone Cancer Center.
"The observation that RSK1 can interact with a regulatory subunit (PKARI alpha) of PKA, thereby allowing for cAMP-independent activation of PKA while attenuating RSK1 action, is a previously unrecognized connection between these two key kinases," he said.
Dr. Bertics explained that the observation provided "significant insight into potential mechanisms of abnormal cell survival and carcinogenesis in disorders such as Carney complex, wherein there is a mutation (heterozygous loss of function) in the PKARI alpha gene" (see Fact box).
According to Dr. Bertics, "the knowledge gained from the present studies opens up the possibility that novel therapeutic modalities targeting this newly identified PKA/PKARI alpha/RSK1 axis may hold great promise in the treatment of the growth abnormalities associated with diseases exhibiting dysfunctional PKARI alpha levels/function."