Marsha Rosner, Ph.D.

Cancer is characterized by loss of normal cellular growth control.   Intracellular signal transduction pathways are critical to the proper interpretation and integration of growth regulatory stimuli, and intricate mechanisms have evolved for ensuring the fidelity of cell replication.  Small changes that alter the magnitude of these signals can significantly impact cellular outcomes.  Thus, elucidating the nature of these signaling pathways and how they are modulated is central to understanding cell cycle control and the maintenance of genomic integrity. The focus of our laboratory is to determine the critical mechanisms that regulate cell growth and differentiation in response to growth factor or oncogenic stimulation and identify key targets for therapeutic intervention.

One of the main players in the regulation of cell growth is the MAP kinase cascade, an evolutionarily conserved signaling pathway. The extracellular signal regulated kinases (ERKs) are a subfamily of MAP kinases that are activated via a cascade involving Ras, Raf kinase, and MEK/ERK kinase (MEK).  Activation of the MAP kinase pathway is tightly controlled, and Raf-1 activation is a key regulatory step in this process.  Multiple events are involved in Raf-1 activation including dephosphorylation of inhibitory sites and subsequent phosphorylation at activating sites such as serine 338 (S338) and tyrosine 341 (Y341).  Raf-1 is also regulated by a number of proteins that modulate its activity, leading to different physiological outcomes.  One of the regulators of Raf-1 signaling is Raf Kinase Inhibitory Protein (RKIP), also known as Phosphatidylethanolamine Binding Protein (PEBP).  RKIP is a ubiquitously expressed and highly conserved protein, and many of its homologs regulate growth and differentiation signaling pathways (reviewed in (Trakul and Rosner, 2005)).  In mammalian cells, RKIP inhibits Raf-1 signaling to MAP kinase, suppressing both Raf-1-induced transformation and AP-1-dependent transcription.  RKIP was also reported to inhibit TNF-a induced activation of IKKb in the NFkB cell survival pathway.  RKIP potentiates apoptosis induced by chemotherapeutic agents.  Finally, RKIP functions as a metastasis suppressor in a well-characterized model of human prostate cancer, and this phenotype correlates with inhibition of Raf-1.

Work from our laboratory showed that RKIP is phosphorylated on S153 by Protein Kinase C (PKC), causing dissociation of RKIP from Raf-1 (Corbit et al., 2003) and Raf-1 activation (Trakul et al., 2005).  Phosphorylated RKIP inhibits G protein coupled Receptor Kinase 2 enhancing G protein-coupled receptor signaling.  RKIP depletion also increases the amplitude and dose response of MAP kinase activation and DNA synthesis in EGF-treated cells (Trakul et al., 2005; Trakul and Rosner, 2005).  These studies indicate that RKIP acts as an endogenous modulator of the MAPK signaling cascade, limiting the response of the cell to growth factor stimuli.  Our recent results have shown that RKIP regulates mitotic progression in mammalian cells ((Eves et al., 2006)). These data indicate that RKIP regulates Aurora B kinase and the spindle checkpoint via the Raf/MEK/ERK cascade, and demonstrate that small changes in the MAP kinase pathway can profoundly impact the fidelity of the cell cycle.  Using approaches ranging from NMR to animal knockout models, future studies are focused on the molecular interactions that govern the inhibitory functions of RKIP, and the mechanism by which RKIP acts to suppress tumor progression and metastasis.

The epidermal growth factor receptor (EGFR), which is nearly universally expressed in squamous cell carcinomas of the head and neck (SCCHN), has been associated with cancer cell growth, survival, and metastasis. In nonmalignant tissues, the EGFR signaling pathway has been shown to require specific protein kinase C (PKC) isoforms for effective signaling. The PKC family of serine/threonine kinases consists of 10 members that are classified by their requirements for activation  – classical PKCs (a, b, and g) require both Ca2+ and diacylglycerol (DAG); novel PKCs (d, e, h, and q) are Ca2+- independent but still require DAG; and atypical PKCs, z and i/l, are Ca2+, DAG, and phorbol ester-independent.

Relatively little is known regarding PKC expression, function, and effects of inhibition in SCCHN. Previously we have shown, in a neuronal model, that PKCz is necessary for epidermal growth factor (EGF) induced MAP kinase activation while PKCd is required for basic fibroblast growth factor (bFGF) stimulation (Corbit et al., 2000).  However, the role of PKCz or other isoforms in SCCHN signaling had not been determined. Therefore, we undertook a recent study to characterize PKCz expression, activation and function in normal and malignant head and neck tissues (Cohen et al., 2006). We showed that PKCz is highly expressed in head and neck tumors, and inhibition of PKCz reduces MAP kinase activation in normal human adult epidermal keratinocytes (NHEK) and five of seven head and neck tumor cell lines. Furthermore, SCCHN cell proliferation and viability is reduced by inhibition of PKCz.  Finally, PKCz inhibition potentiates the action of other growth inhibitors in SCCHN.  The findings of this study thus implicate PKCz as a relevant target in SCCHN and suggest that PKCz inhibition is a viable therapeutic strategy.  In future studies, we are focusing on other PKC isoforms such as PKCalpha, and using bioinformatics analyses of cells and human tissue, animal models and molecular approaches to determine how PKCs promote cancer and to test PKC inhibitors as potential cancer treatments.