Role of Raf Kinase in Cancer: Therapeutic Potential of Targeting the Raf/MEK/ERK Signal Transduction Pathway

doi:10.1053/j.seminoncol.2006.04.002

Seminars in Oncology

Volume 33, Issue 4, August 2006, Pages 392-406

https://doi.org/10.1053/j.seminoncol.2006.04.002 Get rights and content

Improvements in our understanding of the molecular basis of cancer have led to the clinical development of protein kinase inhibitors, which target pivotal molecules involved in intracellular signaling pathways implicated in tumorigenesis and progression. These novel targeted agents have demonstrated activity against a wide range of solid tumors, are generally better tolerated than standard chemotherapeutics, and may revolutionize the management of advanced refractory cancer. The ubiquitous Raf serine/threonine kinases are pivotal molecules within the Raf/mitogen extracellular kinase (MEK)/extracellular signal-related kinase (ERK) signaling pathway, which regulates cellular proliferation and survival. Raf kinase isoforms (wild-type Raf-1 or the b-raf V600E oncogene) are overactivated in a variety of solid tumor types, including renal cell carcinoma (RCC), hepatocellular carcinoma (HCC), non-small cell lung cancer (NSCLC), melanoma, and papillary thyroid carcinoma. In this review, the role of Raf in normal cells and in cancer is discussed, and an overview is given of Raf inhibitors currently in development, focusing on sorafenib tosylate (BAY 43-9006 or sorafenib). Sorafenib is the first oral multi-kinase inhibitor to be developed that targets Raf kinases (Raf-1, wild-type B-Raf, and b-raf V600E), in addition to receptor tyrosine kinases associated with angiogenesis (vascular endothelial growth factor receptor [VEGFR]-2/-3, platelet-derived growth factor receptor [PDGFR]-β) or tumor progression (Flt-3, c-kit). Preclinical and clinical sorafenib data that led to its recent approval for the treatment of advanced RCC are summarized, along with current thinking on sorafenib’s mechanism of effect on the tumor and tumor vasculature in melanoma and RCC.

Section snippets

Raf Kinase Isoforms: Gatekeepers to the Raf/MEK/ERK Signaling Pathway

The ubiquitous Raf/MEK/ERK pathway is activated by the binding of growth factors, such as transforming growth factor-alpha (TGF-α), epidermal growth factor (EGF), VEGF, and PDGF-β to their cognate receptors (eg, EGFR, VEGFR-2/-3, and PDGFR-β).19, 20, 21, 22, 23 This results in autophosphorylation by RTKs and activation of a kinase cascade that relays growth factor signals from the cell surface to the nucleus.19, 20, 21, 22, 23 As pivotal members of the Raf/MEK/ERK signaling pathway, the Raf

Oncogenic raf-1 Mutations and Malignant Transformation In Vitro

Activated oncogenic forms of raf-1 (eg, v-raf), mutant Raf proteins, and Raf-1 fusion proteins all induce transformation of cell lines in vitro. Raf-1 has been shown to mediate transformation via the MEK/ERK mitogenic pathway, and also by a MEK/ERK-independent route.⁴⁴ Despite the ease with which oncogenic raf-1 mutants transform cells in vitro, tumor-specific oncogenic raf-1 mutations have been observed only rarely in human tumor-derived cell lines.45, 46 Genetic rearrangements at the Raf-1

Raf Isoforms and Tumor Progression

Dysregulated overexpression of Raf isoforms is also associated with tumor progression. Raf isoforms are involved as downstream effectors of angiogenesis, which is essential for the establishment of the necessary vasculature to facilitate solid tumor growth beyond 2 to 3 mm³. Furthermore, Raf overexpression has been associated with a more aggressive and invasive tumor phenotype and has, therefore, been implicated in promoting tumor metastasis.

Raf: Role in Radio- and Chemoresistance

An essential strategy for any novel targeted anticancer agent involves the abrogation of tumor cell resistance to treatment-induced growth inhibition and cell death. Such resistance has hindered the clinical effectiveness of standard radiotherapy and cytotoxic chemotherapies. However, resistance has also been observed with targeted agents like imatinib, which act by blocking a single target within the tumor (bcr-abl in chronic myelogenous leukemia, c-kit in gastrointestinal stromal tumors

Raf as a Target for Anticancer Agents

The concept of Raf-targeted disruption of cell signaling for therapeutic purposes was first advanced in 1989 with the demonstration of tumor growth inhibition in athymic mice.¹¹⁶ Several agents have been designed that target Raf and are in various stages of clinical development.

Targeting Melanoma With Sorafenib

The high incidence of activating B-Raf mutations in melanoma makes sorafenib an attractive agent for the treatment of this disease, especially because preclinical data suggest that B-Raf inhibition can affect melanoma growth and survival. However, in a small phase II trial of sorafenib monotherapy, in which the drug was given to patients with metastatic melanoma at a dose of 400 mg orally twice per day in 12-week cycles, there was only one PR and three cases of SD among 20 patients.¹³⁰ Although

Molecular Basis of RCC: Rationale for Targeting Raf With Sorafenib

Loss of function of the VHL tumor-suppressor gene, which resides on the short arm of chromosome 3, and consequent HIF-α upregulation have been observed in most cases of clear-cell RCC.¹³³ The normal function of the VHL gene product (pVHL) is to downregulate expression of the HIF-α transcription factors under normoxic conditions.133, 134, 135, 136, 137, 138 This downregulation is achieved by pVHL forming a multi-protein complex that ubiquitinates the α-subunit of HIF, thereby targeting it for

Summary

Isoforms of the Raf kinase function both as gatekeepers to the ubiquitous Raf/MEK/ERK signaling pathway, and as direct mediators of MEK/ERK-independent cellular proliferation and survival. Dysregulated signaling through Raf-1 (NSCLC, RCC, and HCC) or through oncogenic b-raf V600E (malignant cutaneous melanoma and PTC) has been implicated in the pathogenesis and malignant phenotype of several human solid tumors that are notoriously resistant to conventional chemotherapy and radiation. Therefore,

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The contribution of abnormal metabolic targets to hepatocellular carcinoma (HCC) progression and the associated regulatory mechanisms are attractive research areas. High-density lipoprotein binding protein (HDLBP) is an important transporter that protects cells from excessive cholesterol accumulation, but few studies have identified a role for HDLBP in HCC progression.
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An integrated analysis confirmed that HDLBP expression was significantly elevated in HCC compared with noncancerous liver tissues. The knockdown or overexpression of HDLBP substantially inhibited or enhanced, respectively, HCC proliferation and sorafenib resistance. Subsequently, a mass spectrometry screen identified RAF1 as a potential downstream target of HDLBP. Mechanistically, when RAF1 was stabilized by HDLBP, MEKK1 continuously induced RAF1^Ser259-dependent MAPK signaling. Meanwhile, HDLBP interacted with RAF1 by competing with the TRIM71 E3 ligase and inhibited RAF1 degradation through the ubiquitin–proteasome pathway.
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Role of Raf Kinase in Cancer: Therapeutic Potential of Targeting the Raf/MEK/ERK Signal Transduction Pathway

Section snippets

Raf Kinase Isoforms: Gatekeepers to the Raf/MEK/ERK Signaling Pathway

Oncogenic raf-1 Mutations and Malignant Transformation In Vitro

Raf Isoforms and Tumor Progression

Raf: Role in Radio- and Chemoresistance

Raf as a Target for Anticancer Agents

Targeting Melanoma With Sorafenib

Molecular Basis of RCC: Rationale for Targeting Raf With Sorafenib

Summary

Semin Oncol

J Urol

Cancer Lett

J Biol Chem

J Biol Chem

J Biol Chem

Mol Cell Neurosci

Cell Signal

Immunobiology

Mol Immunol

J Biol Chem

Trends Biochem Sci

Clin Colorectal Cancer

J Invest Dermatol

Biochem Pharmacol

Hepatol Res

Cell

Cancer Cell

Curr Opin Genet Dev

Semin Oncol

Lancet

Cancer statistics, 2005

CA Cancer J Clin

Prevention of lung cancer. The new millennium

Chest Surg Clin North Am

Survival of patients with metastatic breast cancerTwenty-year data from two SEER registries

BMC Cancer

Long-term follow-up of patients with metastatic renal cell carcinoma treated with intravenous recombinant interleukin-2 in Europe

Cancer J Sci Am

Five-year survival results of subcutaneous low-dose immunotherapy with interleukin-2 alone in metastatic renal cell cancer patients

Urol Int

Are increasing 5-year survival rates evidence of success against cancer?

JAMA

Issues and progress with protein kinase inhibitors for cancer treatment

Nat Rev Drug Discov

The role of Raf kinases in malignant transformation

Expert Rev Mol Med

Targeting the mammalian target of rapamycin (mTOR)A new approach to treating cancer

Br J Cancer

Imatinib mesylate—Gold standards and silver linings

Clin Exp Med

Meaningful relationshipsThe regulation of the Ras/Raf/MEK/ERK pathway by protein interactions

Biochem J

BAY 43-9006 exhibits broad spectrum oral anti-tumor activity and targets the Raf/MEK/ERK pathway and receptor tyrosine kinases involved in tumor progression and angiogenesis

Cancer Res

The role of Mcl-1 down-regulation in the pro-apoptotic activity of the multi-kinase inhibitor BAY 43-9006

Oncogene

BAY 43-9006 (Sorafenib) inhibits ectopic and orthotopic growth of a murine model of renal adenocarcinoma (Renca) predominantly through inhibition of tumor angiogenesis

Clin Cancer Res

The role of B-RAF in melanoma

Cancer Metastasis Rev

Raf-1 serine 338 phosphorylation plays a key role in adhesion-dependent activation of extracellular signal-regulated kinase by epidermal growth factor

Mol Cell Biol

Raf-1 regulates Rho signaling and cell migration

J Cell Biol

Raf and VEGFEmerging therapeutic targets in Kaposi’s sarcoma-associated herpesvirus infection and angiogenesis in hematopoietic and nonhematopoietic tumors

Leukemia

Analysis of the transcriptional program induced by Raf in epithelial cells

Genes Dev

Serine and tyrosine phosphorylations cooperate in Raf-1, but not B-Raf activation

EMBO J

Positive and negative regulation of Raf kinase activity and function by phosphorylation

EMBO J

Activation of Raf by ionizing radiation

Nature

Embryonic lethality and fetal liver apoptosis in mice lacking the c-raf-1 gene

EMBO J

MEK kinase activity is not necessary for Raf-1 function

EMBO J

BH3-only proteins—Evolutionarily conserved proapoptotic Bcl-2 family members essential for initiating programmed cell death