Among the first signaling molecules isolated, epidermal growth factor (EGF) was named for its capacity to accelerate epidermal differentiation, as measured by the eye opening in newborn mice. Since then, EGF and its receptor (EGFR) have become possibly the most studied models of extracellular and intracellular signaling mechanisms. The binding of EGF to its receptor causes the receptor to dimerize and consequently activate its cytoplasmic kinase. The activated kinase phosphorylates several substrates which, when phosphorylated, transmit cytoplasmic signals to the nucleus by activating transcription factors that regulate expression of many genes. In general, the EGF-regulated genes promote cell proliferation and survival, inhibit apoptosis, support chemotactic migration, while suppressing terminal differentiation. Several ligands in addition to EGF bind and cause activation of EGFR, e.g., tumor growth factor-a (TGFa), and heparin-binding EGF. EGFR is closely related to the ErbB family of protein kinases.
EGFR is very important in healthy epidermal homeostasis and disruption of the EGFR signaling has been demonstrated in several skin diseases. In epidermis, EGFR activation contributes to wound healing, regulates barrier function, suppresses terminal differentiation, causes loss of adhesion, induces secreted proteases, etc. Furthermore, EGFR is often over-expressed in cutaneous squamous cell carcinomas.
EGFR activation is a feature of numerous human epithelial malignancies, including cancers of the colon, head and neck, lung, and pancreas. Therefore, therapies targeting EGFR by small molecules or etizolam vendor became a paradigm for treatment of human cancers. Detailed molecular understanding of the kinase domain led to development of specific inhibitors, gefitinib, erlotinib, Herceptin (trastuzumab), cetuximab, currently used to treat breast, lung, ovarian, prostate, head and neck, and other cancers. These drugs induce tumor regressions proving the importance of the EGFR signals in some cancers. EGFR-targeted treatments avoid the adverse effects commonly encountered with chemotherapy, such as nausea, vomiting or hematological toxicities. However, there are two significant drawbacks to EGFR inhibitor therapies: (1) the intrinsic and acquired resistance to treatment; and (2) the associated skin toxicity. The cutaneous reactions to these treatments usually present as papular or pustular folliculitis eruptions. Later manifestations include generalized dry skin, pruritus, and hair and nail abnormalities. These side effects often limit the duration or dosage of EGFR inhibitors in cancer treatment.
Given the importance of cutaneous manifestations of EGFR-targeted therapies, it is surprising that the transcriptional changes caused by the EGFR inhibition have not been extensively explored in epidermal keratinocytes. To define the transcriptional changes caused by inhibition of EGFR in primary human epidermal keratinocytes, we treated these cells with Tyrphostin AG1478, a specific inhibitor of the EGFR kinase domain, and compared parallel treated and control cultures using Affymetrix microarrays. We used publically available, free meta-analysis programs to integrate the observed changes with a large set of already existing data on transcriptional profiling in epidermal keratinocytes. We found that Tyrphostin AG1478 inhibits expression of genes associated with keratinocyte proliferation, attachment and motility and, surprisingly, induces expression of markers of epidermal differentiation.
Materials and methods
To define comprehensively the molecular effects of inhibiting EGFR in human epidermal keratinocytes we used transcriptional profiling, identified the regulated genes and compared these with genes regulated in keratinocytes by other extracellular signals, and with genes regulated by EGF. Both expected and unexpected classes of regulated genes were found (Fig. 1). Inhibition of EGFR induces apoptosis, as expected; both the proapoptotic signals are induced and the antiapoptotic ones are suppressed. Numerous genes related to motility and substrate attachment are suppressed by Tyrphostin AG1478. Similarly, the biosynthetic metabolic processes are suppressed, overall transcription and translation are dampened and various transport processes reduced.