Ligand-induced, p38-dependent Apoptosis in Cells Expressing High Levels of Epidermal Growth Factor Receptor and ErbB-2

Abstract

Increased expression of the epidermal growth factor (EGF) receptor (EGFR) and ErbB-2 is implicated into the development and progression of breast cancer. Constant ligand-induced activation of EGFR and ErbB-2 receptortyrosine kinases is thought to be involved in the transformation of fibroblasts and mammary epithelial cells. Data herein show that ligand stimulation of cells that express both the EGFR and the ErbB-2 may result either in cell proliferation or apoptosis depending on the expression levels of EGFR and ErbB-2. Mammary tumor cells that express low levels of both receptors or high levels of ErbB-2 and low levels of EGFR survive and proliferate in the presence of EGF. In contrast, fibroblastic cells or mammary tumor cells, which co-express high levels of EGFR and ErbB-2 invariably undergo apoptosis in response to EGF. In these cells persistent activation of p38 MAPK is an essential element of the apoptotic mechanism. Also, the data implicate a p38-dependent change in mitochondrial membrane permeability as a downstream effector of apoptosis. Ligand-dependent apoptosis in cells co-expressing high levels of EGFR and ErbB-2 could be a natural mechanism that protects tissues from unrestricted proliferation in response to the sustained activation of receptor-tyrosine kinases. EGFR1 /ErbB-1 and ErbB-2 are type I receptor-tyrosine kinases characterized by the presence of an extracellular ligand binding domain, a single transmembrane domain, and a cytoplasmic region that includes a tyrosine kinase domain. The extracellular domain of the EGFR is recognized by seven distinct ligands that are related in primary sequence, which results in tyrosine kinase activation (1). In response to ligand stimulation, the EGFR forms homo-or heterodimers as part of the activation and signaling mechanism (2, 3). ErbB-2 has no known direct ligand but can be activated through heterodimerization with EGFR or other members of the ErbB family. Studies of the interactions between ErbB receptors have shown that ErbB-2 is the preferred partner for heterodimerization and thereby potentiates transformation and pro-survival signaling pathways (2-6).Ligand binding induces the rapid internalization of the EGFR and its subsequent lysosomal degradation (7). In contrast, ErbB-2 transactivation by the activated EGFR does not stimulate rapid internalization of ErbB-2 (8 -11). Removal of transactivated ErbB-2 from the cell surface proceeds slowly through a combination of intracellular degradation mechanisms, including lysosomes, the proteasome, and other intracellular proteases (10, 12-13). Heterodimerization of ErbB-2 and the EGFR actually impedes the rapid ligand-dependent internalization of the EGFR and, thereby, results in increased activation of signaling pathways (4, 9, 14).Increased EGFR or ErbB-2 expression or structural alterations in either receptor are frequent in human malignancies (2, 15-17). Overexpression of the EGFR is observed in a significant proportion of brain tumors (18 -20). Also, glioblastomas and other tumors often express a del...

The Localization of Protein Kinase Cδ in Different Subcellular Sites Affects Its Proapoptotic and Antiapoptotic Functions and the Activation of Distinct Downstream Signaling Pathways

“…However, the role of the subcellular localization of PKCy in this effect was not reported. We found that the PKCy-Cyto induced the phosphorylation of MKK3/MKK6, which lies upstream of p38 (48). Although PKCy-Cyto activated the p38 pathway, it played only a partial role in the apoptosis induced by the PKCy-Cyto because the p38 inhibitor SB203580 and silencing of p38 only moderately inhibited cell apoptosis, suggesting that additional signaling pathways mediate the apoptotic effect of the PKCyCyto.…”

The p38 MAPK stress pathway as a tumor suppressor or more?

“…For example, activation of p38 and/or JNK stress MAPK pathways has been shown to be required for a cell-death response after treatment with cancer therapeutic agents such as adriamycin (98), etoposide (99), cisplatin (100), UV radiation (7,101), gamma-irradiation (102) and even ligand epidermal growth factor (EGF) (103) in various systems. p38 activation, however, was also found to be anti-apoptotic in melanoma (104) and glioma cells (105) in response to UV radiation.…”

Endocytic Trafficking of the Epidermal Growth Factor Receptor in Transformed Cells

“…It is worth noting that EGF-mediated apoptosis is not is something that is commonly associated with EGFR signaling in the context of developmental biology, maintenance of healthy tissue, or cancer biology. However, among cell lines that overexpress the EGFR, such as MDA-MB-468 cells and A431 cells, EGF-mediated apoptosis has been well described (Armstrong et al, 1994;Gill and Lazar, 1981;Kottke et al, 1999;Tikhomirov and Carpenter, 2004). Further, indirect evidence for the role of apoptosis in cancer biology has been suggested by Tikhomirov and Carpenter.…”

“…They note that in the literature, more moderate levels of ErbB receptor tyrosine kinases are associated with more invasive carcinomas. They suggest that the induction of apoptosis in cell lines with higher levels of ErbB receptor expression generates a cancer cell whose growth and metastasis is self limiting (Tikhomirov and Carpenter, 2004). Of course, the easiest explanation for the differences in cell survival following EGF and EGF-bead treatment would be that the EGF-beads were not adequately stimulating the receptor.…”