Decreased mitogenic response to epidermal growth factor in human squamous cell carcinoma lines overexpressing epidermal growth factor receptor owing to limiting amounts of the adaptor protein Grb2: Rescue by retinoic acid treatment

Abstract

Growth factor receptors of the tyrosine kinase family regulate proliferation of a variety of cell types. In some human cancers, the epidermal growth factor receptor (EGFR) and its ligands often are overexpressed, leading to both constitutive and autocrine activation. Intracellular signaling via this receptor takes place through several mechanisms of action, including activation of ras and the mitogen-activated protein kinase (MAPK) pathway. Our previous studies have shown that human squamous cell carcinoma (SCC) lines overexpress EGFR and do not increase proliferation in response to exogenous epidermal growth factor (EGF). The vitamin A metabolite retinoic acid (RA) has been used as a chemotherapeutic drug in the treatment of SCC. RA decreases proliferation of SCC lines, in part owing to inhibition of EGFR expression. However, we previously found that treatment of SCC lines with inhibitory doses of RA sensitized cells to the proliferative effects of EGF. We now present a mechanism of action for this effect. RA inhibited expression of EGFR and proteins in the MAPK signaling pathway. Expression of these molecules returned to basal levels within 24 h after RA withdrawal. RA also inhibited autocrine secretion of EGF, which returned to basal levels with slower kinetics. During this time, addition of exogenous EGF stimulated mitosis in SCC lines. These data suggested that signaling proteins downstream of overexpressed EGFR may have limited the mitotic response in SCC lines. In support of this hypothesis, overexpression of the EGFR adaptor protein Grb2 increased cell proliferation and restored EGF-induced mitosis.

EGFR Mediates Responses to Small-Molecule Drugs Targeting Oncogenic Fusion Kinases

“…4D). The interplay of adaptor binding is likely subject to many other factors, including concentration and competition in its environment, and thus may not automatically occur following phosphorylation (41). The ROS1 + HCC78 cell line provides a strong example of cell signaling plasticity that occurs following loss of ROS1 signaling.…”

Regulation of ERK1 gene expression by coactivator proteins

“…ERK1 activation by growth factor receptors is a signal transduction paradigm [15]. Blocking ERK1 activation inhibits growth factor signalling and cellular proliferation [20,21]. However, the mechanisms by which cells regulate ERK1 mRNA and protein expression are less clear.…”

“…Pharmacological inhibition of MEK, the upstream activator of ERK1, by PD98059 significantly decreased proliferation of SCC lines in vitro. RA inhibited proliferation of these cells by decreasing both growth-factor receptor and ERK1 expression [21,22]. The mechanism by which RA regulates ERK1 expression in these cells is largely uncharacterized.…”