Alan Wells

Membrane proximal ERK signaling is required for M-calpain activation downstream of epidermal growth factor receptor signaling.

Localization of signaling is critical in directing cellular outcomes, especially in pleiotropic signaling pathways. The extracellular signal-regulated kinase (ERK)/microtubule-associated protein kinase, which promotes cell migration, proliferation, and differentiation is found in the nucleus and throughout the cytoplasm. Recently, it has been shown that nuclear translocation of ERK is required for transcriptional changes and cell proliferation. However, the cellular consequences, of cytoplasmic signaling have not been defined. We explored whether cytoplasmic, specifically membrane-proximal, ERK signaling is involved in growth factor-induced cell motility. We previously have demonstrated that increased M-calpain activity downstream of epidermal growth factor receptor (EGFR)-mediated ERK activation is necessary for epidermal growth factor (EGF)-induced motility. Calpain isoforms also have been found in nuclear, cytosolic, and plasma membrane-associated compartments in a variety of cell types. We now employ cell engineering approaches to control localization of the upstream EGFR and ERK activities to examine the spatial effect of upstream signal locale on downstream calpain activity. With differential ligand-induced internalization and trafficking-restricted receptor variants, we find that calpain activity is triggered only by plasma membrane-restricted activated EGFR, not by internalized (although still active) EGFR. Cells transfected with membrane-targeted ERK1 and ERK2, which sequester endogenous ERKs, exhibited normal EGF-induced calpain activity. Transfection of an inactive ERK phosphatase (MKP-3/Pyst1) that sequesters ERK in the cytoplasm prevented calpain activation as well as de-adhesion. These data strongly suggest that EGF-induced calpain activity can be enhanced near sites of membrane-proximal EGFR-mediated ERK signaling, providing insights about how calpain activity might be regulated and targeted to enhance its effects on adhesion-related substrates.

19 Calpain proteases in prostate carcinoma

Publisher Summary This chapter describes the role of calpains in cell functioning. Attachment to the primary cell mass is modulated by cadherin-mediated junctions. Calpain may directly target the cytoplasmic tail of cadherins to disrupt this homotypic adhesion. During cell migration, calpains play a central role in forward spread and tail retraction. These two cell behaviors appear regulated by membrane-associated calpains. Cytosolic-, mitochondrial- and nuclear-localized calpains appear to contribute to survival and proliferation. Calpains were the first defined apoptosis mediators and are still found to induce apoptotic and necrotic cell death under certain circumstances. The role of calpains in proliferation is more unsettled because conflicting reports claim calpains as either promoting or inhibiting cell cycling. Still, the central role of calpains in at least three key attributes of tumor progression is tantalizing for further evaluation.