Epidermal Growth Factor Triggers an Original, Caspase-independent Pituitary Cell Death with Heterogeneous Phenotype

Abstract

Programmed cell death (PCD) is physiologically involved in the regulation of cell division and differentiation. It encompasses caspase-dependent mitochondrial and nonmitochondrial pathways. Additional caspase-independent pathways have been characterized in mitochondrial PCDs but remain hypothetical in nonmitochondrial PCDs. Epidermal growth factor (EGF) has been shown to inhibit division of pituitary somato-lactotrope cells occurring in parallel with EGF-mediated differentiation of these precursors into lactotrope cells. We show here that in somato-lactotrope pituitary cell line GH4C1, EGF triggers a PCD characterized by an apoptosis-like DNA fragmentation, insensitivity to broad-range caspase inhibitors, and absence of either cytochrome c or apoptosis-inducing factor release from mitochondria. Dying cells display loose chromatin clustering and numerous cytoplasmic vacuoles, a fraction of which are autophagic, thus conferring a heterogeneous phenotype to this PCD. Moreover, overexpression of cell death inhibitor Bcl-2 prevented not only the EGF-induced PCD but also its prodifferentiation effects, thus pointing to a mechanistic relationship existing between these two phenomena. Overall, the characterized differentiation-linked cell death represents an original form of caspase-independent PCD. The mechanisms underlying this PCD involve combinatorial engagement of discrete death effectors leading to a heterogeneous death phenotype that might be evolutionary related to PCD seen during the differentiation of some unicellular organisms. INTRODUCTIONProgrammed cell death (PCD) is physiologically involved in the control of proliferation/differentiation balance both in the course of development (e.g., organogenesis in mammals) and during the optimization of adult cell/tissue functions (e.g., thymic maturation of T lymphocytes). More recently, PCD has been associated with some pathological processes such as cancer and neurodegeneration. There is currently no consensus on either definition or classification of PCD (Sloviter, 2002;Golstein et al., 2003). The "programmed" character occurs as a hallmark of PCD, but it refers to different aspects of the phenomenon, i.e., to the programmed occurrence of PCD in the course of development (in developmental PCD) or to the programmed succession of morphological and biochemical events (in nondevelopmental PCDs). However, the term programmed is also considered in the sense of "regulated," leading to a broad classification of PCD into apoptosis, necrosis, and autophagy (Golstein et al., 2003).One of the more restraining classifications is based on a particular criterion of the nuclear morphology and divides PCDs into classical apoptosis, apoptosis-like PCD, and necrosis-like PCD characterized by "crescent-like" (type 2), lumpy (type 1), or absence of chromatin condensation, respectively (Jaattela and Tschopp, 2003).Classical apoptosis is the best known phenotypic expression of PCD. It is related to a series of stereotypic morphological and biochemical alterations resulting from the act...

Role of STAT3 as a Negative Regulator in Mac2- Binding Protein Expression

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Bax activation and translocation to mitochondria mediate EGF-induced programmed cell death

“…Cell death induced by EGF is caspase-independent and involves a loss of MMP, but does not involve the release of cytochrome c or AIF from mitochondria. EGF-induced cell death in GH4C1 cells can be blocked by a Bcl-2 overexpression (Fombonne et al, 2004). The characterization of apoptosis in GH4C1 cells closely resembles the morphological and biochemical features of EGF-induced apoptosis in A431/ErbB-2 and SKBr3/EGFR cells.…”

“…The opening of mitochondrial pores and release of AIF and endonuclease G, which induce nuclear apoptosis, could be responsible for DNA fragmentation and chromatin condensation. However, a recent publication suggests that during EGF-induced caspase-independent apoptosis in GH4C1 cells there is no activation of AIF (Fombonne et al, 2004). Another recent publication reported that caspase inhibition prevents the mitochondrial release of AIF and endonuclease G downstream of Bax-mediated opening of pores in the mitochondrial membrane (Arnoult et al, 2003).…”

“…It has been suggested that EGF-induced apoptosis in A431 cells requires STAT1 activation followed by induction of the cyclin-dependent kinase inhibitor p21 WAF1/CIP1 (Chin et al, 1996). While it was proposed that EGFR activation stimulates expression of caspase 1 (Chin et al, 1997), recent work also suggests that EGF-induced apoptosis may proceed through a caspase-independent pathway (Fombonne et al, 2004).…”

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