Bcl-xL Mediates a Survival Mechanism Independent of the Phosphoinositide 3-Kinase/Akt Pathway in Prostate Cancer Cells

Abstract

Among various molecular strategies by which prostate cancer cells evade apoptosis, phosphoinositide 3-kinase (PI3K)/Akt signaling represents a dominant survival pathway. However, different prostate cancer cell lines such as LNCaP and PC-3 display differential sensitivity to the apoptotic effect of PI3K inhibition in serum-free media, reflecting the heterogeneous nature of prostate cancer in apoptosis regulation. Whereas both cell lines are equally susceptible to LY294002-mediated Akt dephosphorylation, only LNCaP cells default to apoptosis, as evidenced by DNA fragmentation and cytochrome c release. In PC-3 cells, Akt deactivation does not lead to cytochrome c release, suggesting that the intermediary signaling pathway is short-circuited by an antiapoptotic factor. This study presents evidence that Bcl-xL overexpression provides a distinct survival mechanism that protects PC-3 cells from apoptotic signals emanating from PI3K inhibition. First, the Bcl-xL/ BAD ratio in PC-3 cells is at least an order of magnitude greater than that of LNCaP cells. Second, ectopic expression of Bcl-xL protects LNCaP cells against LY294002-induced apoptosis. Third, antisense down-regulation of Bcl-xL sensitizes PC-3 cells to the apoptotic effect of LY294002. The physiological relevance of this Bcl-xL-mediated survival mechanism is further underscored by the protective effect of serum on LY294002-induced cell death in LNCaP cells, which is correlated with a multifold increase in Bcl-xL expression. In contrast to Bcl-xL, Bcl-2 expression levels are similar in both cells lines, and do not respond to serum stimulation, suggesting that Bcl-2 may not play a physiological role in antagonizing apoptosis signals pertinent to BAD activation in prostate cancer cells.The key role of the phosphoinositide 3-kinase (PI3K) 1 /Akt signaling cascade in promoting cell survival downstream of a plethora of trophic signals has been well characterized (1-5). Activation of PI3K leads to an increase in its lipid products, phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3,4-bisphosphate, in plasma membranes. These lipid second messengers facilitate the membrane co-localization of Akt with phosphoinositide-dependent kinases, leading to Akt phosphorylation and activation (6). Activated Akt, in turn, mediates antiapoptotic signaling through the phosphorylating inactivation of a multitude of downstream targets involved in apoptosis regulation, including the two key regulators of the cell death machinery BAD and procaspase 9 (4). The paradigm of this PI3K/Akt signaling cascade provides a framework to account for the ability of extracellular growth factors to promote cell survival (1-5). However, in nearly 50% of prostate tumors, this PI3K/Akt survival pathway is constitutively upregulated because of mutations of the tumor suppressor PTEN (7-10) that functions as a negative regulator of PI3K through its lipid phosphatase activity (11-13). Evidence suggests that dysregulation of the PI3K/Akt signaling cascade furnishes a mechanism whereby prostate...

Celecoxib derivatives induce apoptosis via the disruption of mitochondrial membrane potential and activation of caspase 9

“…Celecoxib induced apoptosis in the prostate cancer cell line also appears to be independent of Bcl-2. 29,30,32 The protection of mitochondrial membrane potential by CsA together with the noninvolvement of Bcl-2 in our oral cell lines suggests that the celecoxib derivatives modulate signaling pathways bypassing Bcl-2 or interact with the mitochondria membrane to induce apoptosis. 26 Further studies are needed to identify and decipher the targets, mechanisms and interactions between pathways which lead to celecoxib derivative induced apoptosis in different cancer phenotypes.…”

“…26 Studies in human prostate cancer cell lines indicate that celecoxib induces apoptosis by blocking Akt and Erk2 activation, through the mitochondrial pathway, independent of COX-2 activity and Bcl-2. 8,29,30 This has led to the design of celecoxib derivatives that optimize the apoptosis-inducing potency, separate from the COX inhibiting activity of celecoxib. 8,[31][32][33] Recent studies with the first and second generation derivatives of celecoxib in the PC-3 human prostate cell line suggests that apoptosis is mediated through the blockade of Akt activation by derivative inhibition of 3-phosphoinositide dependent kinase-1 (PDK1).…”

PGC-1α induces apoptosis in human epithelial ovarian cancer cells through a PPARγ-dependent pathway

“…Cytosolic-specific, mitochondria-free lysates were prepared as described in Miyake et al [30]. After 48 h adenovirus infection, Ho-8910 cells were collected by centrifugation at 600 × g for 5 min.…”

ACYP1 Gene Possesses Two Alternative Splicing Forms That Induce Apoptosis

“…These preliminary results indicated that viability was severely reduced in cells ectopically expressing either CT or CTsv. In order to assess whether this phenomenon may be due to apoptosis we monitored the cells with an ELISA technique which detects the enrichment of mono-and oligonucleosomes in the cytoplasm of the transfected cells (13). Fig.…”

“…These can be detected by an ELISA assay that is quite sensitive and specific for apoptosis (13). Transfected HeLa cells were gently lysed to release mono-and oligo-nucleosomes from apoptotic cells.…”