Protein Kinase C-ε Regulates the Apoptosis and Survival of Glioma Cells

Abstract

In this study, we examined the role of protein kinase C (PKC)-E in the apoptosis and survival of glioma cells using tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-stimulated cells and silencing of PKCE expression. Treatment of glioma cells with TRAIL induced activation, caspasedependent cleavage, and down-regulation of PKCE within 3 to 5 hours of treatment. Overexpression of PKCE inhibited the apoptosis induced by TRAIL, acting downstream of caspase 8 and upstream of Bid cleavage and cytochrome c release from the mitochondria. A caspase-resistant PKCE mutant (D383A) was more protective than PKCE, suggesting that both the cleavage of PKCE and its down-regulation contributed to the apoptotic effect of TRAIL. To further study the role of PKCE in glioma cell apoptosis, we employed short interfering RNAs directed against the mRNA of PKCE and found that silencing of PKCE expression induced apoptosis of various glioma cell lines and primary glioma cultures. To delineate the molecular mechanisms involved in the apoptosis induced by silencing of PKCE, we examined the expression and phosphorylation of various apoptosis-related proteins. We found that knockdown of PKCE did not affect the expression of Bcl2 and Bax or the phosphorylation and expression of Erk1/2, c-Jun-NH 2 -kinase, p38, or STAT, whereas it selectively reduced the expression of AKT. Similarly, TRAIL reduced the expression of AKT in glioma cells and this decrease was abolished in cells overexpressing PKCE. Our results suggest that the cleavage of PKCE and its down-regulation play important roles in the apoptotic effect of TRAIL. Moreover, PKCE regulates AKT expression and is essential for the survival of glioma cells. (Cancer Res 2005; 65(16): 7301-9)

Downregulation of Bid is associated with PKCɛ-mediated TRAIL resistance

“…18,19,23 Increased levels and activity of PKCe were associated with TRAIL resistance, 19 and overexpression of PKCe inhibited TRAILinduced apoptosis in these cells. 18,23 It has been reported that cellular susceptibility to TRAIL cannot be explained by the status of TRAIL receptor expression but does correlate with PKCe level. 23 Furthermore, PKCe overexpression did not modify the cell surface expression of TRAIL receptors in erythromyeloid cell lines.…”

“…3,[12][13][14][15] The presence of antiapoptotic proteins, such as Akt or Bcl-2 family members can also contribute to TRAIL resistance by affecting either the extrinsic or the intrinsic cell death pathway. 2,10,[15][16][17][18] Protein kinase C (PKC), a family of phospholipid-dependent serine/threonine kinases, is emerging as a critical regulator of cellular responses to TRAIL. 12,14,[18][19][20][21][22][23][24] The PKC family consists of 11 isoforms that are classified as conventional (a, b1, b2 and g), novel (d, e, Z and y) or atypical (z and i/l).…”

“…2,10,[15][16][17][18] Protein kinase C (PKC), a family of phospholipid-dependent serine/threonine kinases, is emerging as a critical regulator of cellular responses to TRAIL. 12,14,[18][19][20][21][22][23][24] The PKC family consists of 11 isoforms that are classified as conventional (a, b1, b2 and g), novel (d, e, Z and y) or atypical (z and i/l). 25 PKCs are involved in the regulation of cell proliferation, differentiation as well as cell death.…”

“…The mechanism of TRAIL resistance may vary depending on the PKC isozyme. 14,18,19,24,31 PKCe has been implicated in inhibiting TRAIL-induced apoptosis in glioma and melanoma cells. 18,19,23 Increased levels and activity of PKCe were associated with TRAIL resistance, 19 and overexpression of PKCe inhibited TRAILinduced apoptosis in these cells.…”

“…14,18,19,24,31 PKCe has been implicated in inhibiting TRAIL-induced apoptosis in glioma and melanoma cells. 18,19,23 Increased levels and activity of PKCe were associated with TRAIL resistance, 19 and overexpression of PKCe inhibited TRAILinduced apoptosis in these cells. 18,23 It has been reported that cellular susceptibility to TRAIL cannot be explained by the status of TRAIL receptor expression but does correlate with PKCe level.…”