Relationship of Mcl-1 isoforms, ratio p21WAF1/cyclin A, and Jun kinase phosphorylation to apoptosis in human breast carcinomas

Metformin enhances TRAIL-induced apoptosis by Mcl-1 degradation via Mule in colorectal cancer cells

Metformin is an anti-diabetic drug with a promising anti-cancer potential. In this study, we show that subtoxic doses of metformin effectively sensitize human colorectal cancer (CRC) cells to tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), which induces apoptosis. Metformin alone did not induce apoptosis, but significantly potentiated TRAIL-induced apoptosis in CRC cells. CRC cells treated with metformin and TRAIL showed activation of the intrinsic and extrinsic pathways of caspase activation. We attempted to elucidate the underlying mechanism, and found that metformin significantly reduced the protein levels of myeloid cell leukemia 1 (Mcl-1) in CRC cells and, the overexpression of Mcl-1 inhibited cell death induced by metformin and/or TRAIL. Further experiments revealed that metformin did not affect mRNA levels, but increased proteasomal degradation and protein stability of Mcl-1. Knockdown of Mule triggered a significant decrease of Mcl-1 polyubiquitination. Metformin caused the dissociation of Noxa from Mcl-1, which allowed the binding of the BH3-containing ubiquitin ligase Mule followed by Mcl-1ubiquitination and degradation. The metformin-induced degradation of Mcl-1 required E3 ligase Mule, which is responsible for the polyubiquitination of Mcl-1. Our study is the first report indicating that metformin enhances TRAIL-induced apoptosis through Noxa and favors the interaction between Mcl-1 and Mule, which consequently affects Mcl-1 ubiquitination.

Reduced association of anti-apoptotic protein Mcl-1 with E3 ligase Mule increases the stability of Mcl-1 in breast cancer cells

Background:

Mechanisms that increase resistance to apoptosis help promote cellular transformation. Cancer cells have deregulated apoptotic pathways, where increased expression and stability of anti-apoptotic proteins Mcl-1 and Bcl-2 increases resistance to apoptosis. Pathways that increase the stability of proteins in cancer cells remain poorly understood.

Methods:

Using human mammary epithelial and established breast cancer cell lines, we assessed the mechanisms that increase the stability of anti-apoptotic proteins in breast cancer cells by caspase assay, western blot, small-inhibitory RNA treatment and immunoprecipitation.

Results:

While breast cancer cells were resistant to de novo inhibition of protein synthesis, a rapid proteosome-mediated degradation of Mcl-1 and Bcl-2 induced apoptosis in mammary epithelial cells. Although Mule, an E3 ligase that targets Mcl-1 for degradation was expressed in mammary epithelial and breast cancer cell lines, rapid increase of polyubiquitinated Mcl-1 and Bcl-2 was detected only in mammary epithelial cells. Only transient formation of the Mule–Mcl-1 complex was detected in breast cancer cells. Downregulation of pERK1/2 in breast cancer cells reduced Mcl-1 levels and increased Mcl-1/Mule complex.

Conclusion:

Our findings suggest that reduced Mule/Mcl-1 complex has a significant role in increasing the stability of Mcl-1 in breast cancer cells and increased resistance to apoptosis.

Invasion-associated MMP-2 and MMP-9 are Up-regulated Intracellularly in Concert with Apoptosis Linked to Melanoma Cell Detachment

Matrix metalloproteinases, like MMP-2 and MMP-9 gelatinases, show multiple functions as extracellular/cell-surface enzymes, and are broadly recognised for their matrix-degrading ability and involvement in cell motility. Given that adherent cells have reduced attachment during migration and also detach from their substratum during apoptosis, we now investigated whether extracellular matrix-bound gelatinases and intracellular MMP-2 and MMP-9 are modified with progression of death-inducing stimuli. This report shows that melanoma cells undergoing death in response to 2-acetyl furanonaphtoquinone (FNQ) as evidenced by greater Annexin V binding, increased cytosolic expression of pro-MMP-2 and intracellular activation of particulate MMP-9. These changes were associated with early activation of a substrate-attached 40 kDa gelatinase reciprocal with changes in extracellular matrix-bound activated MMP-2. A subsequent activation of secreted MMP-9 and induction of apoptosis-associated fragmentation of poly ADP-Ribose polymerase (PARP) correlated with cell detachment. Our data suggests that intracellularly activated gelatinases may cleave survival-associated substrates other than gelatin that share the Gly-Leu/Iso-Pro like collagen-binding acetylcholinesterase, thereby linking them to apoptosis associated with cell detachment.