Doxorubicin blocks proliferation of cancer cells through proteolytic activation of CREB3L1

Sensitization of U937 leukemia cells to doxorubicin by the MG132 proteasome inhibitor induces an increase in apoptosis by suppressing NF-kappa B and mitochondrial membrane potential loss

Background

The resistance of cancerous cells to chemotherapy remains the main limitation for cancer treatment at present. Doxorubicin (DOX) is a potent antitumor drug that activates the ubiquitin-proteasome system, but unfortunately it also activates the Nuclear factor kappa B (NF-кB) pathway leading to the promotion of tumor cell survival. MG132 is a drug that inhibits I kappa B degradation by the proteasome-avoiding activation of NF-кB. In this work, we studied the sensitizing effect of the MG132 proteasome inhibitor on the antitumor activity of DOX.

Methods

U937 human leukemia cells were treated with MG132, DOX, or both drugs. We evaluated proliferation, viability, apoptosis, caspase-3, -8, and −9 activity and cleavage, cytochrome c release, mitochondrial membrane potential, the Bcl-2 and Bcl-XL antiapoptotic proteins, senescence, p65 phosphorylation, and pro- and antiapoptotic genes.

Results

The greatest apoptosis percentage in U937 cells was obtained with a combination of MG132 + DOX. Likewise, employing both drugs, we observed a decrease in tumor cell proliferation and important caspase-3 activation, as well as mitochondrial membrane potential loss. Therefore, MG132 decreases senescence, p65 phosphorylation, and the DOX-induced Bcl-2 antiapoptotic protein. The MG132 + DOX treatment induced upregulation of proapoptotic genes BAX, DIABLO, NOXA, DR4, and FAS. It also induced downregulation of the antiapoptotic genes BCL-XL and SURVIVIN.

Conclusion

MG132 sensitizes U937 leukemia cells to DOX-induced apoptosis, increasing its anti-leukemic effectiveness.

Doxorubicin, DNA torsion, and chromatin dynamics

Doxorubicin is one of the most important anti-cancer chemotherapeutic drugs, being widely used for the treatment of solid tumors and acute leukemias. The action of doxorubicin and other anthracycline drugs has been intensively investigated during the last several decades, but the mechanisms that have been proposed for cell killing remain disparate and controversial. In this review, we examine the proposed models for doxorubicin action from the perspective of the chromatin landscape, which is altered in many types of cancer due to recurrent mutations in chromatin modifiers. We highlight recent evidence for effects of anthracyclines on DNA torsion and chromatin dynamics that may underlie basic mechanisms of doxorubicin-mediated cell death and suggest new therapeutic strategies for cancer treatment.

Roles of regulated intramembrane proteolysis in virus infection and antiviral immunity

Regulated intramembrane proteolysis (RIP) is a signaling mechanism through which transmembrane precursor proteins are cleaved to liberate their cytoplasmic and/or luminal/extracellular fragments from membranes so that these fragments are able to function at a new location. Recent studies have indicated that this proteolytic reaction plays an important role in host-virus interaction. On one hand, RIP transfers the signal from the endoplasmic reticulum (ER) to nucleus to activate antiviral genes in response to alteration of the ER caused by viral infection. On the other hand, RIP can be hijacked by virus to process transmembrane viral protein precursors and to destroy transmembrane antiviral proteins. Understanding this Yin and Yang side of RIP may lead to new strategies to combat viral infection. This article is part of a Special Issue entitled: Intramembrane Proteases.

Using fluorescence lifetime imaging microscopy to monitor theranostic nanoparticle uptake and intracellular doxorubicin release

We describe the synthesis of iron oxide nanoparticles (IONPs) with excellent colloidal stability in both water and serum, imparted by carefully designed grafted polymer shells. The polymer shells were built with attached aldehyde functionality to enable the reversible attachment of doxorubicin (DOX) via imine bonds, providing a controlled release mechanism for DOX in acidic environments. The IONPs were shown to be readily taken up by cell lines (MCF-7 breast cancer cells and H1299 lung cancer cells), and intracellular release of DOX was proven using in vitro fluorescence lifetime imaging microscopy (FLIM) measurements. Using the fluorescence lifetime difference exhibited by native DOX (~1 ns) compared to conjugated DOX (~4.6 ns), the intracellular release of conjugated DOX was in situ monitored in H1299 and was estimated using phasor plot representation, showing a clear increase of native DOX with time. The results obtained from FLIM were corroborated using confocal microscopy, clearly showing DOX accumulation in the nuclei. The IONPs were also assessed as MRI negative contrast agents. We observed a significant change in the transverse relaxivity properties of the IONPs, going from 220 to 390 mM(-1) s(-1), in the presence or absence of conjugated DOX. This dependence of MRI signal on IONP-DOX/water interactions may be exploited in future theranostic applications. The in vitro studies were then extended to monitor cell uptake of the DOX loaded IONPs (IONP@P(HBA)-b-P(OEGA) + DOX) into two 3D multicellular tumor spheroids (MCS) grown from two independent cell lines (MCF-7 and H1299) using multiphoton excitation microscopy.

CREB3L1 is a metastasis suppressor that represses expression of genes regulating metastasis, invasion, and angiogenesis

The unfolded protein response (UPR) is activated in response to hypoxia-induced stress such as in the tumor microenvironment. This study examined the role of CREB3L1 (cyclic AMP [cAMP]-responsive element-binding protein 3-like protein 1), a member of the UPR, in breast cancer development and metastasis. Initial experiments identified the loss of CREB3L1 expression in metastatic breast cancer cell lines compared to low-metastasis or nonmetastatic cell lines. When metastatic cells were transfected with CREB3L1, they demonstrated reduced invasion and migration in vitro, as well as a significantly decreased ability to survive under nonadherent or hypoxic conditions. Interestingly, in an in vivo rat mammary tumor model, not only did CREB3L1-expressing cells fail to form metastases compared to CREB3L1 null cells but regression of the primary tumors was seen in 70% of the animals as a result of impaired angiogenesis. Microarray and chromatin immunoprecipitation with microarray technology (ChIP on Chip) analyses identified changes in the expression of many genes involved in cancer development and metastasis, including a decrease in those involved in angiogenesis. These data suggest that CREB3L1 plays an important role in suppressing tumorigenesis and that loss of expression is required for the development of a metastatic phenotype.

How does doxorubicin work?

A new mechanism involving cleavage of a transcription factor called CREB3L1 has been proposed to explain the anti-tumour effects of doxorubicin.