Targeting Mcl-1 for the therapy of cancer

Abstract

Introduction: Human cancers are genetically and epigenetically heterogeneous and have the capacity to commandeer a variety of cellular processes to aid in their survival, growth and resistance to therapy. One strategy is to overexpress proteins that suppress apoptosis, such as the Bcl-2 family protein Mcl-1. The Mcl-1 protein plays a pivotal role in protecting cells from apoptosis and is overexpressed in a variety of human cancers.

Areas covered: Targeting Mcl-1 for extinction in these cancers, using genetic and pharmacological approaches, represents a potentially effectual means of developing new efficacious cancer therapeutics. Here we review the multiple strategies that have been employed in targeting this fundamental protein, as well as the significant potential these targeting agents provide in not only suppressing cancer growth, but also in reversing resistance to conventional cancer treatments.

Expert opinion: We discuss the potential issues that arise in targeting Mcl-1 and other Bcl-2 anti-apoptotic proteins, as well problems with acquired resistance. The application of combinatorial approaches that involve inhibiting Mcl-1 and manipulation of additional signaling pathways to enhance therapeutic outcomes is also highlighted. The ability to specifically inhibit key genetic/epigenetic elements and biochemical pathways that maintain the tumor state represent a viable approach for developing rationally based, effective cancer therapies.

Figure 1

Two proposed hypothetical models of the mechanism of action of the Bcl-2 family of proteins. The “indirect activation model” describes a scenario in which the binding of anti-apoptotic proteins inhibits Bax/Bak oligomerization. Displacement of these anti-apoptotic proteins by a BH3 only protein allows dimers to form and apoptosis to occur. In contrast, the “direct activation model” holds that BH3 only proteins are divided into two classes: activators and sensitizers. The activator proteins bind to Bax or Bak, activating them and leading to apoptosis. The anti-apoptotic proteins function in this model by binding to these activator and sensitizing proteins and sequestering them. The BH3 only sensitizers bind to anti-apoptotic proteins in an attempt to displace the activator BH3 proteins. When enough activator BH3 proteins are free, they are able to activate Bax/Bak and induce apoptosis.

Figure 2

Hypothetical model depicting mda-7/IL-24-induced lethality through induction of an ER stress response. [Adapted from Dash et al. (95)]

Figure 3

The combination of BI-97C1 (Sabutoclax) and mda-7/IL-24 cooperate to induce death in prostate carcinoma cells. A) PC3 cells were infected with the indicated doses of Ad.5/3-mda-7 for 6 h and then treated with DMSO or BI-97C1 (Sabutoclax)for 48 h. B) PC3 cells were infected with Ad.5/3-vec or Ad.5/3-mda-7 for 6 h and then treated with indicated concentrations of BI-97C1 (Sabutoclax)for 48 h. C) PC3 cells were infected with Ad.5/3-vec or Ad.5/3-mda-7 for 6 h and then treated with DMSO or 520 nM of BI-97C1 (Sabutoclax) for the indicated time. In A, B and C, % of apoptosis was evaluated. Bars represent S.D. (n = 3). D) PC3 cells were infected with Ad.5/3-vec or Ad.5/3-mda-7 for 6 h and then exposed to the indicated concentrations of BI-97C1 (Sabutoclax) for 48 h after and Western blotting was performed with the indicated antibodies.[Adapted from Dash et al. (87)]

Figure 4

A combination regimen of mda-7/IL-24 and BI-97C1 (Sabutoclax) additively inhibits prostate tumor growth in athymic and immunocompetent mice. A) Effect of mda-7/IL-24 plus BI-97C1 (Sabutoclax) on the growth of subcutaneous implanted prostate tumor cells in nude mice. M2182-Luc (1 × 10⁶) cells were injected s.c. in the left and right flank of male athymic nude mice. After establishing visible tumors of ~100-mm³, intratumoral injections of Ad.5/3-vec or Ad.5/3-mda-7 were given to the tumors on the left flank at a dose of 0.5 × 10⁸ pfu in 100 μl PBS. The injections were given 3X the first week and then 2X a week for two more weeks for a total of seven injections. BI-97C1 (Sabutoclax) (dissolved in ethanol/Cremophor EL/saline = 10:10:80) was injected i.p. at a sub-toxic level either at 1 mg/kg or 3 mg/kg 3X a week throughout the study (total 9 injections). Tumor growth was visualized by bioluminescence measured by the Xenogen imaging system. B) Suppression of tumor development and growth in Hi-Myc mice using a novel ultrasound targeted microbubble destruction (UTMD) approach with complement-treated microbubbles containing Ad.5/3-mda-7 and administration of BI-97C1 (Sabutoclax) intraperitoneally. The prostatic region of male Hi-Myc mice was sonoporated for 10 min following tail-vein injection of the indicated complement-treated microbubble/Ad complexes. BI-97C1 (Sabutoclax)was administered intraperitoneally at a dose of 3 mg/kg 3X a week for the duration of the study (total 9 injections). At the end of the experiment the mice were sacrificed and the prostates were harvested, weighed and photographed. C) Immunohistochemistry of tumors from combination therapy treated Hi-Myc mice. Hi-Myc mice were treated as described in panel B, paraffin-embedded sections obtained from the prostate were analyzed by immunohistochemistry using ant-Ki-67 and anti-MDA-7/IL-24 antibodies. Apoptosis in the prostate sections was detected by TUNEL assay.[From Dash et al. (87)]