Molecular evolution and secondary structural conservation in the B-cell lymphoma leukemia 2 (bcl-2) family of proto-oncogene products

Bcl-XL is qualitatively different from and ten times more effective than Bcl-2 when expressed in a breast cancer cell line

Background

Bcl-2 and Bcl-XL are anti-apoptotic paralogues that inhibit apoptosis elicited by a wide variety of stimuli, and play critical roles in cancer development and resistance to treatment. Many clinical studies have indicated that expression of these anti-apoptotic proteins in tumours is associated with poor prognosis. It has therefore been assumed that in cells the essential difference between Bcl-2 and Bcl-XL involves regulation of expression and that they are otherwise functionally similar. To examine this issue, we have compared the function of the proteins and of mutants of Bcl-2 and Bcl-XL specifically targeted to different subcellular sites.

Methods

We generated clones of the human breast cancer line MCF-7 stably expressing known amounts of Bcl-2, or Bcl-XL as determined by quantitative immunoblotting. Clones expressing equivalent amounts of wild-type and mutants of Bcl-2 and Bcl-XL with subcellular localization restricted to the cytoplasm, endoplasmic reticulum or outer mitochondrial membrane were studied in both MCF-7 and Rat-1 fibroblasts. In MCF-7 cells we measured the functional activities of these proteins in preventing apoptosis induced by four different agents (doxorubicin, ceramide, thapsigargin, TNF-α). Etoposide and low serum were used to compare the effect of Bcl-2, Bcl-XL and mutants located at the endoplasmic reticulum on induction of apoptosis in fibroblasts.

Results

We noted both qualitative and quantitative differences in the functional activity of these two anti-apoptotic proteins in cells: Bcl-2 localized to the endoplasmic reticulum inhibits apoptosis induced by ceramide and thapsigargin but not by doxorubicin or TNFα, while Bcl-XL at the endoplasmic reticulum is active against all four drugs. In fibroblasts Bcl-2 localized to the ER did not prevent cell death due to etoposide whereas Bcl-XL in the same location did. Finally in MCF-7 cells, Bcl-XL is approximately ten times more active than Bcl-2 in repressing apoptosis induced by doxorubicin. This difference can be manifest as a large difference in clonal survival.

Conclusion

When examined in the same cellular context, Bcl-2 and Bcl-XL differ substantially in the potency with which they inhibit apoptosis, mediated in part by differences in the inhibition of specific subcellular pathways.

New insights regarding pharmacologic approaches for ovarian cancer prevention

The pathogenesis of epithelial ovarian cancer is not completely understood, but it commonly is believed that the process of recurrent ovulation (incessant ovulation) causes genetic damage in ovarian epithelial cells and that sufficient genetic damage can lead to ovarian cancer in susceptible individuals. Under this model, it has been suggested that reproductive and hormonal factors, such as pregnancy and oral contraceptive use, decrease ovarian cancer risk mainly via their inhibitory effects on ovulation. There is mounting evidence that the ovarian epithelium is a hormonally responsive target organ whose biology can be impacted strongly by the local hormonal environment. Progestin-mediated apoptotic effects may be a major mechanism underlying the ovarian cancer protective effects of pregnancy (a high progestin state) and oral contraceptive pill use. Similarly, retinoids, vitamin D, and non-steroidal anti-inflammatory drugs may have biologic effects on the ovarian epithelium that are cancer preventive, whereas androgens may have stimulatory effects on the ovarian epithelium, leading to an increased ovarian cancer risk.

Exon skipping in Mcl-1 results in a bcl-2 homology domain 3 only gene product that promotes cell death

Mcl-1 is a member of the Bcl-2 family that is regulated transcriptionally and post-transcriptionally, with expression of the full-length Mcl-1-encoded gene product resulting in enhanced cell survival. As reported here, the human Mcl-1 gene can also undergo differential splicing, which yields an internally deleted, death-inducing gene product, Mcl-1(s/Delta)(TM). Whereas full-length Mcl-1 derives from three coding exons (instead of the two present in Bcl-2 and other anti-apoptotic members of this family), the Mcl-1(s/Delta)(TM) splice variant results from the joining of the first and third exons with skipping of the central exon. Because of the skipped exon and a shift in the reading frame downstream, the Bcl-2 homology domain (BH3) remains intact, whereas the BH1-, BH2-, and transmembrane-encoding domains do not. Mcl-1(s/Delta)(TM) thus has features similar to BH3 only, pro-apoptotic Bcl-2 family members and, accordingly, was found to promote cell death. In addition to a variety of other types of regulation, the Mcl-1 gene appears ideally designed for the generation of either a Bcl-2-like viability promoting or, as reported here, a BH3 only death-inducing gene product.

Detection of a homologue of bcl-2 in plant cells

An emerging family of bcl-2-like genes has been identified from nematode to humans. These genes play a role in the maintenance of homeostasis. Its members have highly conserved domains that are important for their dimerization. Since nothing is known about the importance of these genes in plant cells, we have investigated their presence in an alga as well as in three higher plants both by Western analysis and by immunocytochemistry. Immunoblots revealed the presence of a protein immunoreacting with the anti-bcl-2 polyclonal antibody in leaves of tobacco plants. Furthermore, immunocytochemical localization has shown that this protein is mainly associated with mitochondria, plastids, and nuclei of plant cells. Taken together, our results suggest that bcl-2 is a protein highly conserved throughout evolution.