{{short description|Transmembrane molecule in the mitochondria}} thumb|X-ray crystal structure of Bcl-xL with 1.76 Å resolution
'''B-cell lymphoma-extra large''' ('''Bcl-xL'''), encoded by the BCL2-like 1 gene, is a transmembrane molecule in the mitochondria. It is a member of the Bcl-2 family of proteins, and acts as an anti-apoptotic protein by preventing the release of mitochondrial contents such as cytochrome c, which leads to caspase activation and ultimately, programmed cell death.<ref>{{cite journal | vauthors = Korsmeyer SJ | title = Regulators of cell death | journal = Trends in Genetics | volume = 11 | issue = 3 | pages = 101–105 | date = March 1995 | pmid = 7732571 | doi = 10.1016/S0168-9525(00)89010-1 }}</ref>
== Function == It is a well-established concept in the field of apoptosis that relative amounts of pro- and anti-survival Bcl-2 family of proteins determine whether the cell will undergo cell death; if more Bcl-xL is present, then pores are non-permeable to pro-apoptotic molecules and the cell survives. However, if Bax and Bak become activated, and Bcl-xL is sequestered away by gatekeeper BH3-only factors (e.g. Bim) causing a pore to form, cytochrome c is released leading to initiation of caspase cascade and apoptotic events.<ref>{{cite journal | vauthors = Finucane DM, Bossy-Wetzel E, Waterhouse NJ, Cotter TG, Green DR | title = Bax-induced caspase activation and apoptosis via cytochrome c release from mitochondria is inhibitable by Bcl-xL | journal = The Journal of Biological Chemistry | volume = 274 | issue = 4 | pages = 2225–2233 | date = January 1999 | pmid = 9890985 | doi = 10.1074/jbc.274.4.2225 | doi-access = free }}</ref>
While the exact signaling pathway of Bcl-xL is still not known, it is believed that Bcl-xL differs highly from Bcl-2 in their mechanism of inducing apoptosis. Bcl-xL is about ten times more functional than Bcl-2 when induced by the chemotherapy drug, Doxorubicin<ref>{{cite journal | vauthors = Fiebig AA, Zhu W, Hollerbach C, Leber B, Andrews DW | title = Bcl-XL is qualitatively different from and ten times more effective than Bcl-2 when expressed in a breast cancer cell line | journal = BMC Cancer | volume = 6 | issue = 213 | page = 213 | date = August 2006 | pmid = 16928273 | pmc = 1560389 | doi = 10.1186/1471-2407-6-213 | doi-access = free }}</ref> and can specifically bind to cytochrome C residues, preventing apoptosis.<ref>{{cite journal | vauthors = Bertini I, Chevance S, Del Conte R, Lalli D, Turano P | title = The anti-apoptotic Bcl-x(L) protein, a new piece in the puzzle of cytochrome c interactome | journal = PLOS ONE | volume = 6 | issue = 4 | article-number = e18329 | date = April 2011 | pmid = 21533126 | pmc = 3080137 | doi = 10.1371/journal.pone.0018329 | bibcode = 2011PLoSO...618329B | doi-access = free }}</ref> It can also prevent the formation of Apaf-1 and Caspase 9 complex by acting directly upon Apaf-1 rather than Caspase 9, as shown in nematode homologs.<ref>{{cite journal | vauthors = Hu Y, Benedict MA, Wu D, Inohara N, Núñez G | title = Bcl-XL interacts with Apaf-1 and inhibits Apaf-1-dependent caspase-9 activation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 95 | issue = 8 | pages = 4386–4391 | date = April 1998 | pmid = 9539746 | pmc = 22498 | doi = 10.1073/pnas.95.8.4386 | bibcode = 1998PNAS...95.4386H | doi-access = free }}</ref> Growth factor independence 1 (Gfi1) upregulates the expression of the pro-survival Bcl-2 family member Bcl-xL in hematopoietic system. Its expression is upregulated by Gfi1 via Hemgn to promote cell survival.<ref name="Binod2025">{{cite journal |last1=G C |first1=Binod |last2=Du |first2=Pei |last3=Zhang |first3=Yangyang |last4=Yang |first4=Li |last5=Dong |first5=Fan |title=Bcl-xL is important for the antiapoptotic activity of Gfi1 and is upregulated by Gfi1 through hemgn |journal=Journal of Immunology |volume=214 |issue=5 |pages=1046–1058 |date=May 2025 |doi=10.1093/jimmun/vkae066 |pmid=40262274 |pmc=12123215}}</ref><ref>{{cite journal |last1=G C |first1=Binod |last2=Hoyt |first2=Laura J. |last3=Dovat |first3=Sina |last4=Dong |first4=Fang |year=2024 |title=Upregulation of nuclear protein Hemgn by transcriptional repressor Gfi1 through repressing PU.1 contributes to the anti-apoptotic activity of Gfi1 |journal=Journal of Biological Chemistry |volume=300 |issue=11 |article-number=107860 |doi=10.1016/j.jbc.2024.107860 |url=https://www.sciencedirect.com/science/article/pii/S0021925824007782 |doi-access=free }}</ref>
300px|thumb|right|Overview of signal transduction pathways
==Clinical significance== Bcl-xL dysfunction in mice can cause ineffective production of red blood cells, severe anemia, hemolysis, and death. This protein has also been shown as a requirement for heme production<ref>{{cite journal | vauthors = Rhodes MM, Kopsombut P, Bondurant MC, Price JO, Koury MJ | title = Bcl-x(L) prevents apoptosis of late-stage erythroblasts but does not mediate the antiapoptotic effect of erythropoietin | journal = Blood | volume = 106 | issue = 5 | pages = 1857–1863 | date = September 2005 | pmid = 15899920 | pmc = 1895223 | doi = 10.1182/blood-2004-11-4344 }}</ref> and in erythroid lineage, Bcl-xL is a major survival factor responsible for an estimated half of the total survival "signal" proerythroblasts must receive in order to survive and become red cells. Bcl-xL promoter contains GATA-1 and Stat5 sites. This protein accumulates throughout the differentiation, ensuring the survival of erythroid progenitors. Because iron metabolism and incorporation into hemoglobin occurs inside the mitochondria, Bcl-xL was suggested to play additional roles in regulating this process in erythrocytes which could lead to a role in polycythemia vera, a disease where there is an overproduction of erythrocytes.<ref>{{cite journal | vauthors = Silva M, Richard C, Benito A, Sanz C, Olalla I, Fernández-Luna JL | title = Expression of Bcl-x in erythroid precursors from patients with polycythemia vera | journal = The New England Journal of Medicine | volume = 338 | issue = 9 | pages = 564–571 | date = February 1998 | pmid = 9475763 | doi = 10.1056/NEJM199802263380902 | doi-access = free }}</ref>
Similar to other Bcl-2 family members, Bcl-xL has been implicated in the survival of cancer cells by inhibiting the function of p53, a tumor suppressor. In cancerous mouse cells, those which contained Bcl-xL were able to survive while those that only expressed p53 died in a small period of time.<ref>{{cite journal | vauthors = Schott AF, Apel IJ, Nuñez G, Clarke MF | title = Bcl-XL protects cancer cells from p53-mediated apoptosis | journal = Oncogene | volume = 11 | issue = 7 | pages = 1389–1394 | date = October 1995 | pmid = 7478561 }}</ref>
Bcl-xL is a target of various senolytic agents. Studies of cell cultures of senescent human umbilical vein endothelial cells have shown that both fisetin and quercetin induce apoptosis by inhibition of Bcl-xL.<ref name="pmid32686219">{{cite journal | vauthors = Kirkland JL, Tchkonia T | title = Senolytic drugs: from discovery to translation | journal = Journal of Internal Medicine | volume = 288 | issue = 5 | pages = 518–536 | date = November 2020 | pmid = 32686219 | pmc = 7405395 | doi = 10.1111/joim.13141 }}</ref> Fisetin has roughly twice the senolytic potency as quercetin.<ref name="pmid32752135">{{cite journal | vauthors = Wyld L, Bellantuono I, Tchkonia T, Morgan J, Turner O, Foss F, George J, Danson S, Kirkland JL | display-authors = 6 | title = Senescence and Cancer: A Review of Clinical Implications of Senescence and Senotherapies | journal = Cancers | volume = 12 | issue = 8 | article-number = e2134 | date = July 2020 | pmid = 32752135 | pmc = 7464619 | doi = 10.3390/cancers12082134 | doi-access = free }}</ref>
== Related proteins == Other Bcl-2 proteins include Bcl-2, Bcl-w, Bcl-xs, and Mcl-1.
== References == {{Reflist}}
{{Fas apoptosis signaling pathway}}
Category:Mitochondria Category:Apoptosis