Oct-4

Expression and function

Oct-4 transcription factor is initially active as a maternal factor in the oocyte and remains active in embryos throughout the preimplantation period. Oct-4 expression is associated with an undifferentiated phenotype and tumors. Gene knockdown of Oct-4 promotes differentiation, demonstrating a role for these factors in human embryonic stem cell self-renewal. Oct-4 can form a heterodimer with Sox2, so that these two proteins bind DNA together.

Mouse embryos that are Oct-4 deficient or have low expression levels of Oct-4 fail to form the inner cell mass, lose pluripotency, and differentiate into trophectoderm. Therefore, the level of Oct-4 expression in mice is vital for regulating pluripotency and early cell differentiation since one of its main functions is to keep the embryo from differentiating.

Orthologs

Orthologs of Oct-4 in humans and other species include:

Structure

Oct-4 contains the following protein domains:

Implications in disease

Oct-4 has been implicated in tumorigenesis of adult germ cells. Ectopic expression of the factor in adult mice has been found to cause the formation of dysplastic lesions of the skin and intestine. The intestinal dysplasia resulted from an increase in progenitor cell population and the upregulation of β-catenin transcription through the inhibition of cellular differentiation.

Pluripotency in embryo development

Animal model

In 2000, Niwa et al. used conditional expression and repression in murine embryonic stem cells to determine requirements for Oct-4 in the maintenance of developmental potency. Although transcriptional determination has often been considered as a binary on-off control system, they found that the precise level of Oct-4 governs 3 distinct fates of ES cells. An increase in expression of less than 2-fold causes differentiation into primitive endoderm and mesoderm. In contrast, repression of Oct-4 induces loss of pluripotency and dedifferentiation to trophectoderm. Thus, a critical amount of Oct-4 is required to sustain stem cell self-renewal, and up- or down-regulation induces divergent developmental programs. Changes to Oct-4 levels do not independently promote differentiation, but are also controlled by levels of Sox2. A decrease in Sox2 accompanies increased levels of Oct-4 to promote a mesendodermal fate, with Oct-4 actively inhibiting ectodermal differentiation. Repressed Oct-4 levels that lead to ectodermal differentiation are accompanied by an increase in Sox2, which effectively inhibits mesendodermal differentiation. Niwa et al. suggested that their findings established a role for Oct-4 as a master regulator of pluripotency that controls lineage commitment and illustrated the sophistication of critical transcriptional regulators and the consequent importance of quantitative analyzes.

The transcription factors Oct-4, Sox2, and Nanog are part of a complex regulatory network, with Oct-4 and Sox2 being capable of directly regulating Nanog by binding to its promoter, and are essential for maintaining the self-renewing undifferentiated state of the inner cell mass of the blastocyst, embryonic stem cell lines (which are cell lines derived from the inner cell mass), and induced pluripotent stem cells. While differential up- and down-regulation of Oct-4 and Sox2 has been shown to promote differentiation, down-regulation of Nanog must occur for differentiation to proceed.

Role in reprogramming

Oct-4 is one of the transcription factors that is used to create induced pluripotent stem cells (iPSCs), together with Sox2, Klf4, and often c-Myc (OSKM) in mice, demonstrating its capacity to induce an embryonic stem-cell-like state. These factors are often referred to as "Yamanaka reprogramming factors". This reprogramming effect has also been seen with the Thomson reprogramming factors, reverting human fibroblast cells to iPSCs via Oct-4, along with Sox2, Nanog, and Lin28. The use of Thomson reprogramming factors avoids the need to overexpress c-Myc, an oncogene. It was later determined that only two of these four factors, namely Oct4 and Klf4, are sufficient to reprogram mouse adult neural stem cells. Finally it was shown that a single factor, Oct-4 was sufficient for this transformation. Moreover, while Sox2, Klf4, and cMyc could be replaced by their respective family members, Oct4's closer relatives, Oct1 and Oct6, fail to induce pluripotency, thus demonstrating the exclusiveness of Oct4 among POU transcription factors. However, later it was shown that Oct4 could be completely omitted from the Yamanaka cocktail, and the remaining three factors, Sox2, Klf4, and cMyc (SKM) could generate mouse iPSCs with dramatically enhanced developmental potential. This suggests that Oct4 increases the efficiency of reprogramming, but decreases the quality of resulting iPSCs. This is possibly due to a mutated OCT4 DBD cysteine residue (Cys48) that has been identified as a central reprogramming determinant. The serine residue in OCT1 is mutated in the presence of the OCT4 N terminus, which conversely causes OCT4's serine (converted from cysteine) to reduce reprogramming efficiency by 60%.

In embryonic stem cells

• In in vitro experiments of mouse embryonic stem cells, Oct-4 has often been used as a marker of stemness, as differentiated cells show reduced expression of this marker.
• Oct3/4 can both repress and activate the promoter of Rex1. In cells that already express high level of Oct3/4, exogenously transfected Oct3/4 will lead to the repression of Rex1. However, in cells that are not actively expressing Oct3/4, exogenous transfection of Oct3/4 will lead to the activation of Rex1. This implies a dual regulatory ability of Oct3/4 on Rex1. At low levels of the Oct3/4 protein, the Rex1 promoter is activated, while at high levels of the Oct3/4 protein, the Rex1 promoter is repressed.
• Oct4 contributes to the rapid cell cycle of ESCs by promoting progression through the G1 phase, specifically through transcriptional inhibition of cyclin-dependent kinase inhibitors such as p21.
• CRISPR-Cas9 knockout of the gene in human embryonic stem cells demonstrated that Oct-4 is essential for the development after fertilisation.
• Oct3/4 represses Suv39h1 expression through the activation of an antisense long non-coding RNA. Suv39h1 inhibition maintains low level of H3K9me3 in pluripotent cells limiting the formation of heterochromatin.

In adult stem cells

Several studies suggest a role for Oct-4 in sustaining self-renewal capacity of adult somatic stem cells (i.e. stem cells from epithelium, bone marrow, liver, etc.).For example:

• Other scientists have produced evidence to the contrary, and dismiss those studies as artifacts of in vitro culture, or interpreting background noise as signal, and warn about Oct-4 pseudogenes giving false detection of Oct-4 expression. Oct-4 has also been implicated as a marker of cancer stem cells. A majority of Oct4 cancer stem-like cell studies (CSC) report a positive association between expression of OCT4 and chemoresistance. Chemotherapy resulting in the enrichment of CSCs showed changes in the phenotypes and increased stem cell markers of OCT4. Various cancers such as lung cancer, bladder cancer, and mesothelioma cells with high OCT4 expressions showed resistance to cisplatin, general drug resistance, and tumor recurrence. Breast cancer patients had tamoxifen resistance and poor clinical outcomes associated with OCT4. OCT4 knock-downs increase sensitivity to cisplatin and irradiation in lung cells, retained tumorigenicity in glioma-initiating cells, and metastasis mediation in ovarian cancer. In in vitro studies looking at cisplatin, knock-down of OCT4 increased their sensitivity and reduced cell proliferation. Further investigation is needed due to the sparsity of stem cells within tumors and their heterogeneity.

References

References

1. (September 1992). "Human Oct3 gene family: cDNA sequences, alternative splicing, gene organization, chromosomal location, and expression at low levels in adult tissues". Nucleic Acids Research.
2. (April 2000). "Quantitative expression of Oct-3/4 defines differentiation, dedifferentiation or self-renewal of ES cells". Nature Genetics.
3. Zeineddine, Dana et al. "The Oct4 protein: more than a magic stemness marker." American journal of stem cells vol. 3,2 74-82. 5 Sep. 2014
4. (December 2002). "Stem cell pluripotency and transcription factor Oct4". Springer Science and Business Media LLC.
5. (2014). "Role of Oct4 in the early embryo development". Springer Science and Business Media LLC.
6. (October 2018). "Systematic expression alteration analysis of master reprogramming factor OCT4 and its three pseudogenes in human cancer and their prognostic outcomes". Springer Science and Business Media LLC.
7. (May 2005). "Ectopic expression of Oct-4 blocks progenitor-cell differentiation and causes dysplasia in epithelial tissues". Elsevier BV.
8. (May 2003). "POU5F1 (OCT3/4) identifies cells with pluripotent potential in human germ cell tumors". Cancer Research.
9. (March 2005). "High-efficiency RNA interference in human embryonic stem cells". Stem Cells.
10. (July 2005). "Transcriptional regulation of nanog by OCT4 and SOX2". The Journal of Biological Chemistry.
11. (May 2005). "Ectopic expression of Oct-4 blocks progenitor-cell differentiation and causes dysplasia in epithelial tissues". Cell.
12. (June 2011). "Pluripotency factors in embryonic stem cells regulate differentiation into germ layers". Cell.
13. Heurtier, V., Owens, N., Gonzalez, I. et al. The molecular logic of Nanog-induced self-renewal in mouse embryonic stem cells. Nat Commun 10, 1109 (2019). https://doi.org/10.1038/s41467-019-09041-z
14. (July 2007). "Generation of germline-competent induced pluripotent stem cells". Nature.
15. (July 2007). "In vitro reprogramming of fibroblasts into a pluripotent ES-cell-like state". Nature.
16. (June 2007). "Directly reprogrammed fibroblasts show global epigenetic remodeling and widespread tissue contribution". Cell Stem Cell.
17. (December 2007). "Induced pluripotent stem cell lines derived from human somatic cells". Science.
18. (July 2008). "Pluripotent stem cells induced from adult neural stem cells by reprogramming with two factors". Nature.
19. (February 2009). "Oct4-induced pluripotency in adult neural stem cells". Cell.
20. (January 2008). "Generation of induced pluripotent stem cells without Myc from mouse and human fibroblasts". Nature Biotechnology.
21. (December 2019). "Excluding Oct4 from Yamanaka Cocktail Unleashes the Developmental Potential of iPSCs". Cell Stem Cell.
22. (May 2024). "Oct4 redox sensitivity potentiates reprogramming and differentiation". Genes & Development.
23. (April 1998). "Rex-1, a gene encoding a transcription factor expressed in the early embryo, is regulated via Oct-3/4 and Oct-6 binding to an octamer site and a novel protein, Rox-1, binding to an adjacent site". Molecular and Cellular Biology.
24. (February 2010). "Oct-4 controls cell-cycle progression of embryonic stem cells". The Biochemical Journal.
25. (October 2017). "Genome editing reveals a role for OCT4 in human embryogenesis". Nature.
26. (July 2022). "OCT4 activates a Suv39h1-repressive antisense lncRNA to couple histone H3 Lysine 9 methylation to pluripotency". Nucleic Acids Research.
27. (October 2007). "Oct4 expression is not required for mouse somatic stem cell self-renewal". Cell Stem Cell.
28. (March 2008). "The pluripotency regulator Oct4: a role in somatic stem cells?". Cell Cycle.
29. (July 2007). "Oct-4 expression in adult human differentiated cells challenges its role as a pure stem cell marker". Stem Cells.
30. (June 2011). "OCT4 Expression Enhances Features of Cancer Stem Cells in a Mouse Model of Breast Cancer". Laboratory Animal Research.
31. (April 2007). "OCT-4, an embryonic stem cell marker, is highly expressed in bladder cancer". International Journal of Cancer.
32. (April 2020). "Role of OCT4 in cancer stem-like cells and chemotherapy resistance". Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease.
33. (April 2017). "Crosstalks between Raf-kinase inhibitor protein and cancer stem cell transcription factors (Oct4, KLF4, Sox2, Nanog)". Tumour Biology.
34. (May 2017). "Expression of embryonal stem cell transcription factors in breast cancer: Oct4 as an indicator for poor clinical outcome and tamoxifen resistance". Oncotarget.