Oct-2 forms a complex with Oct-1 on the iNOS promoter and represses transcription by interfering with recruitment of RNA PolII by Oct-1

Transcriptional regulation is the fundamental process that modulates cell behaviour in response to environmental cues as well as intracellular requirements ultimately leading to the control of cellular responses in the organism. One of these processes is innate immunity, the first line defence against infectious microorganisms and neoplastic cells. A vital component of this system is the inducible nitric oxide synthase (iNOS) that produces high levels of nitric oxide (NO) in response to immune stimuli which is able to kill neighbouring cells and microbes in a non-specific manner. Due to the toxicity of these levels of NO, expression of iNOS is tightly regulated, mainly at the level of transcription. We showed previously that Oct-1 (POU2f1) is an essential transcription factor that binds to the proximal promoter and regulates iNOS, by recruiting RNA Pol II, leading to initiation and pausing. The relevance of this regulatory mechanism is that iNOS is primed for transcription so that when a stimulus triggers activation of transcription factors such as NFkB, transcription elongation can proceed swiftly to generate a rapid response.

Oct-1 is a member of the POU family of transcription factors that includes seven classes of regulators with distinct function and expression patterns. A common feature of this family is a central DNA-binding domain which in many members has affinity for the same octamer element ATGCAAAT. The flanking N- and C- terminus domains confer each member their specific activity. Oct-2 (POU2f2) is another member of this family which is closest to Oct-1 and whose expression is confined to B cells and neurones. Functionally, it has been implicated in B cell development and regulation of B lymphocyte-specific genes.

One of the many puzzles of transcriptional regulation is how different transcription factors with the same DNA binding domain can have different effects on gene expression. Though many of these differences can be explained by cell type-specific expression of these factors, this is not always clear. For example, though Oct-2 is normally restricted to B lymphocytes and neurones, we found it expressed in some cancer cell lines. We observed that in the cell lines that express Oct-2, iNOS is repressed. The mechanism of Oct-2 repression of iNOS consists in the formation of a complex with Oct-1 on the proximal promoter of the iNOS gene. The presence of Oct-2 interferes with Oct-1 activity to recruit and activate PolII thereby precluding transcription.

These findings help understand the mechanisms by which transcription factors control protein expression in a cell-specific manner and how the interplay between them achieves a fine tuning in response to particular circumstances. The wider interaction of the regulators includes the basal transcriptional machinery as well as the chromatin itself. Furthermore, though these mechanisms allow a differential regulation of proteins in specific cell types, for example avoiding the expression of the potentially harmful iNOS in sensitive cells such as neurones, they can also be hijacked by cancer cells to evade death signalling. The mechanisms that control gene expression are varied and versatile and a better understanding of the molecular basis for them will allow us to modulate them in health and disease.