A positive feedback loop between FOSB and miR-133b controls colon cancer cell proliferation

Abstract

FOSB, a member of the FOS gene family, forms heterodimers with JUN family proteins to engage in diverse cellular processes. Its biological impacts vary among different types of tumors, yet its specific function in colon cancer (CC) remains ambiguous. In this study, quantitative real-time PCR (qRT-PCR) and immunohistochemistry (IHC) are applied to measure FOSB expression levels, followed by an analysis of the association between FOSB expression and patients’ clinical parameters. In vitro experiments are performed to assess cell proliferation, including growth rate, cell cycle distribution, and apoptosis. A subcutaneous xenograft model in nude mice is utilized to monitor tumor growth in vivo. Additionally, chromatin immunoprecipitation (ChIP) and luciferase reporter assays are conducted to dissect the interactions among FOSB, miR-133b, and POU2F1. The results indicate that FOSB expression is downregulated in CC tissues relative to normal controls. Overexpression of FOSB suppresses proliferation and promotes apoptosis in CC cells. Mechanistically, FOSB binds to the promoter region of miR-133b, enhancing its transcription and subsequently repressing POU2F1 expression. Notably, decreased POU2F1 expression also alleviates the transcriptional repression of the FOSB promoter region, establishing a FOSB-miR-133b-POU2F1 feedback loop that inhibits CC proliferation. In summary, our findings suggest that FOSB acts as a tumor suppressor gene in CC and may exert its inhibitory effects on CC growth via the FOSB-miR-133b-POU2F1 feedback loop.

1. Introduction

Uncontrolled cell proliferation is a primary factor contributing to colon cancer (CC)-related mortality [1–3]. MicroRNAs (miRNAs) play crucial regulatory roles in the abnormal proliferation of CC cells. miRNAs usually affect cell proliferation and apoptosis by inhibiting target gene expression in CC. For example, miR-133b [HUGO Gene Nomenclature Committee (HGNC): 31759] can directly target the 3′-untranslated region (UTR) of receptor tyrosine kinase, affect the mesenchymal-epithelial transition (MET) signaling pathway and ultimately inhibit proliferation [4]. Moreover, miRNA expression levels are often regulated by transcription factors, such as the JUN family and YY1 family proteins [5,6]. These two proteins often form a TF-microRNA regulatory network and even a feedback regulatory mechanism, which plays an important role in abnormal CC cell proliferation [7].

As a key member of the FBJ murine osteosarcoma viral oncogene homolog B (FOSB) family, FOSB (HGNC: 3797) has been confirmed to be involved in the abnormal expression of a variety of tumors [8–10]. The FOSB expression level varies among tissue types, and FOSB has different biological effects on different tumors. For example, in non-small cell lung cancer, FOSB can exert antitumor effects by acting on PCDHB13 (HGNC: 8684) and dysregulating mitochondrial Ca2+ homeostasis [11]. In papillary thyroid cancer, FOSB recruits the histone acetyltransferase KAT5 (HGNC: 5275) to increase DPP4 (HGNC: 3009) transcription and activate p62 (HGNC: 11280)/Keap1 (HGNC: 23177)/Nrf2 (HGNC: 7782) signaling, thereby increasing growth and metastasis [12]. However, the function of FOSB in CC remains unclear. Pfannschmidt et al. [13] found that high FOSB expression was associated with longer recurrence-free survival after pulmonary metastasectomy in patients with pulmonary metastasis of CC, but the specific roles and mechanisms involved were unclear. Additionally, FOSB can interact with miRNAs and participate in the regulation of cancer proliferation [14,15]. However, the interaction between FOSB and miRNAs in CC requires further investigation.

POU domain class 2 transcription factor 1 (POU2F1) is also known as octamer transcription factor 1 (OCT1). POU2F1 has been implicated in carcinogenesis and cancer progression, particularly in the gastrointestinal tract [16,17]. Studies have shown that POU2F1 is upregulated in human gastric, intestinal, and colon stem cells and that its expression is positively correlated with tumor aggressiveness [18–21]. However, the mechanisms underlying these effects remain unclear.

Here, we showed that FOSB could regulate the expression of miR-133b as a transcription factor, inhibit the expression of POU Class 2 homeobox 1 (POU2F1, HGNC: 9212), and suppress the proliferation of CC cells. Moreover, downregulation of POU2F1 can in turn reduce the transcriptional inhibition of FOSB. This study revealed a FOSB/miR-133b/POU2F1 feedback loop that inhibits the proliferation of CC.