Sp1, Instead of AhR, Regulates the Basal Transcription of Porcine CYP1A1 at the Proximal Promoter

A new insight into the aryl hydrocarbon receptor/cytochrome 450 signaling pathway in MG63, HOS, SAOS2, and U2OS cell lines

Osteosarcoma is the most common malignant tumor of bone, with rapid progressive growth, early distant metastases, and frequent recurrence after surgical treatment. Osteosarcoma is characterized by changes in the ratio and expression of different cytochrome P450 (CYP) isoforms that can affect the effectiveness of anticancer therapies. The inducible expression of CYP1 genes depends on the ligand-dependent functionality of the aryl hydrocarbon receptor (AHR). In this study, we examined the AHR/CYP1 signaling pathway in four osteosarcoma cell lines (MG63, HOS, SAOS2, and U2OS) induced by the known AHR ligands: indirubin, indole-3-carbinol, and beta-naphthoflavone. Using qPCR and Western blot analysis, we explored the effects of these ligands on the expression of the CYP1 genes and studied the correlation between these responses and the changes in the mRNA and protein levels of AHR and the AHR nuclear translocator (ARNT) in these osteosarcoma cell lines. The results show that the AHR/CYP1 signaling pathway retains its function only in MG63 and HOS cells, and is impaired in SAOS2 and U2OS cells. Our data should be taken into account when recommending new strategies for the treatment of osteosarcoma and when evaluating new drugs against osteosarcoma in vitro.

Specificity Proteins (SP) and Krüppel-like Factors (KLF) in Liver Physiology and Pathology

The liver acts as a central hub that controls several essential physiological processes ranging from metabolism to detoxification of xenobiotics. At the cellular level, these pleiotropic functions are facilitated through transcriptional regulation in hepatocytes. Defects in hepatocyte function and its transcriptional regulatory mechanisms have a detrimental influence on liver function leading to the development of hepatic diseases. In recent years, increased intake of alcohol and western diet also resulted in a significantly increasing number of people predisposed to the incidence of hepatic diseases. Liver diseases constitute one of the serious contributors to global deaths, constituting the cause of approximately two million deaths worldwide. Understanding hepatocyte transcriptional mechanisms and gene regulation is essential to delineate pathophysiology during disease progression. The current review summarizes the contribution of a family of zinc finger family transcription factors, named specificity protein (SP) and Krüppel-like factors (KLF), in physiological hepatocyte functions, as well as how they are involved in the onset and development of hepatic diseases.

AhR regulates the expression of human cytochrome P450 1A1 (CYP1A1) by recruiting Sp1

Cytochrome P450 1A1 (CYP1A1) is abundant in the kidney, liver, and intestine and is involved in the phase I metabolism of numerous endogenous and exogenous compounds. Therefore, exploring the regulatory mechanism of its basal expression in humans is particularly important to understand the bioactivation of several procarcinogens to their carcinogenic derivatives. Site-specific mutagenesis and deletion of the transcription factor binding site determined the core cis-acting elements in the human CYP1A1 proximal and distal promoter regions. The proximal promoter region [overlapping xenobiotic-responsive element (XRE) and GC box sequences] determined the basal expression of CYP1A1. In human hepatocellular carcinoma cells (HepG2) with aryl hydrocarbon receptor (AhR) or specificity protein 1 (Sp1) knockdown, we confirmed that AhR and Sp1 are involved in basal CYP1A1 expression. In HepG2 cells overexpressing either AhR or Sp1, AhR determined the proximal transactivation of basal CYP1A1 expression. Via DNA affinity precipitation assays and ChIP, we found that AhR bound to the promoter and recruited Sp1 to transactivate CYP1A1 expression. The coordinated interaction between Sp1 and AhR was identified to be DNA mediated. Our work revealed a basal regulatory mechanism of an interesting human gene by which AhR interacts with Sp1 through DNA and recruits Sp1 to regulate basal CYP1A1 expression.

T-2 toxin upregulates the expression of human cytochrome P450 1A1 (CYP1A1) by enhancing NRF1 and Sp1 interaction

T-2 toxin is a major pollutant in crops and feedstuffs. Due to its high toxicity in a variety of organisms, T-2 toxin is of great concern as a threat to humans and to animal breeding. Overexpression of CYP1A1 may contribute to carcinogenesis, and CYP1A1 may be a promising target for the prevention and treatment of human malignancies. Therefore, it is essential to understand the regulatory mechanism by which T-2 toxin induces CYP1A1 expression in human cells. In this study, we confirmed that T-2 toxin (100 ng/mL) induced the expression of CYP1A1 in HepG2 cells through NRF1 and Sp1 bound to the promoter instead of through the well-recognized Aromatic hydrocarbon receptors (AhR). In cells treated with T-2 toxin, Sp1, but not NRF1, was significantly upregulated. However, T-2 toxin apparently promoted the interaction between NRF1 and Sp1 proteins, as revealed by IP analysis. Furthermore, in T-2 toxin-treated HepG2 cells, nuclear translocation of NRF1 was enhanced, while knockdown of Sp1 ablated NRF1 nuclear enrichment. Our results revealed that the upregulation of CYP1A1 by T-2 toxin in HepG2 cells depended on enhanced interaction between Sp1 and NRF1. This finding suggests the tumorigenic features of T-2 toxin might be related to the CYP1A1, which provides new insights to understand the toxicological effect of T-2 toxin.