Hypoxia and its Master Regulator HIF-1? Regulate the Expression of Calprotectin in Carcinoma Cells

Abstract

Oral squamous cell carcinoma (OSCC) is the most common cancer in the head and neck region. The severity of OSCC has been correlated inversely with the levels of a protein complex called calprotectin in carcinoma cells. Intracellular expression of calprotectin in carcinoma cells has anti-tumor properties such as apoptosis, arrest of cell cycle at G2/M phase, and reduction of size of tumor in vivo, thus contributing to better tumor outcomes. However, in OSCC, there is significant downregulation of calprotectin protein and mRNA levels leading to unfavorable tumor prognosis. Solid tumors such as human OSCC are characterized by abnormal cell growth leading to reduced oxygen availability in the tissue. This creates a state of pathologic hypoxia that has been shown to modify gene transcription in tumor cells that favors the growth and survival of cells. The transcriptional change is regulated by hypoxia inducible factors (HIFs) that selectively upregulate transcription of genes that are essential for the growth and survival of tumor cells. Available evidence suggests an association between hypoxia, its master regulator HIF-1 and calprotectin in several diseases such as inflammatory bowel disease and prostate cancer. The aim of this in vitro study was to determine the effect of hypoxia on the fate of calprotectin in squamous carcinoma cell lines. In addition, we wanted to determine whether there was an association between calprotectin and HIF-1 mRNA and/or protein levels in carcinoma cells. Squamous carcinoma cell lines (TR146 cells and KB HeLa-like cells) were treated with cobalt chloride to induce hypoxia. Treated cells were compared to control cells under normoxia. Our results suggest that intracellular calprotectin protein and mRNA expression levels appear to be independent of hypoxia and HIF-1 mRNA expression. Nevertheless, calprotectin and HIF-1 levels may be co-regulated indirectly through RAGE signaling pathway when calprotectin is released extracellularly and acts as ligand for RAGE receptor activation. In conclusion, the human OSCC microenvironment is a complex network in which many regulatory pathways coexist making it a challenge to reproduce complex conditions in in vitro experiments. Under the experimental conditions used, calprotectin expression on the protein and transcriptional levels is independent of hypoxia-induced pathways. Calprotectin and HIF-1 mRNA may be co-regulated, however, through intracellular and extracellular signaling pathways.