Expression of microRNA-21 in non-small cell lung cancer tissue increases with disease progression and is likely caused by growth conditional changes during malignant transformation

Abstract

MicroRNAs can govern up to hundred different mRNAs and are important regulators of gene expression programs in development and disease. We analyzed the expression of microRNA-21, one of the most common oncomirs, in non-small cell lung cancer (NSCLC). Using northern blots the microRNA-21 expression levels of NSCLC-derived tissue and cell lines were measured. In line with earlier observations we show that mature microRNA-21 expression levels are highly increased in NSCLC-derived tissue compared to normal lung tissue. Additionally, we demonstrate that microRNA-21 levels correlate with malignancy since its expression in higher staged tumors is significantly more elevated compared to stage 1A. Interestingly, microRNA-21 levels in cultured NSCLC-derived cells are comparable to the expression detected in non-malignant lung tissue. Since microRNA-21 levels showed no fluctuation during the cell cycle, accelerated proliferation of tumor cells is not responsible for microRNA-21 upregulation in the tumor compartment. Similarly to NSCLC-derived cancer cells, the tumor-associated fibroblasts show low expression levels of microRNA-21. Together, these data indicate that rather microenviromental and growth conditional changes than intrinsic features of the cancer cells are responsible for the observed increase of microRNA-21 levels in tumor tissues. Subsequently culturing conditions were changed to assess the impact of co-cultivation with fibroblasts, hypoxia and anchorage-independent growth on microRNA-21 expression. While co-cultivation with tumor-associated fibroblasts had no effect on microRNA-21 expression, both hypoxia and anchorage-independent growth cause a microRNA-21 elevation. In summary, our data demonstrate that growth conditions especially expected in more malignant tumors result in microRNA-21 upregulation explaining the observed increase in higher staged lung cancer tissue, but not in lung cancer-derived cells.