Cells will undergo genotypic changes resulting in reduced responsiveness over time in normal cell culture conditions. Genetic instability is a biological phenomenon that occurs in all stably transfected cells. Therefore, it is critical to prepare an adequate number of frozen stocks at early passages.
Hypoxia inducible factor (HIF) is a heterodimeric transcription factor composed of two basic helix-loop-helix proteins-HIF-1a and HIF-1beta (or Arnt). The HIF a/beta dimmer binds to hypoxia-response element and activates the transcription of many genes that code for proteins that are involved in angiogenesis, glucose metabolism, cell proliferation/survival and invasion/metastasis, such as VEGF, FLT-1, EPO, and Leptin. Studies have shown the association of aberrant HIF activities with human diseases such as cancer and heart disease. HIF-1a is overexpressed in human cancers as a result of intratumoral hypoxia as well as genetic alterations, such as gain-of-function mutations in oncogenes (for example, ERBB2) and loss-of-function mutations in tumour-suppressor genes (for example, VHL and PTEN).