Hypoxia represents an important element of the solid tumor microenvironment and contributes to tumorigenesis and resistance to chemo- and radiation therapy. Hypoxia can modulate cell cycle progression although the mechanisms involved remain unclear. The Cdc25A dual specificity phosphatase promotes cell cycle progression by dephosphorylating and activating cyclin-dependent kinases. Cdc25A is the master regulator of the cell cycle and its disruption induces cell cycle arrest in cancer cells. The recent observation that under hypoxic conditions, levels of Cdc25A protein and mRNA are decreased led to the hypothesis that hypoxia-mediated reduction in Cdc25A may represent a novel mechanism in the hypoxic regulation of the cell cycle. Given the prominent role of Cdc25A in regulating the cell cycle and the proposed changes in Cdc25A protein and mRNA under hypoxic conditions, it was hypothesized that Cdc25A plays an essential role in hypoxia-mediated cell cycle arrest in human tumor cells. The specific aims were to: (1) examine the mechanism of Cdc25A downregulation in response to hypoxia, (2) determine the role HIF-1α in Cdc25A regulation and cell cycle arrest, and (3) determine if Cdc25A downregulation is required for hypoxia-induced cell cycle arrest. Under hypoxic conditions, Cdc25A protein levels were specifically and reversibly suppressed.

It was found that Cdc25A mRNA levels are significantly decreased by a p21-dependent mechanism. In addition, suppression of Cdc25A was independent of p53. Loss of Cdc25A protein occurred in the absence of checkpoint activation. Recent evidence has linked the microRNA miR-21 to Cdc25A and hypoxia. It is shown here that miR-21 was required for Cdc25A mRNA suppression in hypoxic colon cancer cells and miR-21 levels were increased under hypoxic conditions. The HIF-1α transcription factor was not required for suppression of Cdc25A but must be present for hypoxia-induced cell cycle arrest. Under hypoxic conditions, cells undergo p21- and miR-21-S-phase cell cycle arrest.

This study proposes a novel mechanism of transient regulation of Cdc25A via the p21- and miR-21-dependent regulation of mRNA levels in hypoxic cells leading to cell cycle arrest. This previously unknown mechanism may confer protection from hypoxic conditions, contributing to cell survival and the observed resistance to chemo- and radiation therapy.