REGULATION OF GROWTH ARREST SPECIFIC (GAS) GENE p20K IN HYPOXIA

Abstract

<p>A microarray analysis of RNA from contact inhibited CEF indicated a hypoxic signature in the contact inhibition program of gene expression (Ghosh <em>et al</em>., 2009). The purpose of this thesis was to investigate whether GAS genes known to be induced during contact inhibition are inducible by hypoxia. The gene p20K was selected as the model for this investigation because it is a growth arrest specific (GAS) gene with a well-characterized promoter (Mao <em>et al</em>., 1993). p20K expression was shown to be positively regulated in hypoxia. It was then determined by transient expression assay that this induction occurred at the promoter level. Interestingly by dissecting the promoter it was found that the quiescent responsive unit (QRU) was required for promoter induction during hypoxia. It has previously been shown that the QRU was required for contact inhibition induction of p20K in a C/EBPβ dependent manner (Mao <em>et al</em>., 1993; Kim <em>et al</em>., 1999).</p> <p>The mechanism behind hypoxic induction of the QRU was then investigated. The kinetics of HIF1α and p20K induction during hypoxia demonstrated that HIF1α was transiently expressed between 2-8 hrs of hypoxia while p20K was induced after 8 hrs of hypoxia. Co-Immuniprecipitation assay was also used to determine if a HIF1α-C/EBPβ interaction occurred, however, this molecular interaction could not be shown. These experiments suggests that HIF1α is not involved with the induction of the QRU. Over-expression of the dominant negative C/EBPβΔ184 repressed p20K induction, thus implicating C/EBPβ in activation in both contact inhibition and hypoxia. We also observed by western blot analysis that the C/EBP family member CHOP was repressed during hypoxia, causing a decrease in the amount of CHOP-C/EBPβ complexes in the cell. It was also found that over-expression of CHOP antagonized the induction of p20K by hypoxia. In conclusion hypoxia represses CHOP levels resulting in an increase of potent C/EBPβ homodimers at the expense of the inactive CHOP-C/EBPβ heterodimers.</p>