Reoxygenation of human coronary smooth muscle cells suppresses HIF-1α gene expression and augments radiation-induced growth delay and apoptosis

Background and Purpose: Catheter-based coronary brachytherapy with β- and γ-radiation is an evidence-based method to prevent restenosis after percutaneous transluminal coronary angioplasty (PTCA) and stent implantation, but the outcome may be PTCA are hypoxic. A lack of oxygen decreases the effect of low LET (linear energy transfer) irradiation. The authors assumed that reoxygenation of hypoxic human coronary smooth muscle cells (HCSMCs) improves the results of coronary brachytherapy. The expression of hypoxia-inducible factor 1α (HIF-1α) gene, and the rates of growth and apoptosis of hypoxic and reoxygenated HCSMCs after γ-iradiation were therefore analyzed. Material and Methods: An in vitro model of megacolonies of HCSMCs was developed. After exposure to chronic hypoxia the HCSMCs were irradiated with graded doses of 2, 4, 8, and 16 Gy using a 60Co source either under hypoxia (pO2<3 mmHg) or after reoxygenation (pO2∼150 mmHg). RT-PCR (reverse transcription-polymerase chain reaction) analysis was used to quantify HIF-1α gene expression and the growth of HCSMC megacolonies was measured serially. The oxygen enhancement ratio (OER) was calculate from the specific growth delay. Apoptosis of HCSMCs was quantified by counting cells with specific DNA strand breaks using the TUNEL assy. Results: HIF-1α gene expression was markedly suppressed in reoxygenated cells versus hypoxic cells 30 min after γ-irradiation at all radiation doses (158±46% vs. 1,675±1,211%; p<0.01). Apoptosis was markedly increased in reoxygenated HCSMCs. The OER was 1.8(95% CI[confidence interval]1.3-2.4). Therefore, reoxygenated HCSMCs require 44% less radiation dose to achieve the equivalent biological radiation effect compared to hypoxic HCSMCs. Conclusion: Reoxygenation of coronary smooth muscle cells should be considered an option to increase efficacy of coronary brachytherapy. This could be used to reduce radiation dose and associated late side effects. (orig.)