Enhancement of tumor growth under short light/dark cycle in mouse lung

We examined whether a change in daily light/dark cycle affected DNA synthesis and tumor growth in mouse lung. Both CD-1 and A/J mice, which were treated with urethane and maintained under the short light/dark cycle (6 h light, 6 h dark), developed significantly larger sizes of lung tumors than the mice maintained under the standard light/dark cycle (12 h light, 12 h dark), indicating that short and frequent light/dark cycles enhanced tumor growth. When 3H-labeled thymidine was injected and incorporated radioactivity in the lung tissue was measured, two peaks of DNA synthesis in the lung were observed in a day under the short light/dark cycle, and both peaks were during light periods, while only one peak was observed during the dark period under the standard light/dark cycle. Although the level of [3H]thymidine incorporation into the lung tissue in the short light/dark cycle is lower than that in the standard cycle, frequent or unbalanced DNA synthesis may have promoted the growth of lung tumors.