Observations on Biological Effects of Carbon-13, Based on the Effects of 13C-Testosterone on Growth of Human Osteoblasts, Human Aortic Endothelial Cells, and Human Umbilical Vein Endothelial Cells in Vitro

Abstract Despite the increasing knowledge of biological isotope effect, comprehensive understanding of heavy isotope effect in the biological contexts has remained far less than expectation. The present study investigated the carbon isotope effect of 13C enriched testosterone on human cells. It was among the rare studies on carbon isotope effect of bioactive compound. Human osteoblasts, human aortic endothelial cells, and human umbilical vein endothelial cells were cultured in vitro and treated with testosterone and 13C enriched testosterone (13C/12C:6.7%). The impacts of physiological to pharmacological concentrations (10-10-10-5mol/L) of the bioactive compound were taken into account. The cell proliferation activities were measured using MTS assay. The levels of alkaline phosphatase and osteocalcin in osteoblasts were tested. Our results established that 13C enriched testosterone exhibited different biological effects from testosterone. At the concentrations of 10-10mol/L and 10-5mol/L, there were significant differences in prompting cell proliferation between testosterone and 13C enriched testosterone. At physiological concentrations, testosterone prompted proliferations of the three kinds of cells; whereas, 13C enriched testosterone did not prompt the cell proliferation, and its effects were not concentration dependent. At supraphysiological concentration (10-5mol/L), testosterone had the trend of inhibiting cell growth; whereas, 13C enriched testosterone had the trend of prompting cell growth. 13C enriched testosterone significantly enhanced osteocalcin secretion in human osteoblasts at supraphysiological concentration. These findings challenged the common view of growth retardation effect of heavy isotope, which imply that biological isotope effects are worthy of further study. The potential applications of 13C enriched compound were discussed.