A possible factor in genetic instability of cancer cells: stress-induced secreted proteins lead to decrease in replication fidelity.

ABSTRACT

Ultraviolet irradiation triggers a response in mammalian cells known as the UV response. Part of the UV response forms the enhanced synthesis of various extracellular proteins able to transmit the response to non-irradiated cells. Because several cancer-prone syndromes with enhanced genetic instability also have an enhanced synthesis of the same set of proteins without prior stimulation it is possible that induction of these stress responsive proteins may be involved in the process of carcinogenesis and genetic instability. To test this hypothesis mouse T lymphoma cells, GRSL13, were treated with the conditioned medium of UV-induced cells under various experimental conditions. Overall the mutation rate is enhanced 1.8-fold (P < 0.01). However, the degree of enhancement is strongly influenced by culture conditions. UV-induced factors only lead to an enhanced mutation rate when cells, both for the production and response to those factors, originate from a similar cell density. In addition, it was found that fresh medium interferes with this response. To eliminate the hindrance of these factors on the effect of the conditioned medium on the mutation rate, serum-starved cells at high density were treated with serum-free medium derived from high-density UV-irradiated cultures. Using these conditions a 2.8-fold (P < 0.002) enhancement of the mutation rate was found. Fluctuation analysis indicated that the enhancement is 10-fold during the first five generations after treatment. UV-induced factors have also been found to induce cell growth, and the degree of induction was linearly correlated with the enhancement in mutation rate. These experiments are in agreement with the hypothesis that induction of stress responses leads to genetic instability.