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The reverse transcriptase inhibitors (RTI) azidothymidine and carbovir can block telomerase function in various cells, whereas dideoxycytidine does not exhibit such activity. RTI induce senescence in 3T3 Swiss, NIH 3T3 cell cultures and in the clones of immortal spontaneously transformed mouse fibroblasts. The RTI-induced senescence of L6 rat myoblasts in culture resembles the senescence of fibroblasts, but the resulting cells acquire sharp morphological peculiarities. The artificial senescence of fibroblasts and myoblasts resulted in both the appearance of corresponding senescent cells and a small portion of cells with the signs of another type of differentiation. The blockade of telomerase function by RTI in the human tumour cell lines U-937 and MeWo leads to the shortening of telomeres, but does not result in senescence. These cells may undergo crisis and after a while the proliferation resumes and resistant cells appear. RTI inhibit spontaneous reactivation of telomerase in the process of spontaneous transformation of mouse embryonic fibroblasts, which leads to the formation of telomerase-free clones. A fraction of these clones may overcome the senescence via the acquisition of telomerase activity. Cells with a very high level of telomerase activity become resistant to RTI. Thus, the blockade of telomerase function in different cells can induce senescence, partial differentiation or crisis. In human tumour cells it induces mainly crisis.

We showed that the telomerase activity (TAC) of human promyelocytic leukemic cells U-937 and HL-60 sharply decreased after induction of macrophagal differentiation. Dedifferentiation which occurred several days after removing the inductor was accompanied by the resumption of proliferation and increase of TAC. Telomerase activity significantly decreased also when U-937 cells ceased to proliferate in response to long-term inhibition of the telomerase function by azidothymidine. TAC was observed to decrease slowly for 6-8 days in the course of transition of mouse fibroblasts 3T3 Swiss to the quiescent state. TAC decreased both in serum-deprived cells and in slowly proliferating high-density inhibited cells. During the exit from quiescence and in dedifferentiation, the increase in TAC preceded the resumption of proliferation. In all the cases described the alterations of TAC correlated with alterations in the nonspecific polymerase activity which we had found earlier (D. N. Chernov, Y. E. Yegorov, and S. S. Akimov, DokL Biochemistry 349:55-58 (1996)). The problems of TAC regulation are discussed.

Two types of spontaneously transformed cells appear in the culture of senescent mouse embryonic fibroblasts. The first type are cells with restricted proliferative potential (up to 30 population doublings); the other type are immortalized cells. Cells of the first type, unlike those of the second, have no telomerase activity and undergo two rounds of senescence. Spontaneous transformation of mouse embryonic fibroblasts in the presence of the reverse transcriptase inhibitors azidothymidine and carbovir led to the formation of telomerase-free clones. A fraction of these clones have the ability to overcome senescence via the acquisition of high telomerase activity. Cells with a very high level of telomerase activity become resistant to azidothymidine and carbovir. Azidothymidine-induced artificial senescence of rat myoblasts in culture resembles the senescence of fibroblasts, but the resulting cells acquire sharp morphological peculiarities. The blockade of telomerase function by azidothymidine in human U-937 and MeWo cells leads to the shortening of telomeres, but does not result in senescence. A hypothesis of the generation of the signal that induces senescence is proposed. This hypothesis suggests a change in DNA conformation during telomere shortening as a result of a change of loop structure of telomeric chromatin.