RESUMEN
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.