The Japanese senescence accelerated mice (SAM) are a group of the low-longevity mouse lines and represent a new convenient model for studying the senescence process. We studied the proliferation of embryo fibroblasts of SAMP1 and SAMR1 mouse lines. It was shown that fibroblasts of the shortest longevity line SAMP1 have a markedly decreased proliferative potential of the mean 8.7 population doublings, whereas fibroblasts of a relatively high-longevity line SAMR1 have an average proliferative potential of 12.3 doublings. The fibroblast senescence in both lines is accompanied by simultaneous lowering of the cell proliferative response to the blood serum, epidermal, fibroblast, and platelet-derived growth factors. At initial stages of the cell culture growth, lines SAMP1 and SAMR1 exhibit the same reactions to growth factors, but already beginning from the fifth doubling, the SAMP1 cell response is sharply decreased as compared with SAMR1. Lowering of the proliferative reaction is accompanied by decreased phosphorylation of tyrosine in the cell proteins responsible for the mitogenic reaction. Thus, the parallel decrease in the proliferative response to different growth factors during fibroblast senescence is most likely due to the emergence of a regulatory block at common stages of the mitogenic signal transduction.

It is known that the longevity of senescene accelerated mouse (SAM) is significantly reduced. We intended to check the relationship of the SAM shortened longevity with some characteristics of their cells in culture. We observed the correlation of longevity of SAMP1 (senescence prone strain), SAMR1 (senescence resistant strain), and CBA mice with proliferative lifespans of their embryo fibroblasts in vitro as well as with the survival time of nondividing senesced embryo fibroblasts. It was shown that azidothymidine AZT (3 muM) increases the survival time of nondividing senesced embryo fibroblasts. The premature senescence of SAM fibroblasts is associated with accelerated accumulation of the P-galactosidase-positive cells. Terminal restriction fragments of DNA of SAMP1 are more heterogeneous than SAMR1 ones. There is relatively high telomerase activity tin comparison with CBA mice) in SAM embryos and cell cultures. This activity rapidly decreases during growth in vitro and is restored after spontaneous transformation occurring at a high frequency in both strains. The hypothesis about common mechanisms underlying decreasing of longevity, proliferative potential, and survival time of senesced nondividing cells is discussed.

It is known that the longevity of senescence accelerated mouse (SAM) is significantly reduced. We intended to check the relationship of the SAM shortened longevity with some characteristics of their cells in culture. Investigations of lifespans of SAM embryo fibroblasts, survival of senescent nondividing cells, endogenous P-galactosidase activity, telomerase activity, and telomere length were conducted. There is correlation of longevity of SAMP1 (senescence prone strain), SAMR1 (accelerated senescence resistant strain), and CBA mice with proliferative lifespans of their embryo fibroblasts in vitro as well as with the survival time of nondividing senesced embryo fibroblasts. The premature senescence of SAMP1 and SAMR1 fibroblasts is associated with accelerated accumulation of the beta -galactosidase-positive cells. Terminal restriction fragments of chromosomes of SAMP1 are more heterogeneous than SAMR1 ones. There is relatively high telomerase activity tin comparison with CBA mice) in SAM embryos and cell cultures. This activity rapidly decreases during growth in vitro and is restored after spontaneous transformation occurring at a high frequency in both strains. SAM is a very promising model for studying the relationships of body aging and cell senescence in culture.

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.

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.