Lack of effect of short-term micronized progesterone on bone turnover in postmenopausal women.

A number of studies suggest that progestogens have beneficial effects on bone in postmenopausal women, particularly in combination with estrogen, although these studies have used derivatives that may have estrogenic and androgenic properties in addition to effects mediated by progesterone receptors. Progesterone itself affects only progesterone and glucocorticoid receptors. However, until the development of micronized progesterone (MP), absorption of progesterone preparations was too low to be clinically useful. MP has similar protective effects on the uterus and fewer effects on the lipid profile than other preparations, but its effects on bone are unknown. We tested the hypothesis that MP would alter bone turnover, as measured by serum and urine biochemical markers, in postmenopausal women. Fourteen women aged 65 or over who were not on estrogen replacement received a 6-week course of daily MP (200 mg). Markers of bone turnover were measured in serum and urine collected at baseline, at 6 weeks on MP, and 6 weeks after termination of MP. We also measured total and high-density lipoprotein (HDL) cholesterol and progesterone levels during the study. Markers of bone resorption were urinary free deoxypyridinoline cross-linked N-telopeptides and C-telopeptides of type I collagen. Markers of bone formation were serum osteocalcin, bone alkaline phosphatase, and type I C-terminal and N-terminal procollagen peptides. Using repeated measures analysis of variance, markers of bone formation and resorption did not change with MP treatment in spite of an increase in progesterone levels in all women. We conclude that 6-week treatment with MP alone does not have an effect on bone turnover in postmenopausal women in spite of high physiological levels. These data suggest that effects on bone demonstrated using other progestogen preparations might be due to androgenic or estrogenic effects or that progesterone may not affect bone in estrogen-deficient women.

Rat embryo fibroblasts immortalized with simian virus 40 large T antigen undergo senescence upon its inactivation.

Introduction of simian virus 40 T antigen into rodent fibroblasts gives rise to cells that can proliferate indefinitely but are dependent upon it for maintenance of their growth once the normal mitotic life span has elapsed. Inactivation of T antigen in these immortalized cells causes rapid and irreversible cessation of growth. To determine whether this growth arrest is associated with entry into senescence, we have undertaken a genetic and biological analysis of conditionally immortal (tsa) cell lines derived by immortalizing rat embryo fibroblasts with the thermolabile tsA58 T antigen. This analysis has identified the following parallels between the tsa cells after inactivation of T antigen and senescent rat embryo fibroblasts: (i) growth arrest is irreversible; (ii) it occurs in G1 as well as G2; (iii) the G1 block can be partially overcome by stimulation with 20% fetal calf serum, but the G2 block cannot be overcome; (iv) 20% fetal calf serum induces c-fos, but c-myc is unaltered; and (v) fibronectin and p21(Waf1/Cip1/Sdi1) are upregulated upon growth arrest. These results suggest that T-antigen-immortalized fibroblasts are committed to undergo senescence but are prevented from undergoing this process by T antigen. Inactivation of T antigen removes this block and results in senescence of the cells. Thus, these cell lines may represent a powerful system for study of the molecular basis of entry into senescence.

The H-2KbtsA58 transgenic mouse: a new tool for the rapid generation of novel cell lines.

The ability to generate expanded populations of individual cell types able to undergo normal differentiation in vitro and in vivo is of critical importance in the investigation of the mechanisms that underly differentiation and in studies on the use of cell transplantation to repair damaged tissues. This review discusses the development of a strain of transgenic mice that allows the direct derivation of conditionally immortal cell lines from a variety of tissues, simply by dissociation of the tissue of interest and growth of cells in appropriate conditions. In these mice the tsA58 mutant of SV40 large T antigen is controlled by the interferon-inducible Class I antigen promoter. Cells can be grown for extended periods in vitro simply by growing them at 33 degrees C in the presence of interferon, while still retaining the capacity to undergo normal differentiation in vivo and in vitro. In addition, it appears that cell lines expressing mutant phenotypes can readily be generated by preparing cultures from appropriate offspring of matings between H-2KbtsA58 transgenic mice and mutant mice of interest.

The biological clock that measures the mitotic life-span of mouse embryo fibroblasts continues to function in the presence of simian virus 40 large tumor antigen.

Normal mammalian fibroblasts cultured in vitro undergo a limited number of divisions before entering a senescent phase in which they can be maintained for long periods but cannot be induced to divide. In rodent fibroblasts senescence can be prevented by expression of simian virus 40 large tumor antigen (T antigen). Cells expressing T antigen can proliferate indefinitely; however, such cells are absolutely dependent upon continued expression of T antigen for maintenance of growth; inactivation of T antigen results in a rapid and irreversible entry into a postmitotic state. To determine when, after the initial expression of T antigen, fibroblasts become dependent upon it for continued growth, we serially cultivated embryonic fibroblasts prepared from H-2Kb-tsA58 transgenic mice. We show that these fibroblasts become dependent upon T antigen for maintenance of proliferation only when their normal mitotic life-span has elapsed and that the biological clock that limits the mitotic potential continues to function normally, even in cells expressing this immortalizing gene. Our results suggest that random accumulation of cellular damage is unlikely to be the factor that limits fibroblast division but support the hypothesis that senescence is regulated via a genetic program.