Growth arrest of proadipocytes at a distinct predifferentiation G1 state associated with cytotoxic responsiveness to 8-bromo cyclic AMP

Biological mechanisms for the loss of the differentiated phenotype by non-terminally differentiated adipocytes

The differentiation of 3T3 T proadipocyte stem cells is controlled at two related yet distinct states in the G1 phase of the cell cycle. They are designated GD and GD'. GD is the G1 state at which cells must growth arrest prior to differentiation, and GD' is the G1 state at which non-terminal differentiation occurs. Cells arrested at the GD and GD' states have distinct characteristics; yet cells at both states can mediate the integrated control of cellular proliferation and differentiation. In this paper we report on studies designed to further characterize the relationship of these two states, specifically to determine whether non-terminally differentiated GD'-arrested cells can be induced to lose the adipocyte phenotype and revert to the GD state. We report that retinoic acid (RA) and methyl isobutyl xanthine (MIX) can induce non-terminally differentiated GD'-arrested cells to lose the adipocyte phenotype without undergoing DNA synthesis. Such cells that have lost the adipocyte phenotype are also shown to remain in the G1 phase of the cell cycle and to reacquire most of the characteristics of GD-arrested cells. Most importantly, they demonstrate the capacity to redifferentiate without DNA synthesis. We therefore conclude that when non-terminally differentiated GD'-arrested cells are induced to lose the adipocyte phenotype they do indeed revert to the GD state and they thereby become more responsive to environmental influences which can further regulate the integrated control of cellular proliferation and differentiation.

Mechanisms for the initiation and promotion of carcinogenesis: a review and a new concept

Carcinogenesis in humans is a multistage process, and the two major stages have been designated initiation and promotion. Although the biochemical basis for initiation and promotion remains to be established, recent research has provided important insights into potentially significant biologic mechanisms. These data are reviewed, and a new concept of carcinogenesis is presented. This concept suggests that the initiation of carcinogenesis may result from cellular immortalization and the development of defects in the integrated control of stem cell proliferation and differentiation and that the promotion of carcinogenesis may result when such initiated stem cells develop aberrant autoregulatory growth-control properties.

Efficient differentiation of proadipocyte stem cells on nonadherent surfaces: evidence for differentiation without DNA synthesis

The differentiation of low density BALB/3T3 T proadipocytes that are cultured in standard tissue culture flasks can be induced by heparinized medium containing human plasma. It has been shown that under these conditions, cells first growth-arrest at a distinct state in the G1 phase of the cell cycle, designated GD, and thereafter differentiate within 8 to 12 days. In this paper, we report that the kinetics of proadipocyte differentiation can be significantly accelerated by culture of cells in differentiation-promoting medium on non-adherent surfaces, such as agarose-coated plates or bacteriological Petri dishes. Data also show that in a nonadherent microenvironment extensive differentiation can occur in the absence of DNA synthesis. This was established most convincingly by the demonstration that placement of mitotic cells in heparinized medium containing human plasma and hydroxyurea on agarose-coated Petri dishes induced 70-80% of the cells to GD arrest and differentiate without traversing the S phase of the cell cycle. It is concluded that under appropriate microenvironmental conditions metabolic events that occur solely in the late M or early G1 phase of the cell cycle can mediate the integrated control of proadipocyte proliferation and differentiation.

Differential mitogenic effects of methyl isobutyl xanthine and a tumor growth factor on G1-arrested 3T3 T proadipocytes at the predifferentiation GD state and the growth-factor deficiency GS state

Two growth-states exist in the G1 phase of the 3T3 T proadipocyte cell cycle. GD is the arrest state at which proadipocytes must growth-arrest prior to differentiation. GS is the arrest state at which proadipocytes growth-arrest following deprivation of serum or growth factors. In an attempt to further distinguish these arrest states, we have compared the relative ability of a variety of mitogens to induce GD- and GS-arrested cells to initiate DNA synthesis. The data show that GD-arrested cells at both high and low densities can be induced to proliferate by methyl-isobutyl-xanthine (MIX), whereas high and low density GS-arrested cells are not. The data also show that a tumor growth factor can stimulate the proliferation of both high and low density GD- and GS-arrested cells, whereas other agents are poor mitogens for high density GD-arrested cells. We conclude that MIX and a tumor growth factor (TUGF) can serve as probes to study the characteristics of the GD arrest state.