The Importance of Being Big

The ultimate stem cell, the oocyte, is frequently very large. For example, Drosophila and Xenopus oocytes are ∼105 times larger than normal somatic cells. Importantly, once the large oocytes are fertilized, the resulting embryonic cells proliferate rapidly. Moreover, these divisions occur in the absence of cell growth and are not governed by normal cell cycle controls. Observations suggest that mitogens and cell growth signals modulate proliferation by upregulating G1-phase cyclins, which in turn promote cell division. Like embryonic cells, the proliferation of cancer cells is largely independent of mitogens and growth factors. This occurs, in part, because many proteins that are known to modulate G1-phase cyclin activity are frequently mutated in cancer cells. Interestingly, we have found that both the expression and the activity of G1-phase cyclins is modulated by growth rate and cell size in yeast. These and other data suggest that proliferative capacity correlates with cell size. Thus, a major goal of our laboratory is to use yeast to investigate the relationship between proliferation rate, G1-phase cyclins, growth rate, and cell size. The elucidation of this relationship will help clarify the role of cell size in promoting proliferation in both normal and cancer cells.