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A 72,000 mol wt protein designated PABP binds to the poly(A)+ track of messenger RNAs with high affinity and has been suggested to play an important role in mRNA metabolism in eucaryotic cells. We have employed a human PABP cDNA probe to study the expression of this gene at the mRNA level in BALB/c3T3 mouse cells under different growth conditions and in exponentially growing HeLa cells throughout the cell division cycle. We describe experiments which establish that in BALB/c3T3 cells the expression of this gene is growth factor regulated. Moreover, the gene behaves like a primary response gene in that its induction in quiescent cells does not require the prior synthesis of other growth factor-regulated proteins. In exponentially growing HeLa cells PABP mRNA is expressed throughout the cell division cycle indicating that the expression of this gene is not limited to a specific phase of the cell cycle.

To test the hypothesis that the omental lymphoid organ (OLO) made by peritoneal milky spots is either a source of haemopoietic (macrophage) progenitors or growth factors we attempted to culture OLO cells in vitro in a variety of assay combinations. With the culture in vitro in semisolid agar it was found that OLO cells do not form granulocyte-macrophage or macrophage colonies in response to stimulants. However, when in the same assay marrow cells were used as the targets and OLO-related preparations as stimulants it was observed that marrow cells formed exclusively macrophage colonies. These marrow cells, in response to stimulants derived from other organs, produced granulocyte-macrophage, granulocyte and macrophage colonies. OLO-related preparations tested for macrophage-colony stimulating activity included partly purified medium conditioned by OLO cells derived from mice, either injected with endotoxin or not, and medium conditioned by OLO cells after 14 days, liquid culture in vitro. While these results were observed in Swiss mice, C3H/W mice, which are genetically endotoxin-unresponsive, failed to show this reaction. These data may suggest that the local production of macrophage-colony stimulating activity in the peritoneal cavity could be one physiological role for OLO. OLO is the first organ in adult mice identified to stimulate exclusively macrophage colony growth, and not granulocyte-macrophage or pure granulocyte colonies.