p19 is a highly conserved 19 kD cytosolic
protein that undergoes phosphorylation in response to diverse extracellular factors in mammalian cells. Its expression is abundant in brain and testis and is developmentally regulated. To gain insights regarding its function, we analyzed the expression of p19
mRNA in a variety of cell types during induction of differentiation. Murine
erythroleukemia cells showed a moderate increase followed by a marked decrease in the abundance of p19
mRNA during induction of differentiation. In murine C2 myoblasts and primary fetal rat osteoblasts, p19
mRNA was abundant in replicating cells and decreased to undetectable levels during differentiation. In resting human peripheral blood lymphocytes, p19
mRNA was virtually undetectable but was strongly induced during blast transformation of both B and T cells. In rat liver, p19
mRNA was abundant on embryonic day 17 and decreased during early postnatal development. Upon fractionation of adult rat liver cells by centrifugal elutriation, p19
mRNA was not detected in hepatocytes while a low level was observed in a fraction enriched in non-parenchymal epithelial cells. CCl4-induced liver regeneration resulted in induction of p19
mRNA in hepatocytes. Primary cultures of embryonic and neonatal rat brain were analyzed by indirect immunofluorescence using co-staining with stage-specific markers. p19 expression was restricted to immature neurons and oligodendrocyte precursors. In contrast to the other cell types examined, the neuronal and glial precursors that express p19 were shown, using
BrdU labeling, to be postmitotic both in primary culture and in vivo. The data demonstrate widespread, stage-specific expression of p19 and suggest that the
protein exerts a general, lineage-independent function during induction of differentiation of mammalian cells. In view of the available evidence on the stimulation of
serine phosphorylation of p19 by several
growth factors, our working hypothesis is that phosphorylation of p19 may be involved in the mechanism by which
growth factors control cell differentiation.