Transforming growth factor β1 (TGF-β1) inhibits retinoblastoma gene expression but not pRB phosphorylation in TGF-β1-growth stimulated colon carcinoma cells

The response of the retinoblastoma (RB) gene and its product (pRB) to transforming growth factor β1 (TGF-β1) was studied in three types of colon carcinoma cells derived from the same parental line. TGF-β1 was a growth inhibitor for two enterocytic-differentiated lines, a growth stimulator for two undifferentiated lines, and had no effect on two goblet cell-differentiated lines. TGF-β1 treatment for 3 days decreased RB gene expression and pRB level two- to threefold in each responsive line. When treated with TGF-β1 beginning in early G₁, enterocytic cells were arrested in G₁ and pRB remained underphosphorylated and in low abundance. Neither goblet cell line exhibited these responses to TGF-β1 because they were shown to lack TGF-β1 type I and II receptors. Thus during colonocyte differentiation goblet cells lose responsiveness to TGF-β1 by down-regulating TGF-β1 receptors, while enterocytic cells retain and exhibit responsiveness to TGF-β1 through modulations of pRB. Both of the undifferentiated lines exhibited mixed responses to TGF-β1: a decrease in total amount of RB mRNA and pRB protein yet an increase in pRB phosphorylation consistent with increased cell cycling. Therefore, TGF-β1 controls RB function by two separable mechanisms, the regulation of pRB phosphorylation and the control of RB mRNA and protein level.