PTF1alpha/p48 transcription factor couples proliferation and differentiation in the exocrine pancreas [corrected].

BACKGROUND & AIMS: The basic helix-loop-helix transcription factor pancreas-specific transcription factor 1alpha (PTF1alpha)/p48 is critical for committing cells to a pancreatic fate and for the maintenance of the differentiated state in acinar cells. The aim was to analyze the ability of p48 to modulate cell proliferation, its relationship with cell differentiation, and the mechanisms involved therein. METHODS: Pancreatic and nonpancreatic cells were transfected with p48 cDNA, and the effects on cell proliferation were examined. The effects on cell cycle regulators were analyzed by Western blotting and RT-PCR; transient transfection assays were used to analyze promoter regulation. RESULTS: p48 Inhibited proliferation of acinar and nonacinar cells by inducing a delay in G1-S progression through the up-regulation of p21 CIP1/WAF1 and p27 KIP1 and the down-regulation of cyclin D2. A 2-fold increase in p21 CIP1/WAF1 mRNA and in the activity of the p21 CIP1/WAF1 promoter was observed. The growth inhibition action of p48 was not associated with exocrine differentiation or with apoptosis. The antiproliferative effects were dependent on the COOH-terminal region of p48 and did not require the bHLH domain. Loss of p48 expression occurring during acinar-to-ductal transitions, characteristic of chronic pancreatitis, was associated with an increase of cell proliferation in ductal complexes. CONCLUSIONS: The results indicate that p48 couples cell proliferation and cell differentiation in the exocrine pancreas, thus contributing to tissue homeostasis. These effects may play a role in the increased risk for pancreatic cancer associated with chronic pancreatitis.

Direct binding of Smad1 and Smad4 to two distinct motifs mediates bone morphogenetic protein-specific transcriptional activation of Id1 gene.

Bone morphogenetic proteins (BMPs) are potent inhibitors of myoblast differentiation and inducers of bone formation both in vivo and in vitro. Expression of Id1, a negative regulator of basic helix-loop-helix transcription factors, is up-regulated by BMPs and contributes to the antimyogenic effects of this family of cytokines. In this report, we have identified a specific BMP-2 immediate early response enhancer in the human Id1 gene. Transcriptional activation of the enhancer was increased by overexpression of BMP-responsive Smads, and Smad4 and was completely abrogated in Smad4-deficient cells. Deletion analysis demonstrates that the responsive region is composed of two separate DNA binding elements, a set of overlapping GC boxes, which bind BMP-regulated Smads upon BMP stimulation, and three repeats of CAGAC boxes. Gel shift and oligonucleotide pull-down assays demonstrated that these two types of motifs were capable of binding their corresponding Smads. However, deletion or mutation of either DNA binding element was nonadditive, since disruption of either GC or CAGAC boxes resulted in complete or severe loss of BMP-2 responsiveness. These data suggest the simultaneous requirement of two independent DNA binding elements to allow functional cooperativity of BMP-regulated Smads and Smad4 in BMP-activated gene promoters.