Rap1 reverses transcriptional repression of TGF‐β type II receptor by a mechanism involving AP‐1 in the human pancreatic cancer cell line, UK Pan‐1

Abstract

The TGF-beta signaling pathway has potent anti-mitogenic effects in epithelial cells and loss of negative growth regulation is often associated with increased tumorigenicity. The human pancreatic ductal adenocarcinoma cell line, UK Pan-1, which expresses DPC4, is not highly responsive to TGF-beta due to transcriptional repression of TGF-beta type II receptor (RII). Here, we show that UK Pan-1 cells transfected with a plasmid to overexpress rap1 protein (UK/rap1) causes an increase in RII transcription and restores sensitivity to TGF-beta growth inhibition. The overexpression of rap1 was associated with diminished ras signaling as measured by ras binding domain (RBD)-binding assays. Electrophoretic mobility shift assays (EMSA) analysis revealed increased binding of nuclear proteins to a previously identified positive regulatory element (PRE1) of the RII promoter in rap1 transfected cells. Competition with an oligo containing the AP-1 consensus site was able to inhibit this binding of nuclear proteins to the PRE1 region. Further EMSA analysis using antibodies to various AP-1 components revealed that junB antibodies partially depleted the increase in binding to the PRE1 seen in UK/rap1 cells while antibodies to other AP-1 constituents such as c-jun, c-fos, and ATF-1 had no effect on binding. Consistent with this data, transient transfection of UK Pan-1 cells with junB resulted in greater RII transcription (twofold) as measured by RII-luciferase assay. Mutation of the AP-1 site inhibited junB-mediated or rap1-mediated increases in RII promoter activity. These data suggest that rap1 signaling may mediate an increase in RII transcription via increased binding of nuclear factors including junB to the PRE1 region of the RII promoter.

Antiestrogens Induce Growth Inhibition by Sequential Activation of p38 Mitogen-Activated Protein Kinase and Transforming Growth Factor-β Pathways in Human Breast Cancer Cells

“…Several transcription factors were shown to participate in the transcriptional control of T␤RII (42)(43)(44)(45)(46). Some of them appear to be targets of p38 and might be involved in antiestrogen induction of T␤RII transcription (47,48).…”

Downstream targets of heterogeneous nuclear ribonucleoprotein A2 mediate cell proliferation

“…A number of genes that play auxiliary roles in cell mitogenicity were also regulated by hnRNP A2. These included IGFBP6 and IGFBP7 , which are believed to modulate the mitogenic activity of insulin‐like growth factor 1 28; TGFBR2 , which encodes the type II receptor for transforming growth factor β, an anti‐mitogenic cytokine in epithelial cells 29; and AKAP12 , which generates a protein kinase C substrate, SSeCKS, a negative mitogenic regulator 30. Identification of these genes downstream of hnRNP A2 suggested a capacity of this protein to regulate cell mitogenicity.…”

Bag‐1 promotes cell survival through c‐Myc‐mediated ODC upregulation that is not preferred under apoptotic stimuli in MCF‐7 cells

“…In addition, direct interaction with Hsp70/Hsc70 system can link Bag‐1 isoforms to c‐Jun, which is a component of a transcription factor complex, Activator Protein‐1 (AP‐1). c‐Jun acts as an oncoprotein in collaboration with another oncoprotein, c‐Fos, by elevating transcription and protein synthesis thus leading to cancer development . Bag‐1 isoforms interact with various NHRs, which are transcription factors that bind to the promoter region of a target gene on DNA, and therefore through these interactions, target gene transcription gets affected either positively or negatively depending on the NHR interaction type .…”