Heterogeneous Nuclear Ribonucleoprotein K Is a Transcription Factor

Abstract

The CT element is a positively acting homopyrimidine tract upstream of the c-myc gene to which the well-characterized transcription factor Sp1 and heterogeneous nuclear ribonucleoprotein (hnRNP) K, a less well-characterized protein associated with hnRNP complexes, have previously been shown to bind. The present work demonstrates that both of these molecules contribute to CT element-activated transcription in vitro. The pyrimidine-rich strand of the CT element both bound to hnRNP K and competitively inhibited transcription in vitro, suggesting a role for hnRNP K in activating transcription through this single-stranded sequence. Direct addition of recombinant hnRNP K to reaction mixtures programmed with templates bearing singlestranded CT elements increased specific RNA synthesis. If hnRNP K is a transcription factor, then interactions with the RNA polymerase II transcription apparatus are predicted. Affinity columns charged with recombinant hnRNP K specifically bind a component(s) necessary for transcription activation. The depleted factors were biochemically complemented by a crude TFIID phosphocellulose fraction, indicating that hnRNP K might interact with the TATA-binding protein (TBP)-TBP-associated factor complex. Coimmunoprecipitation of a complex formed in vivo between hnRNP K and epitope-tagged TBP as well as binding in vitro between recombinant proteins demonstrated a protein-protein interaction between TBP and hnRNP K. Furthermore, when the two proteins were overexpressed in vivo, transcription from a CT element-dependent reporter was synergistically activated. These data indicate that hnRNP K binds to a specific cis element, interacts with the RNA polymerase II transcription machinery, and stimulates transcription and thus has all of the properties of a transcription factor.

Editing of hnRNP K protein mRNA in colorectal adenocarcinoma and surrounding mucosa

“…K protein is upregulated in malignant cells (Dejgaard et al, 1994;Mandal et al, 2001;Li et al, 2004), and it regulates the expression of genes involved in mitogenic responses, indicating that it has a direct role in cancerogenesis (Michelotti et al, 1995(Michelotti et al, , 1996Ostrowski et al, 2004). We have shown that following mitogenic stimulation, K protein is recruited to gene loci known to be upregulated in malignant states, including c-myc .…”

“…As K protein is upregulated in tumour cells (Dejgaard et al, 1994;Mandal et al, 2001;Li et al, 2004) and modulates the expression of genes involved in mitogenic responses (Michelotti et al, 1995(Michelotti et al, , 1996Mandal et al, 2001;Ostrowski et al, 2004), altering its properties and introduction of a new phosphorylation site could make K protein either more or less conducive for the cellular processes that lead to tumorigenic state. If so, identification of this novel tumour-associated K protein isoform may prove to be a useful diagnostic, prognostic and/or therapeutic indicator.…”

Altered patterns of expression of members of the heterogeneous nuclear ribonucleoprotein (hnRNP) family in lung cancer

“…hnRNP proteins are involved in a variety of key cellular functions such as mRNA splicing [12], stabilization [13,14], nucleo-cytoplasmic transport [15 Á/17], and transcriptional control [18]. Some of them can also control the transcription of specific genes [19,20]. Several evidences, summarized in the following paragraph, support the hypothesis that the hnRNP proteins may be relevant in human carcinogenesis.…”

“…Other hnRNP protein potentially relevant in tumorigenesis is hnRNP K. In the nucleus, this protein can bind directly to the promoter region of the human c-myc gene and functions as a transcription factor [19]. When localized to the cytoplasm, hnRNP K inhibits translation of specific mRNAs such as 15-lipoxygenase mRNA [36].…”

Transforming activity of EWS/FLI is not strictly dependent upon DNA-binding activity

“…Increasing evidence shows that some proteins, exempli®ed by hnRNPK (Michelotti et al, 1996) and the Drosophila transcription factor bicoid (Dubnau and Struhl, 1996;Rivera-Pomar et al, 1996), have the ability to act on both levels (Ladomery, 1997). Of particular relevance with respect to EWS/FLI is the ETS family protein PU.1.…”