Differential Expression of Classical Nuclear Transport Factors During Cellular Proliferation and Differentiation

Abstract

We recently cloned six human importin a proteins that transport specific substrates in complex with importin β into the nucleus. We now compared their absolute expression levels in different human cell lines. We examined their expression regulation during human cell proliferation and differentiation by means of specific antibodies. Proliferation inhibition by starvation of HeLa and HaCaT cells led to a marked decrease in the expression of various nuclear transport factors. In contrast, re-addition of serum increased α-importin expression. We analyzed two models for cell differentiation and found differential importin regulation. Stimulation of rat pancreatic AR42J cell differentiation towards a neuroendocrine phenotype with activin A or towards an acinar phenotype with dexamethasone, caused strong upregulation of importin α3 and α4 expression. Phorbol ester-induced differentiation of human leukemia (HL60) cells towards a macrophage phenotype led to downregulation of importin α1 and α4 expression after 72 hours. Similarly, importins α1 and α4 displayed a strong downregulation when HL60 cells were directed towards a neutrophil phenotype by DMSO treatment. This study is the first to assess all the human importin α isoforms in documenting differential nuclear transport factor regulation during cell proliferation and differentiation.

DNA microarray analysis of changes in gene expression induced by 1,25-dihydroxyvitamin D3 in human promyelocytic leukemia HL-60 cells

“…The findings were generally consistent with those reported by Koehler et al who demonstrated that importin proteins except for importin α5 were down-regulated in their expression in TPA-treated cells (11). A and B).…”

“…However, there has been no report describing the differentiation-associated changes in importin gene expression. The present study revealed the changes in gene expression compatible with the reported observations for protein expression (11). Thus, this is the first paper to describe the down-regulated gene expression of importins in association with differentiation toward the monocyte/macrophage lineage in HL-60 cells.…”

“…Importins are protein factors involved in the transport of macromolecules from the cytoplasm to nucleus (11). Previously, changes in importin expression have been examined at the protein level in 12-O-tetradecanoylphorbol-13-acetate (TPA)-treated HL-60 cells (11).…”

“…Previously, changes in importin expression have been examined at the protein level in 12-O-tetradecanoylphorbol-13-acetate (TPA)-treated HL-60 cells (11). DVD and TPA often exhibit similar effects including inducing the differentiation of…”

Identification of Cargo Proteins Specific for Importin-β with Importin-α Applying a Stable Isotope Labeling by Amino Acids in Cell Culture (SILAC)-based in Vitro Transport System*

“…It is widely accepted that Imp-␤ and some other Imp-␤ family carriers are abundant proteins in cells (44), but Imp-␤ is no more abundant than the other members of the family in our observation (data not shown). In addition, cellular contents of Imp-␣s are roughly comparable with that of Imp-␤, although expression levels of each of the family members vary in different cell types and tissues (7,(45)(46)(47). Therefore, it is not only the cellular concentration of carriers that determines import activity of each pathway.…”