Nuclear-cytoplasmatic shuttling of proteins in control of cellular oxygen sensing

Abstract

In order to pass through the nuclear pore complex, proteins larger than ∼40 kDa require specific nuclear transport receptors. Defects in nuclear-cytoplasmatic transport affect fundamental processes such as development, inflammation and oxygen sensing. The transcriptional response to O2 deficiency is controlled by hypoxia-inducible factors (HIFs). These are heterodimeric transcription factors of each ∼100-120 kDa proteins, consisting of one out of three different O2-labile α subunits (primarily HIF-1α) and a more constitutive 1β subunit. In the presence of O2, the α subunits are hydroxylated by specific prolyl-4-hydroxylase domain proteins (PHD1, PHD2, and PHD3) and an asparaginyl hydroxylase (factor inhibiting HIF-1, FIH-1). The prolyl hydroxylation causes recognition by von Hippel-Lindau tumor suppressor protein (pVHL), ubiquitination, and proteasomal degradation. The activity of the oxygen sensing machinery depends on dynamic intracellular trafficking. Nuclear import of HIF-1α and HIF-1β is mainly mediated by importins α and β (α/β). HIF-1α can shuttle between nucleus and cytoplasm, while HIF-1β is permanently inside the nucleus. pVHL is localized to both compartments. Nuclear import of PHD1 relies on a nuclear localization signal (NLS) and uses the classical import pathway involving importin α/β receptors. PHD2 shows an atypical NLS, and its nuclear import does not occur via the classical pathway. PHD2-mediated hydroxylation of HIF-1α occurs predominantly in the cell nucleus. Nuclear export of PHD2 involves a nuclear export signal (NES) in the N-terminus and depends on the export receptor chromosome region maintenance 1 (CRM1). Nuclear import of PHD3 is mediated by importin α/β receptors and depends on a non-classical NLS. Specific modification of the nuclear translocation of the three PHD isoforms could provide a promising strategy for the development of new therapeutic substances to tackle major diseases.

The importin α/β-specific inhibitor Ivermectin affects HIF-dependent hypoxia response pathways

“…Appropriate subcellular localisation of the HIF-subunits is crucial for the regulation of HIF-dependent transcription. HIF-1α, HIF-2α, as well as HIF-1β can shuttle between nucleus and cytoplasm and Brought to you by | University of Georgia Libraries Authenticated Download Date | 10/6/15 3:59 AM nuclear import is mediated by the importin α/β transport receptors (Depping et al, 2015).…”

“…Accordingly, PHD2-mediated hydroxylation of HIF-1α predominantly occurs in the cell nucleus depending on very dynamic subcellular trafficking (Pientka et al, 2012). These data indicate that nuclear translocation of HIF-1α is a very important regulatory step in the cellular response towards reduced oxygen availability (Depping et al, 2015).…”

Filamin A interacts with the coactivator MKL1 to promote the activity of the transcription factor SRF and cell migration

“…In contrast to small molecules that enter the nucleus without further regulation procedures, these types of large proteins, and even MKL1 on its own, require association with transport factors like importins. Importins bind nuclear localization signals (NLS) located on the protein, for example as on MKL1, this way allowing the Importin--NLS--MKL1 complex to interact with the nuclear pore and successfully pass through its channel (Depping R., Jelkmann W. et al, 2015). Based on the stated arguments of size and pore restrictions, a collective MKL1--full--length--FLNA entrance into the nucleus is highly doubtful, however a translocation of FLNA fragments together with the transcription factor androgen receptor has been demonstrated (Loy C., Sim K. et al, 2003).…”

Hypoxic Upregulation of ARNT (HIF-1β): A Cell-Specific Attribute with Clinical Implications

“…Translocation of ARNT from the cytoplasm into the nucleus is mediated by importins as also demonstrated for other HIF family members [30,31]. Blocking of this specific process was proposed as a novel way to suppress HIF signalling [30].…”

“…Blocking of this specific process was proposed as a novel way to suppress HIF signalling [30]. Whether ARNT shuttles, back into the cytoplasm is unknown.…”