Regulation of HTLV-1 transformation

Abstract

Human T-cell leukemia virus type 1 (HTLV-1) is the only identified oncogenic human retrovirus. HTLV-1 infects approximately 5-10 million people worldwide and is the infectious cause of adult T-cell leukemia/lymphoma (ATL) and several chronic inflammatory diseases, including HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), dermatitis, and uveitis. Unlike other oncogenic retroviruses, HTLV-1 does not capture a cellular proto-oncogene or induce proviral insertional mutagenesis. HTLV-1 is a trans-activating retrovirus and encodes accessory proteins that induce cellular transformation over an extended period of time, upwards of several years to decades. Inarguably the most important viral accessory protein involved in transformation is Tax. Tax is a multifunctional protein that regulates several different pathways and cellular processes. This single viral protein is able to modulate viral gene expression, activate NF-κB signaling pathways, deregulate the cell cycle, disrupt apoptosis, and induce genomic instability. The summation of these processes results in cellular transformation and virus-mediated oncogenesis. Interestingly, HTLV-1 also encodes a protein called Hbz from the antisense strand of the proviral genome that counters many Tax functions in the infected cell, such as Tax-mediated viral transcription and NF-κB activation. However, Hbz also promotes cellular proliferation, inhibits apoptosis, and disrupts genomic integrity. In addition to viral proteins, there are other cellular factors such as MEF-2, superoxide-generating NAPDH oxidase 5-α (Nox5α), and PDLIM2 which have been shown to be critical for HTLV-1-mediated T-cell transformation. This review will highlight the important viral and cellular factors involved in HTLV-1 transformation and the available in vitro and in vivo tools used to study this complex process.

Keywords: HTLV-1; Hbz; Tax; oncogenesis; transformation.

Figure 1. The HTLV-1 proviral genome

The structural and enzymatic proteins (Gag, Pro, Pol, and Env), in addition to five viral accessory proteins (Tax, Rex, p12, p13, and p30), are encoded by sense transcripts derived from the 5′ LTR promoter. Hbz is the only viral accessory/regulatory protein encoded by an antisense transcript derived from a 3′ LTR promoter.

Figure 2. Tax-mediated activation of the NF-κB pathway

HTLV-1-infected cells utilize Tax to stimulate both the classical (A) and alternative (B) NF-κB pathways. (A) Tax interacts with NEMO to facilitate recruitment of the IKK subunits α and β, leading to activation of the IKK complex. Tax can also interact with these IKK subunits directly to promote their dimerization. The activated IKK complex then phosphorylates IκB, subsequently releasing p50/p65 and permitting their translocalization to the nucleus. Tax can also bind to p50/p65 subunits in the nucleus to induce dimer formation and recruit co-activators such as CBP/p300. (B) The alternative pathway depends on phosphorylation of the IKK complex by NIK. Once activated, the IKK complex then phosphorylates p100/RelB, stimulating proteasomal processing of p100 to p52. Tax accelerates this process by recruiting IKK subunit α to p100, thereby allowing p52/RelB to translocate to the nucleus. Similar to the classical pathway, Tax may additionally promote the dimerization and DNA binding activity of nuclear p52. (C) Tax-mediated activation of these respective NF-κB pathways leads to several downstream cellular effects, including deregulation of the cell cycle, stimulation of T-cell proliferation, and increased expression of anti-apoptotic proteins, ultimately promoting viral transformation of HTLV-1-infected cells.

Figure 3. HTLV-1 co-culture immortalization assay

Human PBMCs are obtained from a healthy blood donor and immediately isolated through Ficoll density gradient separation. Freshly isolated PBMCs are subsequently co-cultured with lethally irradiated virus producer cells where CD4⁺ T cells will become infected with HTLV-1. Within the infected cell, Tax and Hbz viral proteins are expressed. Over a period of several weeks, these viral proteins play a prominent role in HTLV-1-mediated T-cell transformation by influencing a multitude of different cellular functions, as depicted above.