HIV infection and the risk of cancer: tumorigenicity of HIV-1 auxiliary proteins

The association of viruses with human cancers is an established phenomenon, supported by clear evidence that shows that these viruses can influence key physiological events, leading to the development and advancement of malignancies. In fact, studies show that globally one in six cancers can be attributable to an infectious etiology, with the majority of the infectious agents being viruses [1,2]. This fraction is likely to be an underestimation in developing countries where the population harbor a significantly larger proportion of virus-associated cancers when compared with elsewhere in the world. While there had been tentative reports of viruses as cancer causing agents in humans from the early 1900s, the first oncogenic virus, Epstein–Barr virus (EBV), was reported in 1964 in cultured Burkitt lymphoma cells [3]. To date, at least seven viruses have been named as having established involvement in human cancers, with a few others having possible causative links [2]. In many instances these viruses are part of common infections, establishing chronic long-term associations with the host, and do not cause cancer in a majority of cases. Additionally, viral infection alone is not sufficient for driving the complex multistep process of oncogenesis, but is dependent on cumulative events, with the state of the host immunity being a major determining factor.

Regardless of virus genome type (RNA/DNA), it appears that the induction of tumors is a biological accident rather than an intended outcome of the viral life cycle. Viruses infect cells in order to exploit the host’s cellular machinery and establish viral replication, and the long-term establishment of these viruses in the genome of host cells provide ample time for the accumulation of mutagenic events which eventually lead to the development of tumors. Contrary to expectation, the tumor mass is not laden with infectious virions, and in some cases (for example:  in the case of EBV or Kaposi sarcoma-associated herpes virus [KSHV]) the viruses are in a latent state in most cancer cells. Eventually, the viral genome could be entirely lost to the tumor cells, without the malignant phenotype being affected [4]. The ‘hit-and-run’ theory has been proposed to describe the phenomenon where viruses are responsible for the triggering event in cancer formation but are no longer needed over time as mutations accumulate to support cancer progression. This means that there is a potential underestimation of the number of cancers driven by viruses, if the presence of the virus is used to establish etiology. Nevertheless, oncogenic viruses have played an extremely significant role in the discovery of oncogenes and tumor suppressor genes. There is no doubt that studies on cellular transformation by viruses and the association of viral proteins with host proteins have enhanced our understanding of cancer biology and thus has been pivotal in the field of cancer research. The continued efforts in defining the role of viruses in tumorigenesis is therefore essential.

Although the HIV virus is not classified as oncogenic, people living with the virus are at a significantly higher risk of developing certain malignancies. While virus-induced immune suppression and dysfunctions of the immune system remain the primary causative link, the persistence of malignancy despite successful antiretroviral therapy (ART) has prompted a deeper look into the mechanisms driving HIV-associated cancers. Accumulating evidence reveals that HIV and its auxiliary proteins can directly promote carcinogenesis in a manner similar to mechanisms described for known oncogenic viruses. The emerging concept of HIV as an oncogenic virus is rapidly gaining credibility as more reports emerge supporting this. Viral protein can be detected in the serum/plasma of patients even when HIV RNA levels are undetectable. These proteins have been shown to affect bystander cells within their microenvironment, in ways which can lead to oncogenesis.

HIV-1 Tat

The HIV-1 Tat protein is an essential factor in HIV DNA transcription and survival, with several non-viral replication associated functions described, linked to HIV-related diseases including cancer. This small protein (86–102 amino acids) is secreted from both latently infected cells as well as cells with active HIV replication and has been shown to become internalized by bystander cells [5]. The Tat protein contains multiple domains shown to promote a range of cellular events, one of which being co-operation with transcription factors leading to alterations in gene expression known to support oncogenic events. For instance, exposure of healthy B cells to Tat promotes the oncogenic co-localization of the c-MYC and IGH loci, a genetic hallmark of aggressive lymphomas which are over-represented among HIV-infected individuals [6]. Although not host to HIV, Tat can be detected within the tumor cells of Burkitt lymphoma (BL) patients. The prevalence of BL among people infected with HIV is disproportionately high when compared with the immunocompetent population, with some reports showing no to marginal improvement in incidence despite the rollout of ART [7]. Mechanistically, the Tat protein has the ability to directly affect MYC transcription through complexing with the AP-1 factor JunB, a known regulator of the c-MYC promoter [8]. In Kaposi sarcoma (KS), an AIDS-defining cancer, Tat promotes tumorigenesis via induction of viral IL-6 and the PI3K/phosphate and tensin homolog/AKT/GSK-3b pathway, and can act synergistically with Orf-K1, a KSHV protein, to alter expression of cellular miRNAses [9]. Additionally, genome-wide studies demonstrate that Tat has affinity for binding widely within the cellular genome, at specific sites within gene regulatory regions [10]. Thus, the true impact of Tat on HIV-related oncogenesis remains as yet largely unexplored.

HIV-1 Nef

An oncogenic role for the HIV Nef protein has been demonstrated in several cancer types. This 27-kDa myristoylated protein is abundantly expressed during early stages of infection and is responsible for enhancing the infectivity of progeny virions through mechanisms which are not yet completely understood. Plasma Nef levels correlate with plasma HIV RNA in untreated patients and can be detected in the serum of approximately half of patients on ART, indicating that the source may be from HIV reservoirs [11]. Like Tat, circulating Nef can become internalized by bystander cells where it can alter cellular function. In KS, which remains one of the most common cancers among people living with HIV, Nef has been shown to interact with the KSHV through synergizing with viral IL-6 to enhance angiogenesis and tumorigenesis. The use of in vivo models such as nude mice revealed that mechanistically this is supported via activation of the AKT pathway through induction of cellular miR-718, an inhibitor of phosphate and tensin homolog [12]. Nef has also been linked to the development of lymphoma. Through nanotubule conduits, macrophages shuttle Nef into B lymphocytes and the viral protein has been found to play an active role in the formation of these actin conduits. BL cells expressing Nef has enhanced expression of lymphoma-driving genes, one of which being activation-induced cytidine deaminase, an essential driver of translocations, including the oncogenic MYC/IGH translocation [13]. Despite these reports, the cellular mechanisms of Nef-driven lymphomagenesis remain largely undefined, and lymphoma remains one of the most prevalent cancers among HIV-infected individuals. Non-small-cell lung cancer is one of the most frequent non-AIDS defining malignancies. In a study investigating the role of Nef in HIV-related lung disease, the viral protein was shown to promote changes in metabolism, cell survival and invasion and increase angiogenesis of lung cancer cells [14]. This could be as a result of a global decrease in miRNA and miRNA-biogenesis protein expression brought about by the presence of Nef, thus indicating a direct involvement of Nef in tumorigenesis.

HIV-1 P17

The structural HIV-1 p17 matrix protein, which plays a regulatory role during various phases of the virus life cycle such as viral entry and replication, gets secreted from HIV-1-infected cells into the extracellular space where it accumulates. As is the case for Nef and Tat, p17 can be detected in serum even in patients on highly active antiretroviral therapy who are virally suppressed [15]. This viral protein has been shown to influence the biological function of many different cell types primarily through interactions between its N-terminal AT20 epitope and cell surface receptors. In particular, p17 has a strong association with angiogenesis. Through high affinity binding to CXCR2, which is related to the IL-8 receptor CXCR1, p17 was shown to promote the formation of capillary-like structures on endothelial cells [16]. In lymphomagenesis, p17 was found to bind and become internalized by B lymphocytes, with higher affinity for EBV-infected cells which preferentially express CXCR2 [17]. A natural variant of p17, S75X, bound more efficiently to these cells and specifically and directly impacted on tumorigenesis – the expression of oncogenic latent membrane protein-1 was enhanced, leading to activation of Akt, ERK1/2 and STAT3 signaling, with associated upregulation of cyclin D2 and D3, and enhanced proliferation. Although not a HIV-associated malignancy, breast cancer in HIV-positive patients is generally more aggressive, with poorer outcomes despite these patients being on ART [18]. When the human breast cancer cell line MDA-MB 231 was exogenously exposed to p17, the migratory abilities and invasiveness of the cancer cells were enhanced. Once again, these effects were mediated via the binding of the viral protein to CXCR2, and this led to the activation of the ERK1/2 signaling pathway. P17 may therefore likely be one of the determining factors of the clinical course of breast cancer in HIV-positive patients.

HIV-1 gp120

HIV-1 envelope glycoprotein gp120 is yet another viral protein that has demonstrated the ability to drive oncogenic transformation. Glioblastomas are over-represented among people infected with HIV, as is the case for other cancers mentioned above, with patients having an inferior median survival, despite successful viral control [19]. Studies, both primary as well as established glioma cell lines, found that when exposed to gp120, these cells have higher proliferation rates, migratory abilities and survival. Additionally, these cells had enhanced expression and activity of certain glycolytic enzymes, with increased glucose uptake and oxygen consumption. This shift in glycolysis, also known as the Warburg effect, is a typical feature of cancer cells which enhanced proliferation, migration and promotion of DNA, protein and lipid synthesis. Notably, mice expressing the viral protein in their brain developed larger tumors than their control counterparts, with shorter median survival. A study on the integrity of the mucosal epithelial cell layer of the tonsil, the cervix and the foreskin revealed disruptions in the expression of cell adherence junction proteins upon exposure to HIV-1 viral proteins including gp120 [20]. This was accompanied by enhanced epithelial–mesenchymal transition, a critical step in the metastatic cascade of solid tumors.

Cancer remains a leading cause of death among HIV-infected persons and the evidence for an oncogenic role for HIV and its auxiliary proteins continues to accumulate. Nevertheless, the actions of established oncogenic viruses including KSHV and EBV, the high-risk HPV and others remain the major causative agents, with some of the evidence demonstrating a collaborative link between HIV and these viruses. It has become clear that, even in patients who are virally suppressed, HIV-encoded proteins remain in tissues and in circulation, and while not inherently oncogenic, these factors potentiate oncogenic risk in non-HIV host bystander cells, compounded by an environment of chronic inflammation and immune dysfunction, among others. These represent carcinogenic characteristics which are unique to HIV-associated malignancies and are not present within the same malignancy that develops in an HIV-negative background. Over time, the deeper understanding of this complex landscape will inform new approaches to diagnosis and treatment of cancers in people who are infected with HIV.