We observed in this large cohort study of patients with mild to severe CKD i) that higher apoA-IV concentrations at baseline were associated with a lower odds for a history of cancer and ii) that during 6.5 years of follow-up higher apoA-IV concentrations were associated with a lower risk for incident cancer, particularly fatal cancer outcomes. All these associations were independent of kidney function. To our knowledge this is the largest and the only prospective study up to now which has investigated the association of apoA-IV with cancer, independent whether the literature in CKD or non-CKD patients is considered (Graphical Abstract).
Several proteomic studies have suggested apoA-IV as a biomarker and a tool for detection of various forms of cancer (Supplementary Table
1). Most of these studies investigated only a small number of serum or plasma samples and only few validated the results by an independent and quantitative method such as an ELISA [
24‐
31]. Most data are available for ovarian cancer with quantitatively validated lower apoA-IV concentrations observed in three studies [
24‐
26]. Further data were reported for pancreatic, oral, hepatocellular, and thyroid cancer (Supplementary Table
1). The distribution of cancer types in our GCKD study is comparable to what is reported for patients with moderate to severe CKD. Cancers of the urinary tract (kidney, bladder, urothelial cancer), prostate cancer, cancers of the digestive organs, skin cancers, cancer of the breast and female reproductive organs and lung cancers were among the most frequent cancers among these patients [
4,
39,
40]. Whether apoA-IV is a biomarker of selected cancers or causally associated with cancer is currently not known.
Possible explanations for the inverse association of apoA-IV with cancer
A number of biological entities constitute the so-called hallmarks of cancer, with inflammation being one of them [
41] in turn increasing the probability for metastasis [
42]. The known anti-inflammatory properties of apo-AIV [
14,
15] and the link to endothelial function [
43], a further component of the tumour microenvironment, could lead to the assumption, that ApoA-IV might be an important marker connected to several pathways in the development of cancer.
In patients with early stages of CKD the risk for cancer is already increased, in particular in males, and the risk increases gradually with more advanced stages of CKD [
3]. Two cohort studies reported a stepwise association between severity of CKD and cancer mortality [
3]. In turn, CKD is prevalent in approximately 12–25% of cancer patients [
39]. Systemic inflammation and oxidative stress are common in patients with advanced CKD and increase with disease progression [
44]. About 20–25% of all cancers are discussed to be triggered by chronic infection or inflammation. Several pathways, intracellular signaling cascades and related mechanisms were described that support this hypothesis such as DNA damage, cell damage, proliferation and transformation, immunosuppression, oxidative stress, angiogenesis or abnormal epithelial cell growth [
45‐
47]. Thus, such inflammatory processes might be one of the main drivers for the higher risk for cancer in CKD, comparable to that for atherosclerosis [
48].
To damage pathogens and protect the organism from harmful antigens, the body releases reactive oxygen species (ROS) in inflamed tissue [
46]. Antioxidant processes prevent the accumulation of ROS and reactive nitrogen species. An imbalance between the formation of oxidative compounds and antioxidant defense mechanisms leads to oxidative damage [
44]. ROS generated during inflammation directly damages proteins and lipids [
49], thereby increasing oxidative stress [
47]. Inflammation and oxidative stress influence the immune response with an induction of angiogenesis, tumor growth and metastasis [
48,
49]. The experimental evidence of the antioxidative and anti-inflammatory properties of apoA-IV as shown for atherosclerosis in mice [
13‐
15] may also explain the association of increased apoA-IV concentrations and lower number of cancer outcomes and cancer death in the GCKD study. Direct modulatory effects of apoA-IV on oxidation-induced intracellular redox-dependent cell signalling mechanisms were supported by cell culture studies [
50].
Generalized endothelial dysfunction is evident in early stages of CKD, leads to albuminuria in the glomeruli and worsens with CKD progression [
51]. Endothelial cells release proangiogenic factors which promote coagulation and inflammation [
46]. The large endothelial surface of highly vascularized kidney vessels is particularly vulnerable to local proinflammatory effects. Endothelial activation attenuates the local vasodilatory capacity and thus increases the production of ROS and oxidized low-density- lipoprotein cholesterol [
44,
48]. While healthy endothelial cells limit tumor growth, invasiveness, and metastasis, dysfunctional ones exposed to the inflammatory tumor microenvironment promote cancer progression, death, and even metastasis through NF-kappa B signaling and other cytokines [
46,
48,
52,
53]. ApoA-IV was shown to inhibit NF-κB activity and upregulate the anti-oxidant and anti-apoptotic enzyme 24-Dehydrocholesterol reductase (DHCR24) [
43]. Importantly, in vivo, already very low concentrations of apoA-IV could be considered anti-inflammatory [
43]. This makes it interesting as a potential therapeutic target. Moreover, as the inhibition of vascular cell adhesion molecule 1 but not intercellular adhesion molecule 1 in vitro by lipid-bound apoA-IV was not fully dependent on DHCR24, other pathways besides inhibition of NF-κB might be of relevance for its anti-inflammatory effects [
43]. ApoA-IV in its lipid-free and HDL-bound form was described to interact with scavenger receptor class B type 1 [
54], which might be one possible alternate pathway. Lipid-bound apoA-IV might also increase the bioavailability of nitric oxide in endothelial cells via different pathways. Taken together, Shearston et al. showed that apoA-IV strongly inhibited acute vascular inflammation in vitro and in vivo [
43]. This could be a further link to relevant mechanisms responsible for the lower number of cancer outcomes together with increased apoA-IV concentrations in the GCKD study.
A further explanation for our findings in the GCKD study is the growing evidence that HDL particles and their components including apoA-IV might either be involved in cancer development or might at least have diagnostic utility. A large meta-analysis [
55] found an inverse relationship between plasma HDL-C levels and the risk of developing cancer. Cancer cells have an increased need of cholesterol and a disturbed cholesterol metabolism with an increase of intracellular cholesterol esters. This abnormal lipid and cholesterol metabolism in tumor cells can lead to altered plasma lipid levels in cancer patients. Disturbances in the cholesterol homeostasis of the cancer cells can therefore have an influence on progression of cancer as well as survival of cancer cells (reviewed in [
55]).
In addition to lipid- and LDL-lowering effects, statins exhibit protective effects against DNA damage, improve endothelial function, and exert antioxidant and anti-inflammatory effects, and are associated with a reduced incidence for many cancers, which makes them potentially valuable in cancer prevention [
48]. Importantly, the effect of higher apoA-IV concentration on decreased risk for cancer outcomes in the GCKD study was independent of statin use. Furthermore, in a study in hemodialysis patients we found no significant effect of atorvastatin on apoA-IV concentrations [
22].
Regardless of the degree of renal dysfunction, CKD patients have a higher risk for venous thromboembolism compared to the general population [
56]. Thrombosis-related death is the second most frequent cause in cancer patients [
57]. A recent study has shown that apoA-IV reduced αIIbβ3 mediated-platelet aggregation and thus thrombosis. Aspirin or clopidogrel showed similar inhibitory effects as compared to apoA-IV [
16]. As CKD and thrombosis, as well as cancer are linked, one mechanism how apoA-IV could be protective for cancer outcomes might be this recently demonstrated strong antithrombotic effect.
It is currently unclear whether apoA-IV concentrations measured in serum reflect the concentrations of apoA-IV in the microenvironment of the cancer and how this influences the survival of cancer cells. A recent study in patients with pancreatic ductal adenocarcinoma observed markedly decreased apoA-IV concentrations in plasma but a pronounced increase of mRNA in the diseased pancreatic tissue which was linked with a less favourable outcome of the patients. This was surprising since usually RNA expression of apoA-IV in normal pancreas tissue is almost zero [
58]. Decreased apoA-IV concentrations in pancreatic cancer were recently confirmed by a study in cats [
59].
Strengths and limitations of the study
The main advantage of the current study compared to most of the proteomic studies is the use of a validated ELISA for measurement of apoA-IV and the prospective assessment of cancer outcomes. Particular strengths of the GCKD study are the large sample size based on a well-defined population with a median follow-up of 6.5 years with very low loss to follow-up, the homogeneity of the study population and a centralized assessment of clinical outcomes.
Since the GCKD study is designed as an observational epidemiological study, it cannot determine causality or conclude on possible biological mechanisms. In case the association of apoA-IV with outcomes is not causal it can at least be considered as a promising predictor of risk for cancer. However, the experimental data from earlier studies are in support of a causal involvement of apoA-IV in outcomes involved in inflammatory processes [
14‐
16,
60,
61]. A further limitation is that we do not have a replication cohort available and that mainly CKD patients in stage G3 or A3 are included in the GCKD study and therefore findings might not be generalizable to other stages of CKD or the general population. However, previous proteomic studies mostly done in serum or plasma revealed that apoA-IV is reduced in many cancer types (for overview see Supplementary Table
1). Furthermore, we cannot exclude residual confounding by unknown or unmeasured confounders. Finally, apoA-IV was measured in serum and no organ-specific cancer tissue was available which would have allowed to analyse the apoA-IV expression in cancer tissue.