Possibility of Total Cholesterol/High-Density Lipoprotein Ratio as a Prognostic Indicator through Correlation Analysis with Hyperlipidemia Related Diseases in Dogs

1. Introduction

The importance of hyperlipidemia in human medicine has been emphasized over the years. The complications of hyperlipidemia include endocrine diseases such as diabetes and pancreatitis, but fatal cardiovascular diseases such as stroke and myocardial infarction are also closely related to this condition [1,2]. Therefore, researchers have conducted studies aiming to prevent such complications through efforts to improve hyperlipidemia, and in the process, drugs to improve hyperlipidemia such as Statins and Fenofibrate[3] have been developed [4]. Lipid which is absorbed into the body through lacteal in small intestine after digestion must travel through blood vessels, but since lipid is fat-soluble, not water-soluble, it cannot dissolve in the blood vessels and move on its own. Therefore, it forms a lipoprotein complex and moves throughout the body's organs, changing into various forms including chylomicron, Very Low-Density Lipoprotein (VLDL), Low-Density Lipoprotein (LDL), HDL accordingly (Figure 1). When chylomicrons are absorbed from lacteal lymphatic vessel [5] in the small intestines, they pass through the liver to become VLDLs, then to adipose tissue to become LDLs, and then to muscles and blood vessels and other organs to condense into HDLs, and the low-density, high-volume lipids are removed.

HDL has been found to play a role in removing lipids from blood vessels, which gives rise to its image as good cholesterol. As HDL levels rise, triglyceride (TG) and LDL levels decrease in relative terms, which can prevent cardiovascular diseases such as stroke and myocardial infarction. However, there is growing awareness recently that higher HDL does not always guarantee healthier condition and that it is better to evaluate this biomarker in comprehensive aspect of health condition [6].

In the veterinary field, there are relatively few studies on the role of HDL and its relationship with hyperlipidemia complications. Additionally, the frequency and importance of cardiovascular diseases such as stroke and myocardial infarction are lower than expected. One study explored the normal range of HDL in dogs [7], but HDL is not yet routinely measured in the clinical field. The findings may provide clues as to why strokes and myocardial infarctions occur less frequently in dogs than in humans. In fact, due to shorter life span of dog than human and other critical disease such as myxomatous mitral valve disease and lymphoma, the risk of atherosclerosis is relatively low than in human. Nevertheless, arteriosclerosis and myocardial infarction due to hyperlipidemia in dogs have been reported recently [8].

In this study, we measured HDL in dogs and cats who visited primary veterinary hospitals and examined its relationship with endocrine diseases such as hyperlipidemia, diabetes, Cushing's, and hypothyroidism. Since the number of patients visiting primary hospitals is limited, it is important to collect as much data as possible. There is no choice but to be sampled due to various species and various underlying diseases. This is valuable data for follow-up research with a uniform population in referral hospitals or in university. Also, there has been few research on HDL levels in cats. These samples of cats could provide supplementary data for follow-up research. It also can be a useful resource for follow-up research with amino acid sequence comparison.

2. Materials and Methods

Blood samples were collected from sixteen client-owned dogs and cats visiting Bundang New York animal hospital located in South Korea. All blood samples were collected from the cephalic vein in a fasted state without anesthesia or sedation and centrifuged at 14500RPM for 1 minute, and the sera were directly analyzed using DRI-CHEM NX500 (Fujifilm), a dry chemistry analyzer, immediately. Only when measuring dog HDL, the serum was diluted 5 times with sterilized WFI, and the other samples were measured directly without dilution at room temperature. JASP was used for data analysis and graph creation (https://jasp-stats.org), and amino acid sequence analysis was performed using BioEdit (https://bioedit.software.informer.com/7.2).

3. Results

Upon analyzing the correlation with the TC/HDL ratio by dividing the presence or absence of hyperlipidemia-related endocrine diseases such as hypothyroidism, Cushing's, gallbladder mucocele, and liver cancer into 1 and 0, concurrent disease and TC/HDL value showed strong positive correlation significantly (Data 1). Interestingly, the dogs which had lower TC/HDL value than 3 had no disease but other dogs had various diseases which had higher TC/HDL value than 3. This data provides that TC/HDL cut-off value as a healthy condition biomarker can be 3 which is similar in human medicine [9].

In general, it is known that, the higher the TC/HDL ratio in humans, the higher the likelihood of various cardiovascular diseases. Therefore, this ratio has been used as a prognostic indicator for cardiovascular diseases such as stroke and arteriosclerosis. The lower the ratio is than 5, the better the prognosis; less than 3.5 is considered good, and if it is more than 6, the risk of cardiovascular disease is high [10]. Also, TC/HDL ratio has been excellent indicator for endocrine disease such as insulin resistance diabetes [11].

The originality of this study was to investigate whether the TC/HDL ratio is effective prognostic biomarker for cardiovascular disease and endocrine disease in client-owned dogs and cats through actual measurements and it was first trial at general practice. There was clear limitation to reveal correlation between cardiovascular disease and TC/HDL value because there was no concurrent heart disease in the samples. Nevertheless, in aspect of endocrine disease related to hyperlipidemia such as Cushing’s disease, hypothyroidism, gallbladder mucocele, these data showed excellent correlation between TC/HDL and concurrent diseases. In fact, in human patients with Cushing's, a decrease in the TC/HDL ratio before and after remission can be observed, and thus, this can be used as a prognostic evaluation factor [12]. In addition, studies have shown that the incidence of gallbladder polyps is strongly related to the non-HDL/HDL ratio [13]. Also, in human hypothyroidism, the TC/HDL ratio is lowered after appropriate treatment [14]. In addition, there are research results showing that TC/HDL can be an important prognostic indicator in the case of non-alcoholic fatty liver disease, although not for liver cancer, as in our case [15]. Additionally, the TC/TG ratio was also measured to examine its potential as a prognostic evaluation factor.

Additionally, TC/TG value can be indicator for small, dense LDL, negative correlation in human medicine [16]. In human, small, dense LDL is an emerging biomarker for cardiovascular disease [17,18]. It is also related to metabolic disease including diabetes mellitus [19]. Therefore TC/TG value can also be negatively correlated to small, dense LDL in veterinary medicine. Pleasurably, lowest TC/TG value was shown in hyperlipidemia, hepatocellular carcinoma dog, #9 (Table 1).There are some study related to LDL and hepatocellular carcinoma in human medicine [20,21] but this result needs more investigation because of small sample and difference between human and dog.

Interestingly, low HDL values were shown in dogs which had lipid metabolism related disorder (Cushing, hypothyroidism) #6,11,12 (Table 1). Also, low HDL value was also shown in diabetic cat #2 (Table 2). Low HDL level is strongly related to non-insulin dependent diabetes mellitus (type 2) in human medicine [22] and in cats most of type of diabetes mellitus are type2 [23] so it is plausible data.

4. Discussion

This study showed TC/HDL value can be excellent biomarker of lipid metabolism related endocrine disease and its cut off value can be 3. the role of HDL in the veterinary field has not yet been fully elucidated and it requires attention in additional research because of small number of samples in this study. This study is significant as the first attempt to measure HDL in actual patients using general biochemical equipment in a primary general practice rather than a laboratory. Also, comparing the amino acid (Figure 2) related to HDL (Cdkal1) [24], which is identical between humans, dogs, cats, and pigs, shows that the role of HDL in cardiovascular disease may be similar among species. The reason why strokes and myocardial infarctions in dogs occur less frequently than in humans could be multifactorial[25] rather than solely genetic, and involve the environment, food, and stress[26]. Additionally, if we compare the incidence of stroke and arteriosclerosis in Schnauzers, which genetically tend to have hyperlipidemia more than other species [27], and also measure the normal range of HDL in Schnauzers, we will be able to gain a deeper understanding of the role of HDL in cardiovascular disease. Cats are more likely to develop blood clots [28] than humans, and dogs [29]. The amino acid sequence of feline ApoA-1, the related protein, is slightly different to that in other species (Figure 2). This may be related to HDL, which plays a role in transporting lipids from blood vessels to other organs [30]. To date, there has been no research on the correlation and mechanism between HDL levels and thromboembolism in cats. Further study may reveal the role of HDL in cardiovascular disease and thromboembolism in felines.

Data 1

(Pearson’s r 0.599, p-value 0.031)