Leucine-rich diet supplementation modulates foetal muscle protein metabolism impaired by Walker-256 tumour

Pregnancy concomitant with cancer occurs in approximately one in one thousand women, but this combination is responsible for one-third of all maternal deaths [1, 2]. Relatively few relevant reports exist in the scientific literature regarding the consequences of tumour growth for the maternal host and the foetal body. Our previous studies showed that tumour-bearing dam rats displayed approximately 70% reabsorption and 50% foetal death [3‐6]; in this work, we focused on foetal growth because there is little data about the effects on foetal homeostasis and damage when the mother suffers from a tumour [7].

Tumour growth promotes extensive biochemical and metabolic changes in the host organism, leading to the process of cachexia and culminating in deep catabolism of lean mass [8‐10]. As a result, the host demonstrates marked weight loss, which is not minimised by forced feeding. To study these all these processes experimentally, Walker-256 tumour is a good experimental model of cachexia [9, 10]. In addition, pregnancy causes physiological changes that ensure foetal growth and development until the term pregnancy and ensures an adequate energy supply to the foetus and after birth [11]. Additionally, during pregnancy, increased oxidative stress can cause pre-eclampsia and intrauterine foetal growth retardation [12], which may be promoted by tumour development [13].

Branched-chain amino acids (BCAAs) are essential for humans by promoting cell function and regulating protein metabolism [14]. BCAAs, especially leucine (Leu), have been studied because of their cell-signalling properties; they can stimulate protein synthesis [14] and decrease protein catabolism [6, 14, 15]. The oral administration of exogenous leucine stimulates protein synthesis in isolated or perfused rat muscles in vivo [16]. Moreover, clinical trials and studies in animals have shown that supplementation with BCAAs increases the balance of nitrogen and improves health after tissue damage, such as that due to burns, radiation, post-surgical stress, sepsis and cancer [17]. Studies have shown that the use of complementary therapies, as nutritional supplementation, improves the clinical status and especially the quality of life of cancer patients [18‐21]. Meanwhile, maternal metabolism is directed to ensure the nutritional supply for the foetus and adjust foetal growth, but the combination of the metabolic and biochemical changes during pregnancy and cancer jeopardises the foetus and mother. Based on this, and more importantly because there are few studies about the foetal consequences of cancer development, this study investigated the effects of Walker-tumour growth on protein catabolism in foetal muscle, especially on proteolytic enzymes, anti-oxidant enzymes and lipid peroxidation and examined whether nutritional supplementation with leucine modulated the damage effects on foetal growth.

In the present study, the foetuses of tumour-bearing pregnant rats suffered similar effects due to tumour growth as those observed in tumour-bearing animals [5, 6, 15, 25, 27]. The weight of the foetuses in the W and LW groups was reduced (Figure 1A), and in this case the tumour effects account for 83% of the total variation, indicating that this low foetal weight is extremely significant. Consistent with this reduced foetal weight, the ratio of the foetal weight to the placental weight, which is indicative of the efficiency of placental transport, was decreased in both tumour-bearing groups and in the leucine pair-fed foetuses, especially when compared to the control rats (Figure 1B). These data show that the tumours and the pair-feeding scheme accounted for this impaired effect (P < 0.0002). The reduction in foetal weight occurred in parallel with the group with the lowest concentration of total foetal serum protein, group W, which was 27% lower than that of the control group, C (Figure 1C). In contrast, the total foetal serum protein of the LW group was similar to that of the control group (C) and higher than that of the W group (P = 0.0029). The serum albumin levels of the rats in the tumour-bearing group (W) were reduced by approximately 31% compared to those in group LW (P < 0.0001), which had a similar value to that of the control group (Figure 1D). Although the albumin-serum content was 20% higher in group L (under nutritional supplementation) than in group C, the leucine-rich diet did not prevent the reduction in albumin levels in the foetuses from tumour-bearing rats, as observed when comparing the LW and L groups (Figure 1D, P < 0.0001). Similarly, we observed a reduction in glucose levels in tumour-bearing foetuses, and those in the W groups showed a severe decrease in glucose levels (Figure 1E). Furthermore, the haematocrit was reduced only in the W group (Figure 1F). These data show that diet accounted for 46.7% of the total variance (P < 0.0001), modulating the effect of Walker tumour development.

Tumour growth affected anti-oxidative enzyme activities and increased lipid peroxidation (malondialdehyde) in foetal gastrocnemius muscles (Figure 2). Alkaline phosphatase activity could be used to test muscle-cell function [37], which was decreased in the W foetuses, but leucine supplementation led to a similar level in the LW group. In contrast, the L, Cp and Lp groups displayed increases in alkaline phosphatase activity compared to the control or tumour-bearing groups (Figure 2A). In comparison to the C group, glutathione-S-transferase activity was reduced in the W group but was similar in the LW group and increased in the L, Cp and Lp groups (Figure 2B). Catalase activity increased significantly only in the W group (Figure 2C), and glutathione levels tended to increase only in the W group (Figure 2D). The product of oxidative stress, as expressed by the malondialdehyde level, increased only in the W group (Figure 2E), and the MDA/GST ratio was increased 3-fold in the W group compared to the LW group (Figure 2F). These data show that diet accounted for 27% of the total variance in enzyme activity, modulating the tumour changes to foetal antioxidant enzyme activities.

The total muscle protein content (Figure 3A) was decreased in both of the Walker tumour-bearing groups (W and LW) compared to the control groups (C and L). Despite the food restriction that was imposed by pair feeding, no differences in the foetal muscle protein content in the pair-fed groups (Cp and Lp) were observed. Foetal muscle protein synthesis, as determined by the incorporation of phenylalanine (L-[³H]-Phe), was approximately 27% lower in the foetuses of tumour-bearing pregnant rats (W) than in the LW group (P = 0.0103) and other groups (Figure 3B). As a result of leucine supplementation, muscle protein synthesis recovered in the LW group, and this change was significantly different from that of the W group; diet significantly modulated the tumour effect (P = 0.0005). In addition, the malnutrition factor that was imposed by food restriction, measured in the pair-fed groups (Cp and Lp), had no effect on the protein synthesis process (Figure 3B).

Foetal muscle protein degradation was evaluated by the amount of released tyrosine and was approximately 31% higher in the W group than in the control group C (Figure 3C); in this case, the diet also accounted for a significant modulation of tumour effect (P = 0.0038). The chymotrypsin-like enzymatic activity, which is the catalytic portion of the ubiquitin-proteasome, was increased approximately 3.9-fold in group W, and despite having tumours, the foetuses from pregnant rats that received a leucine-rich diet (LW) showed a less-pronounced increase in the chymotrypsin-like activity compared to the rats in group W. These differences were near statistical significance (Figure 4A; P = 0.0803). The pair-fed groups (Cp and Lp) showed an increase in chymotrypsin-like enzyme activity that was similar to that observed in the LW group when compared to the L group. Furthermore, the expression of the 20S subunit of the ubiquitin-proteasome was decreased in the tumour-bearing groups (W and LW) and the pair-fed groups (Cp and Lp) (Figure 4B). Cathepsin H, which is one of the enzymes involved in the lysosomal proteolytic pathway, showed significantly increased activity in the W group, but this increase was less pronounced in the LW group (Figure 4C) (P < 0.047); in this case, the diet modulated the effect of tumour growth. The calcium-dependent proteolytic pathway showed no difference in its calpain activity between the tumour-bearing groups when compared to the C group, but the L and Lp groups showed decreased activity in relation to the values observed in group C rats (Figure 4D).

Maternal physiological adaptations are impaired by cancer development and can lead to impaired foetal growth and development. Here, we clearly show that in the presence of Walker-256 tumour growth, foetal development was jeopardised, especially with respect to muscle protein metabolism linked to increased proteolysis and inhibited protein synthesis. These effects were likely associated with the lack of an anti-oxidative response, which reduced foetal weight. More importantly, we observed that nutritional supplementation with leucine led to a marked modulatory effect that minimised foetal proteolysis, maintained the balance of similar levels of protein synthesis and degradation, and restored the anti-oxidant responses.

Previous studies found that the presence of a tumour affected foetal weight [4, 5]. This decrease in foetal weight was related to a reduction in placental weight, which may be related to changes in placental tissue, and decreased the foetal/placental weight ratio (Figure 1), leading to impaired foetal development. Foetal growth in rats is more intense during the third phase of pregnancy (19^th - 21^st day), which coincides with the greatest tumour weight [3, 5]. The high activity of tumour cells that promotes exponential growth occurs during the same period of foetal growth. Thus, we verified that, at the time points when the tumour weighed approximately 10 to 12% of the maternal body weight, the W foetuses also displayed reductions in body weight, total serum protein, albumin and glucose that was coupled with a decline in muscle protein synthesis and increased proteolysis. All of these findings are also the main metabolic changes that occur during tumourigenesis [25], as was found in the group W foetuses. Proteolysis is the main process during cancer-cachexia, and this can be assessed by a decrease in albumin content, followed by the catabolism of skeletal muscle cells, promoting atrophy and apoptosis [43, 44]. Although the total muscle protein decreased in both of the tumour-bearing groups (W and LW), a leucine-rich diet maintained protein synthesis and reduced protein degradation in the LW group. This observation suggests that the LW group had a less intense catabolism of lean body mass, as was verified in the W group, which was likely related to the degradation of foetal muscle protein that was enhanced by oxidative stress [38, 45]. According to our previous studies, lean body mass was reduced during tumour growth, and dietary supplementation with leucine minimised this loss [3, 5, 8, 24]. Intense protein catabolism, primarily in the skeletal muscle, may have provided substrates, predominantly amino acids and gluconeogenic substrates, to be used by neoplastic cells [25, 46, 47]. Consequently, it is thought that tumour development induces a deviation of nutrients from the placenta and foetus (which is the reason for the decrease in foetal weight and reduction in the foetus/placenta ratio that is associated with increased protein degradation and oxidative stress). In parallel, the foetuses from mothers that were subjected to graduated food restriction (Cp and Lp) showed reduced weight, but their serum parameters and total muscle protein were similar to those of the control group animals. Nutritional supplements are particularly important for foetal viability and foetus/placenta unity, but foetuses have the capacity to adapt metabolically to acute and chronic changes in maternal and foetal serum nutrients content [48, 49]. In contrast, we show that foetuses from tumour-bearing mothers, primarily those in group W, were not able to efficiently adapt their metabolism under cancer conditions compared to those under food restriction, as found in pair-fed foetuses (Cp). This fact may be related to the action of substances that are produced by cancer cells, as we have previously reported [4, 7, 50, 51]. Carbo and colleagues [52] found deficits in the amino acid mobilisation of foetuses in pregnant rats that received tumour necrosis factor during pregnancy, and this was associated with decreased placental blood flow due to the action of this cytokine. We previously verified that pregnant rats that were injected with ascitic liquid (obtained from tumour-bearing animals without tumour cells) exhibited an increase in foetal reabsorption and a reduction in foetal and placental weight; similar deleterious effects were found in tumour-bearing mothers [3, 4, 50]. In these experiments, we proposed that these changes were induced by substances produced by cancer cells and did not result from only the effect of reducing nutrients, as was observed in this study when comparing the W and Cp foetuses.

Pregnancy changes, especially the increase in muscle protein breakdown, are related to the effects of the tumour on the body. In the cancer host, the catabolism that is associated with the reduction and/or maintenance of the protein synthesis process is increased [24, 53] and is related to increased oxidative stress [38, 45, 50]. In this case, the foetuses from the rats in group W displayed reduced protein synthesis, which was also associated with an increase in proteolysis caused by the activation of proteolytic systems and a high oxidative stress process. The ubiquitin-proteasome pathway can be modulated by a leucine-rich diet [6, 8]. As has been reported in many other studies, the activity of this process was higher and more widely observed in foetal cells. The 20S subunit, which comprises the catalytic core of the ubiquitin-proteasome proteolytic pathway, is most likely involved in protein turnover, leading to differentiation and development of embryonic and foetal tissues. However, we showed that although the 20S subunit was expressed at lower levels in the W and LW foetuses, the greater chymotrypsin-like activity and oxidative stress that were found in group W were sufficient to promote protein wasting in those foetuses. More importantly, under leucine nutritional supplementation, the LW foetuses showed less increased protein degradation because leucine diminished the increase in chymotrypsin activity and maintained the cathepsin enzyme while increasing the antioxidant response; these results suggest a beneficial effect of leucine in minimising proteolysis. Moreover, tumour growth produces alterations that affect placental metabolism and interact with the placenta-foetus, as evidenced by intrauterine growth restriction with increased foetal-muscle catabolism [3, 4, 6, 24, 52].

Nutritional supplementation with leucine provides several benefits by activating cellular synthesis processes [6, 24, 25]. These data suggest that the foetuses that received nutritional leucine supplementation, even those that were suffering from their mother’s tumours, showed diminished deleterious effects due to tumour growth compared to those that did not receive leucine; the foetuses in the LW group had similar muscle protein synthesis to the control foetuses, and more importantly, the process of protein catabolism was minimised and was equal to that of the control group [4, 5, 54]. Further studies are underway in our laboratory to determine whether leucine supplementation can counteract the damage to foetal growth during cancer development and the mechanism underlying this effect. These data indicate the importance of co-adjuvant therapy in improving the state of host responses to clinical treatment and particularly the importance of leucine supplementation, which has a positive effect on the host mother and on foetal development.

The nutritional supplementation of a leucine-rich diet induced protein synthesis in the skeletal muscle of foetuses of pregnant, tumour-bearing rats. This effect may be due to minimisation of protein catabolism and enhancement of the anti-oxidative response.