Low dietary inorganic phosphate stimulates lung tumorigenesis through altering protein translation and cell cycle in K-ras(LA1) mice

Spontaneous Tumor Regression and Reversion: Insights and Associations with Reduced Dietary Phosphate

Tumors that spontaneously shrink from unknown causes in tumor regression, and that return to normal cells in tumor reversion, are phenomena with the potential to contribute new knowledge and novel therapies for cancer patient survival. Tumorigenesis is associated with dysregulated phosphate metabolism and an increased transport of phosphate into tumor cells, potentially mediated by phosphate overload from excessive dietary phosphate intake, a significant problem in Western societies. This paper proposes that reduced dietary phosphate overload and reregulated phosphate metabolism may reverse an imbalance of kinases and phosphatases in cell signaling and cellular proliferation, thereby activating autophagy in tumor regression and reversion. Dietary phosphate can also be reduced by sickness-associated anorexia, fasting-mimicking diets, and other diets low in phosphate, all of which have been associated with tumor regression. Tumor reversion has also been demonstrated by transplanting cancer cells into a healthy microenvironment, plausibly associated with normal cellular phosphate concentrations. Evidence also suggests that the sequestration and containment of excessive phosphate within encapsulated tumors is protective in cancer patients, preventing the release of potentially lethal amounts of phosphate into the general circulation. Reducing dietary phosphate overload has the potential to provide a novel, safe, and effective reversion therapy for cancer patients, and further research is warranted.

Resveratrol is an inhibitor of sodium-dependent inorganic phosphate transport in triple-negative MDA-MB-231 breast cancer cells

Metastasis is a major cause of death in patients with breast cancer. A growing body of evidence has demonstrated the antitumour effects of resveratrol, a non-flavonoid polyphenol. Resveratrol inhibits metastatic processes, such as the migration and invasion of cancer cells. In several cancer types, the importance of inorganic phosphate (Pi) for tumor progression has been demonstrated. The metastatic process in breast cancer is associated with Na⁺ -dependent Pi transporters. In this study, we demonstrate, for the first time, that resveratrol inhibits the Na⁺ -dependent Pi transporter. Results from kinetic analysis shows that resveratrol inhibits Na⁺ -dependent Pi transport non-competitively. Resveratrol also inhibits adhesion/migration in MDA-MB-231 cells, likely related to inhibition of the Na⁺ -dependent Pi transporter.

The Roles of Sodium-Independent Inorganic Phosphate Transporters in Inorganic Phosphate Homeostasis and in Cancer and Other Diseases

Inorganic phosphate (Pi) is an essential nutrient for the maintenance of cells. In healthy mammals, extracellular Pi is maintained within a narrow concentration range of 0.70 to 1.55 mM. Mammalian cells depend on Na⁺/Pi cotransporters for Pi absorption, which have been well studied. However, a new type of sodium-independent Pi transporter has been identified. This transporter assists in the absorption of Pi by intestinal cells and renal proximal tubule cells and in the reabsorption of Pi by osteoclasts and capillaries of the blood–brain barrier (BBB). Hyperphosphatemia is a risk factor for mineral deposition, the development of diseases such as osteoarthritis, and vascular calcifications (VCs). Na⁺-independent Pi transporters have been identified and biochemically characterized in vascular smooth muscle cells (VSMCs), chondrocytes, and matrix vesicles, and their involvement in mineral deposition in the extracellular microenvironment has been suggested. According to the growth rate hypothesis, cancer cells require more phosphate than healthy cells due to their rapid growth rates. Recently, it was demonstrated that breast cancer cells (MDA-MB-231) respond to high Pi concentration (2 mM) by decreasing Na⁺-dependent Pi transport activity concomitant with an increase in Na⁺-independent (H⁺-dependent) Pi transport. This Pi H⁺-dependent transport has a fundamental role in the proliferation and migratory capacity of MDA-MB-231 cells. The purpose of this review is to discuss experimental findings regarding Na⁺-independent inorganic phosphate transporters and summarize their roles in Pi homeostasis, cancers and other diseases, such as osteoarthritis, and in processes such as VC.

Modulating phosphate consumption, a novel therapeutic approach for the control of cancer cell proliferation and tumorigenesis

Phosphorus, often in the form of inorganic phosphate (Pi), is critical to cellular function on many levels; it is required as an integral component of kinase signaling, in the formation and function of DNA and lipids, and energy metabolism in the form of ATP. Accordingly, crucial aspects of cell mitosis - such as DNA synthesis and ATP energy generation - elevate the cellular requirement for Pi, with rapidly dividing cells consuming increased levels. Mechanisms to sense, respond, acquire, accumulate, and potentially seek Pi have evolved to support highly proliferative cellular states such as injury and malignant transformation. As such, manipulating Pi availability to target rapidly dividing cells presents a novel strategy to reduce or prevent unrestrained cell growth. Currently, limited knowledge exists regarding how modulating Pi consumption by pre-cancerous cells might influence the initiation of aberrant growth during malignant transformation, and if reducing the bioavailability or suppressing Pi consumption by malignant cells could alter tumorigenesis. The concept of targeting Pi-regulated pathways and/or consumption by pre-cancerous or tumor cells represents a novel approach to cancer prevention and control, although current data remains insufficient as to rigorously assess the therapeutic value and physiological relevance of this strategy. With this review, we present a critical evaluation of the paradox of how an element critical to essential cellular functions can, when available in excess, influence and promote a cancer phenotype. Further, we conjecture how Pi manipulation could be utilized as a therapeutic intervention, either systemically or at the cell level, to ultimately suppress or treat cancer initiation and/or progression.

Inorganic phosphate transporters in cancer: Functions, molecular mechanisms and possible clinical applications

Inorganic phosphate is one of the most essential nutrients for the maintenance of cell life. Because of its essential role in nutrient supplementation, the study of plasma membrane inorganic phosphate transporters in cancer biology has received much attention in recent years. Several studies suggest that these transporters are up-regulated in tumor cells and thus have been considered to be important promoters of tumor progression. Altered expression levels of inorganic phosphate transporters, such as NaPi-IIb (SLC34A2) and PiT-1 (SLC20A1), have been demonstrated. The purpose of this review article was to gather the relevant experimental records on inorganic phosphate transporters in tumors and to demonstrate the importance of these proteins in clinical applications. In this work, we demonstrate that for decades, the potential use of the inorganic phosphate transporter as an antigen for the diagnosis of tumor subtypes remained unknown. With the advancement in molecular biology techniques, phosphate transporters have been identified as being associated with cancer. In addition to their altered expression in cancer, several studies have demonstrated other functions of inorganic phosphate transporters, such as transceptors, rearrangements with oncogenes and modifications in the expression of ABC transporters, aiding in the process of proliferation and drug resistance.

The effects and mechanisms of SLC34A2 in tumorigenesis and progression of human non-small cell lung cancer

BACKGROUND

SLC34A2 with highest expressions in lung, small intestine and kidney encoded a type 2b sodium-dependent phosphate transporter (NaPi-IIb). In lung, SLC34A2 only expressed in the apical membrane of type II alveolar epithelium cells (ATII cells) and played a pivotal role during the fetal lung development and embryonic development. ATII cells acting as multifunctional stem cells might transform into NSCLC after undergoing exogenous or endogenous factors. Increasing evidences showed that the genes performing critical roles during embryogenesis were also expressed during the development of cancer. In addition, recent research found the expression of SLC34A2 had a significant difference between the surgical samples of NSCLC and normal tissues, and SLC34A2 was down-regulated in lung adenocarcinoma cell line A549 and up-regulation expression of SLC34A2 could significantly inhibit cell viability and invasion of A549 in vitro. These results suggested SLC34A2 might play an important role in the development of NSCLC. However, the role of SLC34A2 in tumorigenesis and progression of NSCLC remains unknown.

RESULTS

Our study found that SLC34A2 was also significantly down-regulated in 14/15 of examined NSCLC tissues. Moreover, we found that expressions of SLC34A2 were reduced in six NSCLC cell lines for the first time. Our result also revealed a dramatic inhibitory effects of SLC34A2 on cell growth, migration and invasion of several NSCLC cell lines. SLC34A2 also strongly inhibited tumor growth and metastasis ability in A549 subcutaneous tumor model and lung metastasis model, respectively. Further studies found that the suppressive effects of SLC34A2 on tumorigenesis and progression might be associated with the down-regulation of related protein in PI3K/Akt and Ras/Raf/MEK signal pathway.

CONCLUSIONS

For the first time, our data indicated that SLC34A2 could exert significantly suppressive effects on tumorigenesis and progression of NSCLC. SLC34A2 might provide new insights for further understanding the early pathogenesis of human NSCLC.

Inorganic phosphate and the risk of cancer in the Swedish AMORIS study

Background

Both dietary and serum levels of inorganic phosphate (Pi) have been linked to development of cancer in experimental studies. This is the first population-based study investigating the relation between serum Pi and risk of cancer in humans.

Methods

From the Swedish Apolipoprotein Mortality Risk (AMORIS) study, we selected all participants (> 20 years old) with baseline measurements of serum Pi, calcium, alkaline phosphatase, glucose, and creatinine (n = 397,292). Multivariable Cox proportional hazards regression analyses were used to assess serum Pi in relation to overall cancer risk. Similar analyses were performed for specific cancer sites.

Results

We found a higher overall cancer risk with increasing Pi levels in men ( HR: 1.02 (95% CI: 1.00-1.04) for every SD increase in Pi), and a negative association in women (HR: 0.97 (95% CI: 0.96-0.99) for every SD increase in Pi). Further analyses for specific cancer sites showed a positive link between Pi quartiles and the risk of cancer of the pancreas, lung, thyroid gland and bone in men, and cancer of the oesophagus, lung, and nonmelanoma skin cancer in women. Conversely, the risks for developing breast and endometrial cancer as well as other endocrine cancer in both men and women were lower in those with higher Pi levels.

Conclusions

Abnormal Pi levels are related to development of cancer. Furthermore, the in verse association between Pi levels and risk of breast, endometrial and other endocrine cancers may indicate the role of hormonal factors in the relation between Pi metabolism and cancer.

Inorganic Phosphate as a Novel Signaling Molecule with Antiproliferative Action in MDA-MB-231 Breast Cancer Cells

Inorganic phosphate (P_i) is an essential nutrient for living organisms. It plays a key role in diverse physiological functions, including osteoblast differentiation and skeletal mineralization. Relevantly, P_i is emerging as an important signaling molecule capable of modulating multiple cellular functions by altering signal transduction pathways, gene expression, and protein abundance in many cell types. To our knowledge, the consequences of elevated P_i on behavior of breast cancer cells have been poorly addressed. In this study we investigate the effects of P_i on proliferation of MDA-MB-231 breast cancer cells. We report that P_i inhibits proliferation of MDA-MB-231 cells by slowing cell cycle progression, without apoptosis occurrence. We found that P_i causes cells to accumulate in G1 phase in a time-dependent manner. Accordingly, G1 accumulation was associated with a decrease of cyclin A and cyclin E and an increase of cell cycle inhibitors p21 and p27 protein levels, respectively. Moreover, the P_i-induced antiproliferative effect was dynamically accompanied by profound changes in ERK1/2 and STAT3 protein and phosphorylation levels in response to P_i. Altogether, our data represent the first evidence of P_i acting as a novel signaling molecule in MDA-MB-231 breast cancer cells, capable of eliciting a strong antiproliferative action and suggest that targeting P_i levels at local sites might represent the rationale for developing novel strategies for therapeutic intervention in triple-negative breast cancer.