Dietary protein restriction inhibits tumor growth in human xenograft models

Abstract

Purpose: Data from epidemiological and experimental studies suggest that dietary protein intake may play a role in inhibiting prostate and breast cancer by modulating the IGF/AKT/mTOR pathway. In this study we investigated the effects of diets with different protein content or quality on prostate and breast cancer.

Experimental design: To test our hypothesis we assessed the inhibitory effect of protein diet restriction on prostate and breast cancer growth, serum PSA and IGF-1 concentrations, mTOR activity and epigenetic markers, by using human xenograft cancer models.

Results: Our results showed a 70% inhibition of tumor growth in the castrate-resistant LuCaP23.1 prostate cancer model and a 56% inhibition in the WHIM16 breast cancer model fed with a 7% protein diet when compared to an isocaloric 21% protein diet. Inhibition of tumor growth correlated, in the LuCaP23.1 model, with decreased serum PSA and IGF-1 levels, down-regulation of mTORC1 activity, decreased cell proliferation as indicated by Ki67 staining, and reduction in epigenetic markers of prostate cancer progression, including the histone methyltransferase EZH2 and the associated histone mark H3K27me3. In addition, we observed that modifications of dietary protein quality, independently of protein quantity, decreased tumor growth. A diet containing 20% plant protein inhibited tumor weight by 37% as compared to a 20% animal dairy protein diet.

Conclusions: Our findings suggest that a reduction in dietary protein intake is highly effective in inhibiting tumor growth in human xenograft prostate and breast cancer models, possibly through the inhibition of the IGF/AKT/mTOR pathway and epigenetic modifications.

Figures

Figure 1. Low protein diet attenuates the growth of prostate and breast cancer in the castrate-resistant LuCaP23.1 model and in the WHIM16 model, respectively

Tumor sizes were assessed two times a week by caliper measurements. (A, B) LuCaP23.1-CR and (G) WHIM16 growth curve of tumors already exposed to low protein diet (pre-implantation studies) and endpoint tumor weights. (E, F) LuCap23.1-CR growth curve of tumors exposed to low protein diet after tumors were implanted and established (post-implantation studies), and endpoint tumor weights. (C) LuCaP23.1-CR and (H) WHIM16 mouse body weights. (D) Measurements of serum glucose in LuCaP23.1-CR bearing mice. Results are expressed as the mean +/− SE, n= 7-10; * p

Figure 2. Low protein diet decreases IGF-1 serum levels and inhibit LuCaP23.1-CR growth in combination with everolimus

(A) Measurements of serum IGF-1 in LuCaP23.1-CR bearing animals fed with either 21% or 7% protein diet. (B, C) Mice were acclimatized for four weeks to either 21% or 7% protein diet and after LuCaP23.1-CR xenograft implantation were treated with everolimus (2 mg/kg PO, daily X5 times/week). (D, E) Endpoint PSA and tumor weights were collected. Results are expressed as the mean +/− SE, n=10.

Figure 3. Low protein diet decreases mTOR and proliferation activity in the LuCaP23.1-CR model

(A) At the end of the “pre-implantation” experiment, tumor samples were collected and processed. Paraffin embedded tissue specimens were stained for p-mTOR, p-S6 ribosomal protein and proliferation marker Ki67. (B) Quantification of staining. Results are based on four fields per tissue and are expressed as the mean +/− SE. **p

Figure 4. Epigenetic alterations associated with low protein diet in the LuCaP23.1-CR model

(A) At the end of the “pre-implantation” experiment, tumor samples were collected and processed. Paraffin embedded tissue specimens were stained for the histone methyltransferase EZH2 and the associated histone mark H3K27me3. (B) Quantification of staining. Results are based on four randomly selected fields and are expressed as the mean + SE. **p

Figure 5. Effect of the different sources of protein diet on tumor growth and potential molecular mechanisms

Endpoint LuCaP23.1-CR tumor weights of mice fed with either 21% protein diet (which consist of both plant and animal protein sources), 20% or 10% protein diet (plant based) or 20% or 10% protein diet (animal based). Results are presented as a mean +/− SE, n= 7-8 per group. *p

Figure 6. Representative schema of the potential molecular mechanisms responsible for the biological effects of protein restriction