The high protein expression of FOXO3, but not that of FOXO1, is associated with markers of good prognosis

Abstract

To better define the role of FOXO1 and FOXO3 transcriptional factors in breast carcinogenesis, we performed a comparative study of their expression at both the RNA and protein levels in a series of human breast tumors. We used qRT-PCR assay to quantify mRNA expression and Reverse Phase Protein Arrays (RPPA) to quantify protein expression in 218 breast tumors from patients with known clinical/pathological status and outcome. Weak correlations were observed between mRNA and protein expressions for both FOXO1 and FOXO3 genes. High expression of FOXO3 protein, but not FOXO1 protein, was a good prognostic marker, negatively correlated with KI67 and markers of activity of the PI3K/AKT/mTOR oncogenic pathway, and positively correlated with p53, a marker of apoptosis. Moreover, FOXO3 protein expression, but not FOXO1 protein expression, was also negatively correlated with various proteins involved in different DNA repair mechanisms. FOXO3 protein, but not FOXO1 protein, appears to be a tumor suppressor that inhibits breast cancer by altering DNA damage response (DDR), thereby inducing p53-dependent apoptosis. This antitumor effect appears to be suppressed by excessive activity of the PI3K/AKT/mTOR pathway. High FOXO3 protein expression could be a biomarker of deficient DDR in breast tumors.

Figure 1

Scatter plots and Spearman correlation coefficients (r) between FOXO1 (A) and FOXO3 (B) protein and mRNA levels in a series of 218 breast tumors.

Figure 2

Kaplan-Meier metastasis-free survival curves for FOXO3 and FOXO1 genes, according to protein levels (A) for FOXO3 and mRNA levels (B,C) for FOXO1 and FOXO3 in a series of 218 breast tumors. P-values are estimated using the log-rank test. Patients with breast tumors expressing high levels of FOXO3 protein had significantly better MFS than patients with breast tumors expressing lower levels of this protein (p = 4.1.10⁻²) (A). FOXO3 and FOXO1 mRNA expressions have no prognostic value (B and C respectively).