Role of Runx2 in IGF-1Rβ/Akt- and AMPK/Erk-dependent growth, survival and sensitivity towards metformin in breast cancer bone metastasis.

The mechanisms underlying reprogramming of growth factor signaling and metabolic pathways during bone metastasis of breast cancer are not clear. The Runt-related transcription factor (Runx2) regulates cell signaling during mammary epithelial morphogenesis and promotes invasion; therefore, we investigated its role in cell growth and metabolic signaling in bone-seeking breast cancer cells. We performed systemic inoculation of control or Runx2 knockdown invasive MDA-MB-231 cells in NOD/SCID mice, and compared parental and bone-derived variants for phenotypic and molecular alterations. The Runx2 knockdown showed early (0-2 weeks) inhibition of metastatic spread but late (4-6 weeks) outgrowth, suggesting Runx2-dependent bi-phasic response and reprogramming of metastatic cells. The late-stage tumor outgrowth of bone-derived Runx2 knockdown cells was associated with increased insulin-like growth factor- 1Rβ (IGF-1Rβ) levels. Interestingly, glucose uptake and glycolysis were reduced in the bone-derived Runx2 knockdown cells that could be further reduced by extracellular-regulated protein kinase (Erk1/2) inhibition. Furthermore, the Runx2 knockdown cells displayed activation of AMP-activated protein kinase (AMPKα), the sensor of cellular metabolism. Importantly, the Runx2 knockdown in bone-derived cells resulted in increased sensitivity to both Erk1/2 inhibition and AMPKα activation by PD184161 and metformin, respectively, despite increased IGF-1Rβ and AMPKα levels. Our results reveal that Runx2 promotes metastatic spread of mammary tumor cells. The growth of late-stage tumor cells can be targeted by Runx2 knockdown in combination with Mek-Erk1/2 inhibition and metformin treatment.