SMAD1 and cancer: Furthermore, it was observed that the addition of ER stress inhibitor 4-PBA partially attenuated the promoting effect of CMs from shRRBP1-transduced two-type cancer cells on OCN and p-Smad1/5/9 levels in MC3T3-E1 cells, implicating that RRBP1 depletion-induced ER stress partially aided in the enhancement of osteoblastic phenotype expression in MC3T3-E1 cell.