How long do you seed your cells? and in what kind of cell culture plate (12-well, 6-well, or ..). Usually this amout of protein is acceptable for that amount of cells.

@Fakry many thanks for your response. However, LAD2 cells are suspension cells not adherent hence samples were just spun after being treated

@Sabastian thank you for the well thought explanation. It is true my sample is viscous, I will try sonication going forward. In addition, protein determination was performed using BCA.