Objective  This work aimed to explore the effects of Na⁺/K⁺-ATPase activity and the expression of itsβ1-subunit and P-glycoprotein(P-gp) on the proliferation and invasion of human colorectal cancer parental cells(SW480) and oxaliplatin-resistant cells(SW480/OxR).The molecular mechanisms of ouabain for reversing the multidrug resistance(MDR) of human colorectal cancer oxaliplatin-resistant cells were also examined.

  Methods  SW480 and SW480/OxR cells derived from the same patient were treated with or without ouabain at the physiological concentration of 1 nM.The SW480 and SW480/OxR cell proliferation capacity was assessed by the MTS assay.The invasion capacity was deteremined using a Transwell chamber.The Na⁺/K⁺-ATPase activity was measured by biochemical and enzymological techniques.The expression of theβ1-subunit and P-gp of Na⁺/K⁺-ATPase was determined by real-time quantitative PCR, Western blotting, and flow cytometry.

  Results  The capacity of invasion significantly increased in the SW480/OxR cells compared with the SW480 cells(P < 0.01).There was no difference between the SW480 and SW480/OxR cell proliferation capacities(P > 0.05).The Na ⁺/K⁺-ATPase activity significantly decreased in SW480/OxR cells compared with SW480 cells(P < 0.01).The expression of the Na ⁺/K⁺-ATPaseβ1-subunit in mRNA and protein levels was lower in SW480/OxR cells than in SW480 cells(P < 0.01).However, the expression of P-gp in mRNA and protein levels was higher in SW480/OxR cells than in SW480 cells(P < 0.01).Interestingly, ouabain at the physiological concentration of 1 nM significantly enhanced the activity of Na ⁺/K⁺-ATPase in SW480/OxR cells(P < 0.05).The expression of theβ1-subunit was also upregulated and that of P-gp was downregulated at the protein level, thereby inducing a decrease in the capacity of SW480/OxR cell growth inhibition and invasion.Nevertheless, after ouabain treatment for 48 h, the Na ⁺/K⁺-ATPase activity andβ1-subunit protein expression level in SW480/OxR cells were still lower than those in SW480 cells(P < 0.01).

  Conclusion  Decreased Na ⁺/K⁺-ATPase activity can be attributed to the downregulation of Na⁺/K⁺-ATPaseβ1-subunit expression and may cause the MDR of human colorectal cancer cells.Ouabain could partly reverse the MDR of such cells, which can be attributed to increased Na⁺/K⁺-ATPase activity and P-gp expression downregulation.