Purpose: Small-molecule inhibitors of heat-shock protein 90 (HSP90) have been under development as chemotherapeutic agents. The adverse events reported from early clinical trials included hyponatremia. Given the limited number of patients enrolled, the number of hyponatremia incidents was remarkable and repeatedly, the event was judged as severe. Inappropriate V2 vasopressin receptor stimulation is an established cause of hyponatremia. We explored the hypothesis that HSP90 inhibition produces hypersensitivity to vasopressin by upregulating V2-receptors.
Methods: Experiments were carried out in cell culture using HEK293 cells with heterologous expression of the human V2-receptor and HELA cells with an endogenous V2-receptor complement. We tested the effect of HSP90 inhibition by three structurally unrelated compounds (alvespimycin, luminespib, radicicol) and asserted its specificity in cells depleted of cytosolic HSP90 (by RNA interference). Assays encompassed surface V2-receptor density and vasopressin-stimulated formation of cyclic AMP (cAMP).
Results: The results demonstrate a twofold increase in cell-surface receptor density following pre-incubation with each of the HSP90 inhibitors. The effect had a concentration-dependence consistent with the individual potencies to inhibit HSP90. Similarly, depletion of cytosolic HSP90 increased surface-receptor density and at the same time, reduced the inhibitor effect. Upregulated V2-receptors were fully functional; hence, in culture treated with an HSP90 inhibitor, addition of vasopressin resulted in higher levels of cAMP than in controls.
Conclusion: Since formation of cAMP is the first signalling step in raising water permeability of the collecting duct epithelia, we suggest that V2-receptor upregulation generates hypersensitivity to vasopressin linking HSP90 inhibition to the development of hyponatremia.
Keywords: HELA cells; HSP90 inhibitor; Hyponatremia; Upregulation; V2 vasopressin receptor.