Early downregulation of Mcl-1 regulates apoptosis triggered by cardiac glycoside UNBS1450

Cell Death Dis. 2015 Jun 11;6(6):e1782. doi: 10.1038/cddis.2015.134.


Cardiac glycosides (CGs), prescribed to treat cardiovascular alterations, display potent anti-cancer activities. Despite their well-established target, the sodium/potassium (Na(+)/K(+))-ATPase, downstream mechanisms remain poorly elucidated. UNBS1450 is a hemi-synthetic cardenolide derived from 2″-oxovorusharin extracted from the plant Calotropis procera, which is effective against various cancer cell types with an excellent differential toxicity. By comparing adherent and non-adherent cancer cell types, we validated Mcl-1 as a general and early target of UNBS1450. A panel of CGs including cardenolides ouabain, digitoxin and digoxin as well as bufadienolides cinobufagin and proscillaridin A allowed us to generalize our findings. Our results show that Mcl-1, but not Bcl-xL nor Bcl-2, is rapidly downregulated prior to induction of apoptosis. From a mechanistic point of view, we exclude an effect on transcription and demonstrate involvement of a pathway affecting protein stability and requiring the proteasome in the early CG-induced Mcl-1 downregulation, without the involvement of caspases or the BH3-only protein NOXA. Strategies aiming at preventing UNBS1450-induced Mcl-1 downregulation by overexpression of a mutated, non-ubiquitinable form of the protein or the use of the proteasome inhibitor MG132 inhibited the compound's ability to induce apoptosis. Altogether our results point at Mcl-1 as a ubiquitous factor, downregulated by CGs, whose modulation is essential to achieve cell death.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antineoplastic Agents / pharmacology*
  • Apoptosis / drug effects
  • Apoptosis / physiology*
  • Calotropis / metabolism
  • Cardenolides / pharmacology*
  • Cell Line, Tumor
  • Down-Regulation / drug effects
  • Humans
  • Jurkat Cells
  • Leupeptins / pharmacology
  • MCF-7 Cells
  • Myeloid Cell Leukemia Sequence 1 Protein / biosynthesis*
  • Myeloid Cell Leukemia Sequence 1 Protein / metabolism
  • Proto-Oncogene Proteins c-bcl-2 / metabolism
  • Transcription, Genetic / genetics
  • bcl-X Protein / metabolism


  • Antineoplastic Agents
  • BCL2L1 protein, human
  • Cardenolides
  • Leupeptins
  • MCL1 protein, human
  • Myeloid Cell Leukemia Sequence 1 Protein
  • Proto-Oncogene Proteins c-bcl-2
  • UNBS 1450
  • bcl-X Protein
  • benzyloxycarbonylleucyl-leucyl-leucine aldehyde