Head and neck squamous cell carcinomas (
HNSCC) are frequently characterized by
chemotherapy and radiation resistance, and by overexpression of Bcl-XL, an antiapoptotic member of the Bcl-2
protein family. In this report we examined whether cell-permeable
peptides derived from the BH3 domains of proapoptotic Bax, Bad, or Bak could be used to target Bcl-XL and/or Bcl-2 in
HNSCC cells, and induce apoptotic death in these cells. To render the
peptides cell permeable, Antennapedia (Ant) or
polyarginine (
R8) peptide transduction domains were fused to the amino termini. Fluorescence microscopy of
peptide-treated
HNSCC cells revealed that the BH3
peptides colocalized with mitochondria, the site of Bcl-XL and Bcl-2 expression. By contrast, a mutant
peptide (BaxE BH3) which cannot bind Bcl-XL or Bcl-2 was diffusely localized throughout the cytoplasm. Treatment of three
HNSCC cell lines (1483, UM-22A, UM-22B) with the wild-type BH3
peptides resulted in loss of viability and induction of apoptosis, as assessed by MTS assays and
annexin V staining. In general, Ant-conjugated
peptides were more potent than R8-conjugated
peptides, and Bad
BH3 peptide was typically more potent than Bax BH3 or Bak BH3. Treatment of purified
HNSCC mitochondria with BH3
peptides resulted in robust release of
cytochrome c. Thus, the relative apoptosis resistance of
HNSCC cells is not due to a deficit in this step of the intrinsic, mitochondrial-mediated apoptosis pathway. We conclude that cell-permeable BH3
peptides can be used to target Bcl-XL and/or Bcl-2 in
HNSCC, and targeting of these
proteins may have therapeutic value in the treatment of this disease.