Involvement of retinoblastoma (Rb) and E2F transcription factors during photodynamic therapy of human epidermoid carcinoma cells A431

Oncogene. 1999 Mar 11;18(10):1891-6. doi: 10.1038/sj.onc.1202493.


Photodynamic therapy (PDT), a promising new therapeutic modality for the management of a variety of solid malignancies and many non-malignant diseases, is a bimodal therapy using a porphyrin based photosensitizing chemical and visible light. The proper understanding of the mechanism of PDT-mediated cancer cell-kill may result in improving the efficacy of this treatment modality. Earlier we have shown (Proc. Natl. Acad. Sci. USA; 95: 6977-6982, 1998) that silicon phthalocyanine (Pc4)-PDT results in an induction of the cyclin kinase inhibitor WAF1/CIP1/p21 which, by inhibiting cyclins (E and D1) and cyclin dependent kinases (cdk2 and cdk6), results in a G0/G1-phase arrest followed by apoptosis in human epidermoid carcinoma cells A431. We have also demonstrated the generation of nitric oxide during PDT-mediated apoptosis (Cancer Res.; 58: 1785-1788, 1998). Retinoblastoma (pRb) and E2F family transcription factors are important proteins, which regulate the G1-->S transition in the cell cycle. Here, we provide evidence for the involvement of pRb-E2F/DP machinery as an important contributor of PDT-mediated cell cycle arrest and apoptosis. Western blot analysis demonstrated a decrease in the hyper-phosphorylated form of pRb at 3, 6 and 12 h post-PDT with a relative increase in hypo-phosphorylated pRb. Western blot analysis also revealed that PDT-caused decrease in phosphorylation of pRb occurs at serine-780. The ELISA data demonstrated a time dependent accumulation of hypo-phosphorylated pRb by PDT. This response was accompanied with down-regulation in the protein expression of all five E2F (1-5) family transcription factors, and their heterodimeric partners DP1 and DP2. These results suggest that Pc4-PDT of A431 cells results in a down regulation of hyper-phosphorylated pRb protein with a relative increase in hypo-phosphorylated pRb that, in turn, compromises with the availability of free E2F. We suggest that these events result in a stoppage of the cell cycle progression at G1-->S transition thereby leading to a G0/G1 phase arrest and a subsequent apoptotic cell death. These data provide an evidence for the involvement of pRb-E2F/DP machinery in PDT-mediated cell cycle arrest leading to apoptosis.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Carcinoma, Squamous Cell / metabolism*
  • Carrier Proteins*
  • Cell Cycle Proteins*
  • DNA-Binding Proteins*
  • E2F Transcription Factors
  • E2F1 Transcription Factor
  • Humans
  • Phosphorylation
  • Photochemotherapy*
  • Rabbits
  • Retinoblastoma / metabolism*
  • Retinoblastoma-Binding Protein 1
  • Transcription Factor DP1
  • Transcription Factors / metabolism*
  • Tumor Cells, Cultured


  • Carrier Proteins
  • Cell Cycle Proteins
  • DNA-Binding Proteins
  • E2F Transcription Factors
  • E2F1 Transcription Factor
  • E2F1 protein, human
  • Retinoblastoma-Binding Protein 1
  • TFDP1 protein, human
  • Transcription Factor DP1
  • Transcription Factors