Retinoblastoma-binding Protein 2-homolog 1: A Retinoblastoma-Binding Protein Downregulated in Malignant Melanomas

Mod Pathol. 2005 Sep;18(9):1249-57. doi: 10.1038/modpathol.3800413.

Abstract

In malignant melanomas, the loss of cell cycle control is thought to be due to a lack of retinoblastoma protein (pRb)-activity. Members of the previously described family of retinoblastoma-binding proteins (RBPs) are supposed to act as pRb-modulating factors. Based on RNA-fingerprinting of normal human melanocytes, we previously described a new family member with high sequence homology to the retinoblastoma-binding protein-2 (RBP-2), termed RBP2-Homolog 1 (RBP2-H1). Based on its UVB responsiveness, it was hypothesized that this gene may also play a role in melanocytic tumors. In the present study, we can confirm by real-time RT-PCR (six common melanocytic nevi, five advanced nodular melanomas and seven melanoma metastases) and immunohistochemistry (tissue microarrays: 52 melanocytic nevi, 60 melanomas, 60 metastases; and conventional sections: five common nevi, four advanced nodular melanomas, five melanoma metastases) that RBP2-H1 expression is progressively downregulated in advanced and metastatic melanomas in vivo with a certain intratumoral heterogeneity. Whereas benign melanocytic nevi are RBP2-H1 positive in about 70% of the cases, a lack of RBP2-H1 expression was found in 90% of the primary malignant melanomas and 70% of the melanoma metastases, respectively. Interestingly, a similar deficiency can be found in glioblastomas, but not epithelial cancers. In accordance to the in vivo data, established melanoma cell lines exhibit low but heterogeneous levels of RBP2-H1 expression. By co-immunoprecipitation, we provide the first evidence that a subfraction of total RBP2-H1 can bind to pRb, which makes this protein a true pRb-interacting factor. We conclude that loss of RBP2-H1 is a common finding in the progression of malignant melanomas. Since a direct interaction of RBP2-H1 and pRb seems possible, the loss of RBP2-H1 may possibly contribute to uncontrolled growth in malignant melanomas.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Base Sequence
  • Blotting, Western
  • Cell Line, Tumor
  • Down-Regulation
  • Humans
  • Immunohistochemistry
  • Immunoprecipitation
  • Intracellular Signaling Peptides and Proteins / deficiency*
  • Intracellular Signaling Peptides and Proteins / genetics
  • Melanoma / metabolism*
  • Molecular Sequence Data
  • Nevus / metabolism
  • Phylogeny
  • RNA, Messenger / analysis
  • Retinoblastoma Protein / metabolism
  • Retinoblastoma-Binding Protein 2
  • Reverse Transcriptase Polymerase Chain Reaction
  • Skin Neoplasms / metabolism*
  • Tissue Array Analysis
  • Tumor Suppressor Proteins / deficiency*
  • Tumor Suppressor Proteins / genetics

Substances

  • Intracellular Signaling Peptides and Proteins
  • RNA, Messenger
  • Retinoblastoma Protein
  • Tumor Suppressor Proteins
  • KDM5A protein, human
  • Retinoblastoma-Binding Protein 2