Cancers develop as a result of inactivation of tumor suppressor genes and activation of oncogenes. PTEN is one of the most frequently inactivated tumor suppressor genes in human cancer. The PTEN gene is approximately 100,000 bp in size and is located on chromosome 10q23. PTEN mRNA is transcribed from a single promoter, spliced together from 9 exons, and varies from 2-6 kb in length based upon the length of its 3' untranslated region. The transcripts all contain a 403 amino acid open reading frame that encodes the PTEN protein, which is composed of a phosphatase, C2 and tail domains. The PTEN protein encodes a lipid phosphatase that removes the 3' phosphate from the inositol ring of phosphoinositol-3, 4, 5-trisphosphate (PIP3). PTEN can be inactivated in cancers by a variety of alterations that include missense mutation, homozygous deletion, transcript silencing, and enzyme inhibition. Recent examination of the 5' end of the PTEN transcript cDNA indicated the potential for a CTG initiation codon capable of translating a 576 amino acid open reading frame in the same reading frame as classical PTEN protein, such that it would contain an amino-terminal 173 amino acid alternatively translated domain followed by a phosphatase, C2 and tail domain. Comparison of different animal species indicated that the alternatively translated domain was highly conserved among vertebrates. Expression of the full length cDNA in cells led to the expression of 75 kDa and 56 kDa protein bands, which were dependent upon intact initiation codons for their respective expression. Examination of the 173 amino acid alternatively translated domain revealed that it had a secretion signal sequence and a cell penetration polyarginine sequence. The 75 kDa form of PTEN, which we have named PTEN-Long, could be secreted from cells and displayed cell penetrating activity. Once inside the cell, PTEN-Long was able to inhibit signaling to AKT and regress tumor growth. Moreover, a PTEN-Long Red Fluorescent Protein fusion protein was able to enter cells. Based upon these preliminary data, we propose to better understand how tumor mutations affect PTEN-Long secretion, cell binding, uptake and PIP3 signaling and examine its tumor suppressive and fused cargo delivery capacities.
PTEN is one of the most commonly mutated tumor suppressors in human cancer. Here we propose to study a heretofore unrecognized isoform of PTEN, which we call PTEN-Long, to determine how it functions. PTEN- Long is highly relevant to our understanding of cancer biology because it is secreted and can travel in the body to enter other cells where it is able to inhibit PIP3 signaling and induce tumor cell death. Because it has the ability to ferry other proteins into cells, it provides a new tool for delivering proteins intocells.
