Downregulation of PHLPP Expression Contributes to Hypoxia-Induced Resistance to Chemotherapy in Colon Cancer Cells

Abstract

Hypoxia is a feature of solid tumors. Most tumors are at least partially hypoxic. This hypoxic environment plays a critical role in promoting resistance to anticancer drugs. PHLPP, a novel family of Ser/Thr protein phosphatases, functions as a tumor suppressor in colon cancers. Here, we show that the expression of both PHLPP isoforms is negatively regulated by hypoxia/anoxia in colon cancer cells. Interestingly, a hypoxia-induced decrease of PHLPP expression is attenuated by knocking down HIF1␣ but not HIF2␣. Whereas the mRNA levels of PHLPP are not significantly altered by oxygen deprivation, the reduction of PHLPP expression is caused by decreased protein translation downstream of mTOR and increased degradation. Specifically, hypoxiainduced downregulation of PHLPP is partially rescued in TSC2 or 4E-BP1 knockdown cells as the result of elevated mTOR activity and protein synthesis. Moreover, oxygen deprivation destabilizes PHLPP protein by decreasing the expression of USP46, a deubiquitinase of PHLPP. Functionally, downregulation of PHLPP contributes to hypoxia-induced chemoresistance in colon cancer cells. Taken together, we have identified hypoxia as a novel mechanism by which PHLPP is downregulated in colon cancer, and the expression of PHLPP may serve as a biomarker for better understanding of chemoresistance in cancer treatment. Hypoxia is a condition commonly occurring in most solid tumors. Numerous studies have demonstrated that hypoxia plays a pivotal role in tumor progression and metastasis and is often associated with increased malignancy and poor prognosis (1, 2). Serving as the primary modulators of hypoxic stress, hypoxia-inducible factors (HIFs) are rapidly induced in response to oxygen deprivation to regulate the expression of genes that facilitate adaptation to hypoxic conditions (2, 3). The HIF transcription factors are heterodimers consisting of an O 2 -sensitive ␣ subunit and a stable ␤ subunit. There are three isoforms of mammalian HIF␣, of which HIF1␣ and HIF2␣ are the most structurally similar and best characterized (3). Under normoxic condition, HIFs are modified at two conserved proline residues by prolyl hydroxylase domain proteins (PHDs; consisting of PHD1, PHD2, and PHD3). The modified HIFs are subsequently recognized and ubiquitinated by an E3 ligase, the von Hippel-Lindau (VHL) protein, resulting in the degradation of HIF proteins via the proteasome pathway. Under hypoxic conditions, the hydroxylase activity of PHDs is inhibited, and stabilized HIF transcription factors can translocate to the nucleus and regulate the transcription of hypoxia-associated genes (3-5).Previous studies have shown that hypoxia inhibits mTOR and cap-dependent protein translation via a TSC2-dependent mechanism upon the induction of REDD1 (6, 7). mTOR is an evolutionarily conserved serine/threonine protein kinase that functions in two distinct complexes, mTORC1 and mTORC2, in cells. mTORC1 serves as a nutrient and energy sensor by controlling protein translation through phosphorylation of 4E-BP1 and S6K1 (8)....

Oncogenic Ras differentially regulates metabolism and anoikis in extracellular matrix-detached cells

“…Studies from a number of labs have now implicated loss of PHLPP1 activity as an important facet of cancer cell survival. 23,[36][37][38] That being said, despite the fact that PHLPP1 has a predicted Ras-associated domain, 39 our study is the first (to our knowledge) to directly link downregulation of PHLPP1 to oncogenic Ras signaling. In addition, the majority of studies aimed at understanding PHLPP1 in cancer cells have focused on PHLPP1's ability to dephosphorylate Akt.…”

Pleckstrin Homology (PH) Domain Leucine-rich Repeat Protein Phosphatase Controls Cell Polarity by Negatively Regulating the Activity of Atypical Protein Kinase C

“…Previous studies have shown that PHLPP expression is frequently down-regulated in various cancer types and PHLPP loss promotes tumor progression by promoting cell growth and proliferation and inhibiting apoptosis (17,24,25,46,47). Because loss of epithelial polarity is an important hallmark of advanced malignant tumors, the ability of PHLPP to maintain proper polarization of epithelial cells likely contributes to its tumor suppressor function.…”

Pleckstrin homology domain leucine-rich repeat protein phosphatases set the amplitude of receptor tyrosine kinase output

“…32), and genetic deletion of one isoform, PHLPP1, is sufficient to cause prostate tumors in a mouse model (33). Their down-regulation is associated with hypoxiainduced resistance to chemotherapy (34), further underscoring their role in cancer. Consistent with their tumor-suppressive function, PHLPP1 and PHLPP2 are on chromosomal loci (18q21.33 and 16q22.3, respectively) that frequently are deleted in cancer (33).…”