Accomplices in crime: The diabolical liaison between protein tyrosine phosphatase 1B and ras in hepatocellular carcinoma
Authors: Diego F. Calvisi
Journal: Hepatology (2016)

Potential conflict of interest: Nothing to report. See article on Page 1528 Hepatocellular carcinoma (HCC) is one of the most frequent and lethal cancers, ranking fifth in incidence and second in lethality worldwide.1 Therapeutic options for HCC are limited.2 Although surgical resection and liver transplantation are potentially curative treatments for this disease, they can be applied only to early‐stage or localized HCC.2 In addition, the only approved drug for advanced HCC, the multikinase inhibitor, sorafenib, has shown limited efficacy and poor tolerability.2 In order to establish novel, effective therapeutic approaches against HCC, a better understanding of its molecular pathogenesis is mandatory. Ras proteins, expressed in virtually every nucleated cell, are molecular switches for signaling cascades that regulate many aspects of the cell biology.3 These proteins are characterized by two distinct conformations: an inactive GDP‐bound and an active GTP‐bound.3 Two classes of proteins control the active and inactive states of Ras proteins: Ras guanine nucleotide exchange factors (GEFs) and Ras GTPase‐activating proteins (GAPs).4 In particular, Ras GEFs promote the activation of Ras by increasing the exchange of GDP for GTP, whereas Ras GAPs trigger Ras inactivation by converting GTP into GDP.6 A number of Ras GAPs have been identified, including p120GAP or RASA1, disabled homolog 2 interacting protein (DAB2IP), nGAP, SynGAP1, neurofibromin (NF1), RASA2, RASA3, RASA4, and Ras GTPase‐activating‐like protein 1 (RASAL1).6 Activating mutations in Ras genes have been identified in ∼30% of human cancers.4 These mutations render the GTPase domain of Ras insensitive to inactivation by Ras GAPs, thus resulting in unrestrained activity of Ras.4 Nonetheless, accumulating evidence indicates that aberrant activation of Ras signaling also occurs in the absence of Ras mutations in cancer.5 Indeed, uncontrolled activity of the Ras pathway is often the consequence of mutations in upstream inducers and downstream effectors or inactivation of Ras/Raf/Mek inhibitors, such as RASSF1A, NORE1A, RKIP, and SPRY2 5 An alternative and emerging mechanism leading to unconstrained Ras activation in cancer is the suppression of Ras GAPs.5 The importance of inactivation of Ras GAPs as another route of Ras aberrant activation has been demonstrated by the finding that mutation of Ki‐Ras gene and inactivation of Ras GAPs by promoter hypermethylation are mutually exclusive events in breast, lung, colorectal, and pancreatic cancers.7 In human HCC, activation of the Ras pathway represents a very frequent oncogenic event.6 Indeed, although Ras mutations are relatively rare in this tumor type, a sustained activity of the Ras cascade is ubiquitous and inversely correlates with length of patient survival in liver cancer.6 In particular, a systematic analysis has shown that, in the absence of Ras mutations, at least one component of the Ras GAP family (RASAL1, DAP2IP, or NF1) is down‐regulated mainly by promoter hypermethylation and/or loss of heterozygosity in all liver tumor specimens.7 In addition, the master upstream inducer of Ras GAPs, paired‐like homeodomain transcription factor 1 (PITX1),8 was found to be inactivated in a subset of HCC patients characterized by an extremely poor outocome.7 In the present issue of Hepatology, Tai et al. take a further step in understanding the molecular mechanisms that lead to deregulation of Ras GAPs in human liver cancer.9 The researchers investigated the role of protein tyrosine phosphatase 1B (PTP1B), a negative regulator of receptor tyrosine kinases that promotes activation of Ras signaling through suppression of p120GAP,10 in HCC. Through a series of elegant experiments, Tai et al. uncovered the mechanism whereby PTP1B down‐regulates levels of p120GAP in this tumor type.9 Specifically, the researchers found that PITX1 is a direct substrate of PTP1B in HCC cell lines. In these cells, PITX1 is dephosphorylated by PTP1B at Y160, Y175, and Y179 residues, an event that primes PITX1 for proteasome‐mediated degradation.9 Importantly, the impaired transcriptional activity of PITX1 resulted in down‐regulation of two Ras GAPs, namely, p120GAP and RASAL1, thus triggering unrestrained activation of the Ras cascade and consequent in vitro proliferation of HCC cell lines.9 Subsequent investigations showed that sorafenib decreases PTP1B activity and promotes the expression of PITX1 and p120GAP by PITX1 hyperphosphorylation.9 Furthermore, the researchers found that forced overexpression of PTP1B impairs sensitivity to sorafenib in in vitro and in vivo models of liver cancer, thus implying that PTP1B has an important effect on sorafenib‐dependent growth restraint.9 The importance of the data obtained in experimental models was further substantiated in a vast collection of human HCC specimens by immunohistochemistry. In the latter clinical samples, in fact, PTP1B was found to be consistently overexpressed in tumor samples when compared with corresponding nontumorous surrounding livers, whereas an opposite staining pattern was detected for p120GAP and PITX1.9 The significant study by Tai et al. provides novel insights of high relevance to understand the molecular mechanisms responsible for aberrant activation of the Ras cascade in HCC harboring wild‐type Ras. In addition, because of the pathogenic role of deregulated Ras signaling in various types of cancer, the present data might have a broad impact not limited to this tumor type. In particular, the data shown here strongly support a previously hypothesized tumor‐suppressive and oncogenic role of the Ras GAPs/PITX1 axis7 and PTPB1,10 respectively, in cancer. However, although the study by Tai et al. has significantly improved our knowledge on the role, or roles, of PTPB1 and Ras GAPs/PITX1 in cancer, further investigation is required on this topic. Indeed, in addition to regulation of Ras activity, it has been previously shown that the tumor‐suppressive functions of Ras GAPs and PITX1 can involve positive and negative modulation of other signaling pathways, such as the ASK1/JNK/p38 MAPK and VEGFR/Akt/PLC‐γ cascades.7 The relevance of the modulation of the latter pathways by Ras GAPs and PITX1 on tumor development and progression remains to be determined. Furthermore, the findings of the present investigation might have important clinical implications for treatment of HCC. Indeed, the researchers identified PTP1B overexpression as a previously unknown mechanism responsible for sorafenib resistance in HCC cells, whose disruption might open innovative therapeutic approaches against this deadly disease. Notably, because PTP1B is a well‐established negative regulator of the insulin and leptin pathways, inhibitors of PTP1B able to overcome the resistance to leptin and insulin for treatment of obesity and type 2 diabetes have been identified11 and might be promptly tested in HCC patients exhibiting elevated levels of PTP1B. Based on the present preliminary data, it is conceivable that use of PTP1B inhibitors either alone or, most likely, in association with sorafenib, might be beneficial for treatment of human HCC.Figure 1: Schematic representation of the molecular crosstalk between PTP1B and PITX1 in human HCC. (A) In hepatocytes with low levels of PTP1B, an oncogenic stimulus triggers the rapid activation/phosphorylation (p) of PITX1 at three tyrosine residues (Y160, Y175, and Y179). Once activated, PITX1 translocates into the nucleus where it induces activation of p120GAP and RASAL1, two Ras GAPs. Transcriptional activation of p120GAP and RASAL1 results in Ras inactivation through GTP hydrolysis, thus inducing tumor suppression. (B) In HCC cells with elevated PTP1B, PITX1 is dephosphorylated (black dots) at the aforementioned tyrosine residues by PTP1B, leading to inhibition of the transcriptional activity of PITX1 (red crossing bars) and its proteosomal‐mediated degradation. Because of inactivation of PITX1 and its downstream effectors, Ras activity is unrestrained and oncogenesis is promoted. (C) Treatment of HCC cells exhibiting high levels of PTP1B with sorafenib and/or PTP1B inhibitors (inh) blocks (blunted red arrow) the detrimental dephosphorylation activity of PTP1B over PITX1, which can consequently migrate into the nucleus and exert its tumor‐suppressive effects by activation of p120GAP and RASAL1.