Levels of protein tyrosine phosphatase 1B determine susceptibility to apoptosis in serum‐deprived hepatocytes

Abstract

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of tyrosine kinase growth factor signaling. To assess the importance of PTP1B in the balance between death and survival in the liver, we have developed immortalized neonatal hepatocyte cell lines lacking (PTP1B−/−) or overexpressing (PTP1B+/+PTP1B) PTP1B. Early activation of caspase‐3 occurred in PTP1B+/+PTP1B hepatocytes but was nearly abolished in PTP1B−/− cells. At the molecular level, PTP1B overexpression/deficiency altered the balance of pro‐(Bim) and anti‐(Bcl‐xL) apoptotic members of the Bcl‐2 family upon serum withdrawal. Likewise, cytosolic cytochrome C increased rapidly in PTP1B+/+PTP1B hepatocytes whereas it was retained in the mitochondria of PTP1B−/− cells. DNA fragmentation and the increase of apoptotic cells induced by serum withdrawal in wild‐type (PTP1B+/+) hepatocytes were absent in PTP1B−/− cells. Conversely, overexpression of PTP1B accelerated DNA laddering and increased the number of apoptotic cells. In serum‐deprived PTP1B+/+PTP1B hepatocytes, a rapid entry of Foxo1 into the nucleus and an earlier activation of caspase‐8 was observed. However, both events were suppressed in PTP1B−/− hepatocytes. Moreover, PTP1B deficiency conferred resistance to apoptosis induced by activation of Fas and constitutively active Foxo1. Rescue of PTP 1B in deficient hepatocytes recovered the phenotype of wild‐type cells whereas reduction of PTP1B by siRNA suppressed apoptosis. Our results reveal a unique role for PTP1B as a mediator of the apoptotic pathways triggered by trophic factors withdrawal in hepatocytes. This novel mechanism may represent an important target in the design of therapeutic strategies for human liver regeneration after pathological damage as well as for treatment of hepatocarcinomas. J. Cell. Physiol. 212: 76–88, 2007. © 2007 Wiley‐Liss, Inc.

Protein-tyrosine Phosphatase 1B (PTP1B) Deficiency Confers Resistance to Transforming Growth Factor-β (TGF-β)-induced Suppressor Effects in Hepatocytes

“…Among them, protein-tyrosine phosphatase 1B (PTP1B) is a widely expressed member of the nonreceptor PTPs, which is located mainly associated with intracellular membranes such as the endoplasmic reticulum (6). This phosphatase plays important roles regulating insulin signaling and susceptibility to apoptosis because its deficiency confers resistance to cell death triggered by different stimuli (7,8). These cellular effects are dependent on the modulation of several prosurvival pathways directly controlled by PTP1B, such as the EGFR, IGF-1R, or the IR (9).…”

Alx3-deficient mice exhibit decreased insulin in beta cells, altered glucose homeostasis and increased apoptosis in pancreatic islets

“…Several days later, mice were treated intraperitoneally with insulin (0.75 U/kg), and after 15 min they were killed. A fragment of liver and soleus muscle were immediately frozen in liquid nitrogen, and then homogenised in ice-cold lysis buffer [21]. Extracts were centrifuged at 15,000 g for 40 min at 4°C, and the supernatant fractions were collected and stored at -80°C.…”

“…Membranes were incubated with anti-thymoma viral proto-oncogene 1 (AKT [also known as AKT1]) [21] or anti-phospho-AKT (Ser473; sc-7985; Santa Cruz Biotechnology, Santa Cruz, CA, USA) primary antibodies.…”

Protein Tyrosine Phosphatases: Emerging Regulators of Apoptosis

“…The level of PTP-1B expression was recently correlated with the sensitivity of hepatocytes to growth factor deprivation-induced apoptosis. 26 The pro-survival PI3K/Akt cascade (known to be regulated by PTP-1B) was shown to influence the nuclear localization of Foxo1, a transcription factor that regulates the expression of several pro-apoptotic genes. 26,27 Interestingly, hepatocytes lacking PTP-1B were resistant to apoptosis induced by a constitutively active mutant of Foxo1, which implicates an alternate Akt independent, pathway.…”

“…26 The pro-survival PI3K/Akt cascade (known to be regulated by PTP-1B) was shown to influence the nuclear localization of Foxo1, a transcription factor that regulates the expression of several pro-apoptotic genes. 26,27 Interestingly, hepatocytes lacking PTP-1B were resistant to apoptosis induced by a constitutively active mutant of Foxo1, which implicates an alternate Akt independent, pathway. 26 Finally, PTP-1B was recently identified as a pro-oncogenic molecule, 11,28,29 which implies an anti-apoptotic function for this enzyme.…”