Allosteric Activation of Acid α-Glucosidase by the Human Papillomavirus E7 Protein

Abstract

Changes in the cellular carbohydrate metabolism are a hallmark of malignant transformation and represent one of the earliest discernible events in tumorigenesis. In the early stages of certain epithelial cancers, a metabolic switch is regularly observed, in which slowly growing glycogenotic cells are converted to highly proliferating basophilic cells. This step is accompanied by a rapid depletion of the intracellular glycogen stores, which in liver carcinogenesis results from the activation of the enzyme acid ␣-glucosidase by an as yet unknown mechanism. We show here that acid ␣-glucosidase is a target for the E7 protein encoded by human papillomavirus type 16, a human tumor virus that plays a key role in the genesis of cervical carcinoma. We show that expression of E7 induces the catalytic activity of acid ␣-glucosidase in vivo and wild type E7, but not transformation-deficient mutants bind directly to acid ␣-glucosidase and increase the catalytic activity of the enzyme in vitro. The data suggest that the E7 protein encoded by human papillomavirus type 16 can act as an allosteric activator of acid ␣-glucosidase.It has long been known that tumor cells display characteristic alterations of the carbohydrate metabolism (for review see Ref. 1), which represent one of the earliest discernible events in tumorigenesis (for recent review see Ref.2). In certain epithelial cancers, such as liver cancer or kidney cancer, one of the first detectable alterations is a metabolic switch, in which slowly growing glycogenotic cells, also referred to as clear cells, are converted to highly proliferating basophilic cells (for review see Ref. 3). This step is accompanied by a rapid depletion of the intracellular glycogen stores, which in liver carcinogenesis results from the activation of acid ␣-glucosidase (4), whereas the activity of glycogen phosphorylase, the other cellular glycogendegrading enzyme, is reduced throughout hepatocarcinogenesis (5). The actual content of glycogen in a cell is controlled through the balance of glycogen-synthesizing enzymes and glycogen-degrading enzymes, which themselves are under allosteric control by various metabolites (6). It is unknown at present how the activity of these enzymes is modulated in early carcinogenesis, to first build up the clear cell phenotype and then trigger its disappearance at later stages. According to the current concept (7), the deregulation of metabolic enzymes in early tumor cells reflects changes in cellular signal transduction which lead to tumor-specific alterations of the metabolic apparatus.Reduced glycogen storage is regularly observed in early lesions of the cervix, a finding that has been used for clinical diagnosis of cervical dysplasia for more than 60 years (8). Cervical neoplasia is tightly linked to infection by human papillomaviruses (HPV)1 of the high risk group, e.g. HPV-16 (9), and two viral genes, E6 and E7, are required for papillomavirus-associated carcinogenesis (for review see Refs. 9 and 10). The E7 oncogene of HPV-16 cooperates with HPV-16 E6 to ...

Destabilization of the Retinoblastoma Tumor Suppressor by Human Papillomavirus Type 16 E7 Is Not Sufficient To Overcome Cell Cycle Arrest in Human Keratinocytes

“…Alternatively, the failure of E7 CVQ68-70AAA or ⌬79-83 to override cell cycle control in keratinocytes may be related to other factors. ⌬79-83 does not transform rodent cells (47), and the region encompassed by this mutation has been implicated in binding to TATA-binding protein (47), Mi-2␤ (8), M2 pyruvate kinase (73), and acid ␣-glucosidase (72). The interaction of E7 with one or several of these factors, or as yet unidentified factors, may be important for deregulating keratinocyte cell cycle control.…”

The immortalizing and transforming ability of two common human papillomavirus 16 E6 variants with different prevalences in cervical cancer

“…Changes in cellular carbohydrate metabolism are a hallmark of malignant transformation and represent one of the earliest discernible events in tumorigenesis (Zwerschke et al, 2000). HPV oncoproteins have previously been shown to alter carbohydrate metabolism (Zwerschke et al, 2000;Mazurek et al, 2001).…”

“…HPV oncoproteins have previously been shown to alter carbohydrate metabolism (Zwerschke et al, 2000;Mazurek et al, 2001). HPV16 E7 is shown to target the glycolytic control enzyme, pyruvate kinase M2 (a key player in reprogramming cellular carbohydrate metabolism in tumors), such that pyruvate kinase M2 is shifted to the tumor specific, dimeric form that possesses decreased substrate affinity (Mazurek et al, 2001).…”

Critical Roles for Non-pRb Targets of Human Papillomavirus Type 16 E7 in Cervical Carcinogenesis

“…Two E7 mutants deficient in binding to pRb can cooperate with E6 in the immortalization of primary human keratinocytes (20), and the ability of E7 to transactivate certain E2F-responsive promoters may depend on binding to p107 rather than binding to pRb as shown with the B-myb promoter (21). Furthermore, the E7 D79-83 mutant exhibits a decreased ability to transform baby rat kidney cells, although it efficiently binds and degrades pRb (22)(23)(24)(25). Another E7 mutant, E7 CVQ68-70AAA , which is also able to bind and destabilize pRb but is deficient in p21 inactivation, fails to overcome differentiation-dependent cell cycle withdrawal or DNA damage-induced cell cycle arrest in human keratinocytes (24,26).…”