LHPP Human

What is LHPP and what is its primary function in human cells?

LHPP (phospholysine phosphohistidine inorganic pyrophosphate phosphatase), also known as HDHD2B, is a histidine phosphatase protein that was originally identified in bovine liver. It is highly conserved from worms to humans and plays a pivotal role in histidine dephosphorylation. LHPP belongs to a small family of known histidine phosphatases that includes PHPT1, PGAM5, and LHPP. While the importance of the former two in tumor progression has been well documented, LHPP's functions have only recently gained significant research attention . The primary function of LHPP appears to be regulating histidine phosphorylation, which has implications for epigenetic regulation and carcinogenic activity suppression. It was first confirmed as a tumor suppressor in 2018 and has since been implicated in controlling multiple cellular processes related to cancer development .

How is LHPP expression distributed across normal human tissues?

LHPP shows variable expression patterns across normal human tissues. According to immunohistochemistry data from The Human Protein Atlas, LHPP is significantly expressed in normal liver tissues and is widely distributed in both cytoplasmic and membrane compartments . Expression analysis across multiple tissue types demonstrates that LHPP is constitutively expressed in most normal tissues with some variation in expression levels. In the oral cavity, LHPP exhibits high expression in normal oral keratinocytes (HOK cells) compared to oral squamous cell carcinoma (OSCC) cells . This consistent expression across normal tissues suggests a fundamental role in maintaining cellular homeostasis.

What cellular compartments contain LHPP protein?

High-magnification immunofluorescence studies reveal that LHPP is expressed in both the cytoplasm and nucleus of cells . According to predictions from The Human Protein Atlas, the main subcellular localizations of LHPP protein are indeed the cytoplasm and nucleus. This dual localization pattern suggests that LHPP may have different functions depending on its subcellular compartmentalization. When LHPP is overexpressed using lentiviral vectors, stronger nuclear fluorescence is often observed, possibly due to the integration of the LHPP gene into the host chromosome . This subcellular distribution pattern provides important context for understanding LHPP's molecular functions.

How does LHPP expression differ between cancer and normal tissues?

Comprehensive analyses across multiple cancer types demonstrate that LHPP expression is significantly reduced in most human tumors compared to matched normal tissues. Pan-cancer analysis using TCGA and GTEx databases shows that LHPP gene expression levels are markedly reduced in tumor tissues compared with normal tissues in multiple cancer types, including sarcoma (SARC) and testicular germ cell tumors (TGCT) .

In specific cancer types:

• In oral squamous cell carcinoma (OSCC), LHPP is significantly downregulated compared to normal oral mucosa tissues, as confirmed by both TCGA database analysis and experimental validation .

• In hepatocellular carcinoma (HCC), analysis of the Oncomine database shows that LHPP expression is significantly downregulated in HCC tissues compared to paired normal tissues .

• Western blot analyses of patient samples consistently demonstrate lower LHPP expression in cancer tissues compared to adjacent normal tissues .

This widespread downregulation across multiple cancer types strongly suggests that LHPP functions as a tumor suppressor gene.

What is the relationship between LHPP expression and clinical outcomes in cancer patients?

LHPP expression levels show significant correlations with clinical outcomes across several cancer types. Higher LHPP expression is generally associated with better prognosis:

How does LHPP expression correlate with tumor differentiation status?

LHPP expression shows a strong correlation with tumor differentiation status, particularly in OSCC. Immunohistochemistry (IHC) staining demonstrates that LHPP expression is highest in normal oral mucosas with a mean optical density (MOD) of 0.033, decreased in highly differentiated OSCC tissues (MOD of 0.024), and almost absent in moderately and poorly differentiated OSCC tissues (MOD of 0.006 and 0.0018, respectively) .

Statistical analysis of 23 OSCC cases revealed a highly significant correlation between LHPP expression and differentiation status (P=0.000002), while other clinical parameters (age, sex, tumor size, tumor location, and muscle invasion) showed no significant correlations with LHPP expression . This strong association with differentiation status suggests that LHPP could serve as a key biomarker for determining tumor grade and potentially guide therapeutic strategies.

What signaling pathways does LHPP regulate in cancer cells?

LHPP influences several key signaling pathways in cancer cells, with the PI3K/Akt pathway being particularly well-documented. In OSCC, GO and KEGG enrichment analyses combined with experimental validation demonstrate that LHPP promotes cancer cell apoptosis by decreasing the transcriptional activity of phosphorylated PI3K (p-PI3K) and phosphorylated Akt (p-Akt) . This modulation of the PI3K/Akt pathway, which is a central regulator of cell survival and proliferation, appears to be a key mechanism underlying LHPP's tumor suppressor function.

Additionally, gene set enrichment analysis (GSEA) of RNA-seq data from HCC patients in TCGA reveals that LHPP expression negatively correlates with cell cycle progression and metastasis pathways . This suggests that LHPP may act through multiple signaling mechanisms to suppress tumor growth and metastatic potential.

How does LHPP affect cancer cell proliferation, migration, and invasion?

Functional studies across multiple cancer types provide strong evidence that LHPP inhibits cancer cell proliferation, migration, and invasion:

• In OSCC: Cell Counting Kit-8 tests, EdU proliferation tests, scratch assays, invasion tests, monoclonal formation tests, and mouse xenograft tumor models all demonstrate that LHPP inhibits OSCC growth, proliferation, and migration both in vivo and in vitro .

• In HCC: CCK8 assays show that overexpression of LHPP in Huh7 and LM3 hepatocellular carcinoma cells significantly inhibits cell viability. Similarly, colony formation assays demonstrate reduced colony-forming ability in LHPP-overexpressing cells .

These consistent findings across different cancer types and experimental approaches strongly support LHPP's role in suppressing multiple aspects of cancer cell behavior critical for tumor progression.

Which oncogenes are regulated by LHPP expression?

LHPP has been shown to regulate the expression of several key oncogenes involved in cancer progression. In HCC, real-time PCR assays demonstrate that forceful expression of LHPP suppresses the expression of several oncogenes, including:

• CCNB1 (Cyclin B1): A critical regulator of cell cycle progression

• PKM2 (Pyruvate Kinase M2): A key enzyme in cancer metabolism

• MMP7 (Matrix Metalloproteinase-7): Involved in extracellular matrix degradation and metastasis

• MMP9 (Matrix Metalloproteinase-9): Important for cancer cell invasion and metastasis

This downregulation of oncogenes provides a molecular mechanism explaining how LHPP exerts its tumor-suppressive effects on cell proliferation and metastasis.

What types of genetic alterations affect LHPP in human cancers?

Analysis of genetic alteration patterns across different cancer types from the TCGA project reveals four main types of genetic alterations affecting LHPP:

• Mutation

• Structural variant

• Amplification

• Deep deletion

Among these, amplification is the most common genetic alteration type, occurring primarily in:

• Stomach adenocarcinoma

• Uterine corpus endometrial carcinoma

• Ovarian serous cystadenocarcinoma

• Adrenocortical carcinoma

• Esophageal carcinoma

• Pheochromocytoma and paraganglioma

The highest frequency of LHPP genetic alteration (3.86%) is observed in stomach adenocarcinoma, with amplification as the primary alteration type (2.50%). In brain lower grade glioma (LGG), deep deletion is the most common alteration, with a frequency of 1.95% . These patterns of genetic alteration provide insight into potential mechanisms of LHPP dysregulation in different cancer types.

How do specific LHPP mutations impact protein function?

In brain lower grade glioma (LGG), missense mutations represent the main genetic alteration affecting LHPP . The R45H mutation has been specifically identified, although its functional consequences are not fully elucidated in the provided search results. The presence of these mutations, particularly in a cancer type where high LHPP expression correlates with better survival outcomes, suggests that these mutations may impair LHPP's tumor suppressor function.

The impact of specific mutations on LHPP protein structure, stability, and enzymatic activity represents an important area for future research. Understanding the functional consequences of these mutations could provide valuable insights into the molecular mechanisms underlying LHPP's tumor suppressor activities and potentially identify targetable vulnerabilities in cancers with altered LHPP function.

What cell and animal models are effective for studying LHPP function?

Several effective experimental models have been established for studying LHPP function:

Cell Models:

• OSCC cell lines (SCC15 and SCC25) with lentivirus-mediated LHPP overexpression

• HCC cell lines (Huh7 and LM3) with lentivirus-mediated LHPP overexpression

• Normal oral keratinocytes (HOK cells) as controls for OSCC studies

• Normal liver cell line (LO2) as control for HCC studies

Animal Models:

• Mouse xenograft tumor models have been successfully used to validate LHPP's tumor-suppressive effects in vivo

These models provide complementary approaches for investigating LHPP function at both cellular and organismal levels. Cell lines enable detailed mechanistic studies of LHPP's effects on proliferation, migration, and signaling pathways, while animal models allow assessment of LHPP's impact on tumor growth and progression in a physiologically relevant context.

What methods are most effective for measuring LHPP expression and activity?

Multiple complementary methodologies have been employed to assess LHPP expression and functional activities:

Expression Analysis:

• RT-PCR and real-time PCR for mRNA expression analysis

• Western blotting for protein expression quantification

• Immunohistochemistry (IHC) for tissue expression patterns

• Immunofluorescence staining for subcellular localization

Functional Assays:

• Cell Counting Kit-8 (CCK8) assays for cell viability

• EdU proliferation assays

• Scratch assays for cell migration

• Transwell invasion assays

• Colony formation assays

• Flow cytometry for apoptosis assessment

Bioinformatic Approaches:

• Differential gene expression (DGE) analysis using RNA-seq data

• Gene Ontology (GO) and KEGG pathway enrichment analysis

• Gene Set Enrichment Analysis (GSEA)

• Survival analysis using Kaplan-Meier methods

This multi-modal approach to studying LHPP provides comprehensive insights into its expression patterns, subcellular localization, and functional effects across different experimental systems.

What are the key unresolved questions about LHPP function in human diseases?

Despite significant progress in understanding LHPP's role as a tumor suppressor, several important questions remain unresolved:

• The precise biochemical mechanism by which LHPP's histidine phosphatase activity influences cancer-related signaling pathways

• The complete spectrum of LHPP substrates in human cells

• The role of LHPP in non-cancer human diseases

• The potential of LHPP as a therapeutic target

• The mechanisms regulating LHPP expression in normal and cancer tissues

Addressing these questions will require interdisciplinary approaches combining biochemistry, molecular biology, cell biology, and translational research methodologies.

How might LHPP research translate to diagnostic or therapeutic applications?

The consistent downregulation of LHPP across multiple cancer types and its correlation with clinicopathological parameters and survival outcomes suggest several potential translational applications:

Diagnostic Applications:

• LHPP expression could serve as a diagnostic biomarker, particularly for cancer differentiation status in OSCC

• LHPP expression patterns might help distinguish tumor subtypes and predict prognosis

Therapeutic Applications:

• Strategies to restore LHPP expression or function could represent a novel therapeutic approach for cancers with LHPP downregulation

• Understanding the signaling pathways regulated by LHPP may identify alternative therapeutic targets in cancers where LHPP is inactivated

• LHPP's negative regulation of oncogenes like CCNB1, PKM2, MMP7, and MMP9 suggests that these downstream effectors might represent druggable targets in LHPP-deficient tumors

Development of these applications will require further validation in diverse patient populations and comprehensive preclinical studies to assess efficacy and safety.