**5. Microbiome Metabolism for Therapeutic Applications in HCC**

Table 3 has summarized the microbiome in HCC. The third most common cause of cancer-related deaths globally is HCC, which carries a heavy disease burden [106]. HCC is notorious for being highly aggressive and is associated with frequent progression and recurrence. Numerous immune checkpoint inhibitors, mainly anti-PD-1/anti-PD-L1 monoclonal antibodies, have been studied and approved for HCC over the past few years. However, only a small portion of patients (20%) benefit [107–109]. To date, there are no known indicators that can accurately predict the clinical outcome of anti-PD-1/anti-PD-L1 immunotherapy [110].

**Table 3.** HCC-related changes in the microbiota's regulation in animal, rat, and human models. Predominant microbiota present on various sites and their regulation with HCC.


Notes and abbreviations: ↑, increased bacterial metabolism; ↓, decreased bacterial metabolism; HCC, hepatocellular carcinoma; DEN, Diethyl nitrosamine; HFD, high-fat diet; NASH, nonalcoholic steatohepatitis; Ref, Reference.

Additional changes to the microbiome occur during the cirrhosis–HCC transition period. The microbial bacteria of *Veillonella, Streptococcus, Clostridium,* and *Prevotella* were more prominent in the cirrhosis patients. *Eubacterium, Alistipes*, and *Faecalibacterium praus-* *nitzii* were comfortable in the healthy gut–liver axis [123]. The probiotics are lactic acid bacteria, including species of *Lactobacillus, Streptococcus,* and *Enterococcus,* as well as yeast, *Bifidobacterium, Propionibacterium, Bacillus, Escherichia coli,* and *Bifidobacterium,* which can both encourage the growth of helpful bacteria and inhibit the growth of harmful bacteria [124]. *Actinobacteria* were shown to be more prominent in early HCC vs. cirrhosis [125]. The correct diagnosis of liver cancer may be achieved by using three biomarkers (*Enterococcus, Phyllobacterium,* and *Limnobacter*). Among HCC patients, the abundances of the genera that produce butyrate (*Clostridium, Ruminococcus,* and *Coprococcus*) decreased, while the abundances of the genera that produce LPS (*Neisseria, Enterobacteriaceae,* and *Veillonella*) increased [121]. Figure 3 explains the principal mechanisms of liver cancer and liver damage and the metabolic alterations involved.

**Figure 3.** Molecular processes connected to both alcoholic and nonalcoholic HCC. The main risk factors for the development of NAFLD and AFLD, respectively, are a high-calorie diet and excessive alcohol intake. The clinical spectra of liver damage in supporting HCC development in NAFLD and AFLD have comparable molecular mechanisms despite the diverse pathogenic genesis. Within the intestinal tract, microbe-dependent reactions transform primary bile acids into secondary bile acids.

An initial piece of evidence that a gut microbe may influence liver cancer was discovered in mice that had spontaneously acquired *Helicobacter hepaticus (H. hepaticus)* infections. The *H. hepaticus* infection has been linked to chronic hepatitis and fibrosis in male BALB/c mice. The *H. hepaticus* is a spiral bacterium that also lives in the bile *canaliculi,* the cecal mucosa, and the colonic mucosa and causes chronic active hepatitis and liver tumors. The *H. hepaticus* is the prototypical carcinogenic bacteria for mice, and experimental infection has already been employed as a model of microbial tumor promotion in the liver [126–128]. Later, it was revealed that *H. hepaticus* intestinal colonization was sufficient to produce aflatoxin B1 (AFB1)- and HCV transgene-induced HCC. This information prompted crucial research. In addition to activating Wnt/-catenin, hepatocyte turnover, and the impaired phagocytic clearance of injured cells, the processes also implicated stimulated NF-B-regulated networks linked to innate and adaptive immunity [129].
