Assessment of Mouse Liver Histopathology Following Exposure to HFPO-DA With Emphasis on Understanding Mechanisms of Hepatocellular Death.

Ammonium 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)-propanoate (HFPO-DA) is a short chain member of per- and polyfluoroalkyl substances (PFAS). To better understand the relevance of histopathological effects seen in livers of mice exposed to HFPO-DA for human health risk assessment, histopathological effects were summarized from hematoxylin and eosin (H&E)-stained sections in several repeat-dose toxicity studies in mice. Findings across studies revealed histopathological changes consistent with peroxisomal proliferation, whereas two reports of steatosis could not be confirmed in the published figures. In addition, mechanisms of hepatocellular death were assessed in H&E sections as well as with the apoptotic marker cleaved caspase-3 (CCasp3) in newly cut sections from archived liver blocks from select studies. A comparison of serially CCasp3 immunolabeled and H&E-stained sections revealed that mechanisms of hepatocellular death cannot be clearly discerned in H&E-stained liver sections alone as several examples of putatively necrotic cells were positive for CCasp3. Published whole genome transcriptomic data were also reevaluated for enrichment of various forms of hepatocellular death in response to HFPO-DA, which revealed enrichment of apoptosis and autophagy, but not ferroptosis, pyroptosis, or necroptosis. These morphological and molecular findings are consistent with transcriptomic evidence for peroxisome proliferator-activated receptor alpha (PPARα) signaling in HFPO-DA exposed mice.

Assessment of the mode of action underlying development of liver lesions in mice following oral exposure to HFPO-DA and relevance to humans.

HFPO-DA (ammonium, 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)propanoate) is a short-chain polyfluorinated alkyl substance (PFAS) used in the manufacture of some types of fluorinated polymers. Like many PFAS, toxicity studies with HFPO-DA indicate the liver is the primary target of toxicity in rodents following oral exposure. Due to the structural diversity of PFAS, the mode of action (MOA) can differ between PFAS for the same target tissue. There is significant evidence for involvement of peroxisome proliferator-activated receptor alpha (PPARα) activation based on molecular and histopathological responses in the liver following HFPO-DA exposure, but other MOAs have also been hypothesized based on limited evidence. The MOA underlying the liver effects in mice exposed to HFPO-DA was assessed in the context of the Key Events (KEs) outlined in the MOA framework for PPARα activator-induced rodent hepatocarcinogenesis. The first 3 KEs (ie, PPARα activation, alteration of cell growth pathways, and perturbation of cell growth/survival) are supported by several lines of evidence from both in vitro and in vivo data available for HFPO-DA. In contrast, alternate MOAs, including cytotoxicity, PPARγ and mitochondrial dysfunction are generally not supported by the scientific literature. HFPO-DA-mediated liver effects in mice are not expected in humans as only KE 1, PPARα activation, is shared across species. PPARα-mediated gene expression in humans produces only a subset (ie, lipid modulating effects) of the responses observed in rodents. As such, the adverse effects observed in rodent livers should not be used as the basis of toxicity values for HFPO-DA for purposes of human health risk assessment.

A review of mammalian in vivo genotoxicity of hexavalent chromium: implications for oral carcinogenicity risk assessment.

Assessment of genotoxicity is a critical component of mode of action (MOA) analysis and carcinogen risk assessment due to its influence on quantitative risk extrapolation approaches. To date, clear guidance and expert consensus on the determination of a mutagenic MOA remains elusive, resulting in different estimates of carcinogenic risk for the same chemical among different stakeholders. Oral toxicity criteria for hexavalent chromium [Cr(VI)], for example, differ by orders of magnitude due largely to the interpretation of in vivo genotoxicity data. Herein, we review in vivo genotoxicity studies for Cr(VI) to inform the MOA for Cr(VI)-induced tumors observed in a two-year cancer bioassay in mice and rats exposed via drinking water. Overall, genotoxicity results in carcinogenic target tissues (viz., oral cavity and duodenum) are negative. Results in the intestine are consistent with imaging data indicating little to no chromium present in the crypt compartment following oral exposure. Positive genotoxicity results in nontarget tissues have been reported at high doses mostly following nonphysiological routes of exposure. Given the negative genotoxicity results in carcinogenic target organs from oral exposure to Cr(VI), there is scientific justification to support the use of nonlinear low-dose extrapolation methods in the derivation of oral toxicity criteria for Cr(VI). These results highlight important differences between genotoxicity testing for hazard identification purposes and quantitative risk assessment.