Passage
The lumen of the human gut is lined by a monolayer of epithelial cells that acts as a selectively permeable barrier, preventing the passage of harmful intraluminal foreign antigens, flora, and toxins into the circulation while allowing digestion and absorption of essential dietary nutrients along with the transfer of electrolytes and water.Proteins in the tight junctions of intestinal epithelial cells maintain barrier integrity, but barrier dysfunction occurs when these cells are damaged in the setting of infection, burns, shock, or hypoxia (low oxygen levels) . The transcription factor HIF-1, a heterodimer composed of the macromolecules HIF-1α and HIF-1β, regulates the adaptive cellular response to hypoxia and the consequent expression of tight junction proteins.Researchers assessed the concentration of HIF-1 heterodimer components in human intestinal Caco-2 cells subjected to hypoxia/reoxygenation (H/R) in vitro. Caco-2 cells, a colon-derived cell line, were cultured under specific conditions to mimic the functional and morphological phenotype of wild-type enterocytes lining the small intestine. These cells were prepared and grown as a monolayer on a collagen-coated membrane.Next, the monolayer was cultured in hypoxic conditions and then exposed to atmospheric oxygen levels (normoxia) for 30, 60, and 120 minutes. Protein levels were quantified using direct enzyme-linked immunosorbent assay (ELISA) , in which an antibody linked to a reporter enzyme was utilized to bind and detect expression of the target molecule (analyte) in a sample. When the colorless substrate of the reporter enzyme was added, the enzyme generated a visible colored product that could be quantified based on color intensity.
Figure 1 Concentration of (A) HIF-1α and (B) HIF-1β in Caco-2 cells subjected to H/R (Note: 0 minutes = cells cultured in hypoxic conditions) Emodin, an anthraquinone compound that prevents hypoxia-induced epithelial cell disruption, was used in conjunction with a chemical HIF-1α inhibitor (HIF-1α-I) to treat Caco-2 cells in a separate experiment. Transepithelial electrical resistance (TEER) was assessed as a measure of barrier function, with higher TEER values indicating a more intact epithelial cell barrier. HIF-1α-I was found to block emodin's protective effect on epithelial barrier integrity.
Adapted from Lei Q, Qiang F, Chao D, et al. Int J Mol Med. 2014;34(6) :1629-39.
-Which of the following statements about HIF-1 is most likely to be true?At low oxygen levels, HIF-1 is nonfunctional.At low oxygen levels, HIF-1 is functional.At normal oxygen levels, HIF-1 concentration is decreased.At normal oxygen levels, HIF-1 concentration is increased.

Question

Passage
The lumen of the human gut is lined by a monolayer of epithelial cells that acts as a selectively permeable barrier, preventing the passage of harmful intraluminal foreign antigens, flora, and toxins into the circulation while allowing digestion and absorption of essential dietary nutrients along with the transfer of electrolytes and water.Proteins in the tight junctions of intestinal epithelial cells maintain barrier integrity, but barrier dysfunction occurs when these cells are damaged in the setting of infection, burns, shock, or hypoxia (low oxygen levels) . The transcription factor HIF-1, a heterodimer composed of the macromolecules HIF-1α and HIF-1β, regulates the adaptive cellular response to hypoxia and the consequent expression of tight junction proteins.Researchers assessed the concentration of HIF-1 heterodimer components in human intestinal Caco-2 cells subjected to hypoxia/reoxygenation (H/R) in vitro. Caco-2 cells, a colon-derived cell line, were cultured under specific conditions to mimic the functional and morphological phenotype of wild-type enterocytes lining the small intestine. These cells were prepared and grown as a monolayer on a collagen-coated membrane.Next, the monolayer was cultured in hypoxic conditions and then exposed to atmospheric oxygen levels (normoxia) for 30, 60, and 120 minutes. Protein levels were quantified using direct enzyme-linked immunosorbent assay (ELISA) , in which an antibody linked to a reporter enzyme was utilized to bind and detect expression of the target molecule (analyte) in a sample. When the colorless substrate of the reporter enzyme was added, the enzyme generated a visible colored product that could be quantified based on color intensity.

Figure 1 Concentration of (A) HIF-1α and (B) HIF-1β in Caco-2 cells subjected to H/R (Note: 0 minutes = cells cultured in hypoxic conditions) Emodin, an anthraquinone compound that prevents hypoxia-induced epithelial cell disruption, was used in conjunction with a chemical HIF-1α inhibitor (HIF-1α-I) to treat Caco-2 cells in a separate experiment. Transepithelial electrical resistance (TEER) was assessed as a measure of barrier function, with higher TEER values indicating a more intact epithelial cell barrier. HIF-1α-I was found to block emodin's protective effect on epithelial barrier integrity.
Adapted from Lei Q, Qiang F, Chao D, et al. Int J Mol Med. 2014;34(6) :1629-39.
-Which of the following statements about HIF-1 is most likely to be true?At low oxygen levels, HIF-1 is nonfunctional.At low oxygen levels, HIF-1 is functional.At normal oxygen levels, HIF-1 concentration is decreased.At normal oxygen levels, HIF-1 concentration is increased.

Quizplus · Accepted Answer

11ecba2d_0f2c_0390_a3bf_0ba0656b2824_MD0008_00 The passage states that HIF-1α and HIF-1β comprise the HIF-1 heterodimer, a protein complex composed of two different subunits. Based on this definition, HIF-1 requires assembly of these subunits to function. The graphs in Figure 1 show that both HIF-1α and HIF-1β are expressed and functional at 0, 30, 60, and 120 minutes, suggesting that the HIF-1 dimer can be assembled at all these time points. Figure 1A shows that HIF-1α concentration is greatest under hypoxic conditions (low oxygen, time 0). However, when exposed to normal oxygen concentrations (30, 60, 120 minutes), HIF-1α is degraded and its levels decrease in a time-dependent manner. In contrast, HIF-1β levels are relatively unaffected in both low and normal oxygen concentrations, as shown in Figure 1B. Oxygen promotes degradation of HIF-1α, but not HIF-1β. As a result, HIF-1 would be able to form and function, but its concentration would be reduced due to the decreased amount of HIF-1α available to dimerize with an unchanged amount of HIF-1β (Numbers II and III). (Numbers I and IV) Although HIF-1 is decreased at normal oxygen levels, it is still functional at both low and normal oxygen levels because the subunits that comprise the dimer are present at all time points assessed. Educational objective: A heterodimer is composed of two different protein subunits, and its assembly and activity are dependent on the expression and proper function of these subunits. Oxygen concentration, among other factors (eg, temperature, pH), has been shown to influence protein expression.