Hormone Transport, Receptors, Signal Transduction (3A)
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MCAT Biological and Biochemical Foundations of Living Systems › Hormone Transport, Receptors, Signal Transduction (3A)
In a cell line expressing a GPCR for hormone M, investigators measure second messengers after acute M exposure. M increases cAMP but does not change IP$_3$. A point mutation is introduced into the receptor’s cytosolic region that prevents coupling to heterotrimeric G proteins while leaving ligand binding intact. After mutation, M still binds the receptor at the cell surface, but cAMP no longer increases.
Which outcome would be expected if the receptor is blocked (or uncoupled) as described?
IP$_3$ increases because loss of G protein coupling shifts signaling toward PLC activation
Gene transcription increases immediately because the receptor becomes a nuclear transcription factor when uncoupled
cAMP increases normally because adenylyl cyclase is activated directly by M binding to the extracellular receptor domain
cAMP does not increase because G protein coupling is required to transmit the signal from receptor binding to adenylyl cyclase
Explanation
This question tests understanding of the essential role of G protein coupling in GPCR signal transduction, demonstrating that receptor-G protein interaction is required for transmitting the binding signal to downstream effectors. Signal transduction through GPCRs requires functional coupling between the receptor and heterotrimeric G proteins; ligand binding causes conformational changes that activate G proteins, which then modulate effector enzymes like adenylyl cyclase. The mutation that prevents G protein coupling while preserving ligand binding creates a non-functional receptor - hormone M can still bind but cannot activate G proteins to stimulate adenylyl cyclase, so cAMP doesn't increase (option B is correct). This elegantly demonstrates that ligand binding alone is insufficient; the receptor must couple to G proteins to transduce the signal. The distractor A incorrectly suggests direct receptor-adenylyl cyclase interaction bypassing G proteins, while C wrongly proposes a compensatory switch to PLC signaling. When analyzing GPCR mutations, remember that the signaling cascade is sequential and obligate - disrupting any step (binding, G protein coupling, effector activation) blocks all downstream events without creating alternative pathways.
In thyroid follicular cells, investigators track transport of two hormones: TSH (a glycoprotein peptide) and T3 (a thyroid hormone). In serum-like medium containing albumin, T3 is mostly protein-bound, whereas TSH remains largely unbound. Cells are exposed to equal total concentrations of TSH and T3. TSH produces a rapid increase in intracellular cAMP, while T3 produces no immediate change in cAMP but increases transcription of a metabolic gene after several hours.
Which statement best accounts for these observations?
Both hormones act through receptor tyrosine kinases, but only TSH activates phospholipase C to generate IP$_3$
TSH signals through a membrane receptor coupled to cAMP, whereas T3 enters cells and alters gene transcription via an intracellular receptor
T3 acts primarily through a cell-surface GPCR to stimulate cAMP, but albumin binding blocks receptor access
TSH requires intracellular binding proteins to cross the membrane, delaying its effect relative to T3
Explanation
This question tests understanding of peptide versus steroid hormone transport, receptor localization, and distinct signal transduction mechanisms. Signal transduction pathways fundamentally differ based on hormone hydrophobicity: hydrophilic peptides like TSH cannot cross membranes and must bind cell-surface receptors, while lipophilic hormones like T3 diffuse across membranes to bind intracellular receptors. The experimental observations - TSH causing rapid cAMP increase and T3 causing delayed transcriptional changes - perfectly match option C's explanation of TSH using membrane receptor-cAMP signaling while T3 enters cells for genomic effects. The albumin binding detail is a distractor element, as it affects free hormone concentration but doesn't change the fundamental signaling mechanisms. Common mistakes include option B suggesting T3 uses a GPCR (contradicted by no cAMP response) or option D proposing both use RTKs (inconsistent with the cAMP data). When analyzing hormone comparisons, use the response timing (seconds/minutes vs. hours) and second messenger involvement (cAMP presence/absence) to distinguish between membrane receptor signaling and genomic pathways.
In cardiomyocytes, a hormone (E) increases contractility within seconds. E binds a cell-surface receptor; downstream, intracellular $Ca^{2+}$ rises and phosphorylation of L-type calcium channels increases. When cells are pretreated with a G$_s$ inhibitor, E no longer increases channel phosphorylation. When cells are pretreated with an inhibitor of phospholipase C (PLC), E still increases channel phosphorylation.
Which cellular response is most consistent with the signal transduction pathway described?
Direct phosphorylation of L-type calcium channels by the receptor’s intrinsic tyrosine kinase domain
Increased phosphorylation of membrane proteins by PKA due to G$_s$-dependent cAMP production
Decreased cAMP due to G$_i$ activation, leading to reduced $Ca^{2+}$ influx through L-type channels
Increased IP$_3$ formation and ER $Ca^{2+}$ release as the primary driver of channel phosphorylation
Explanation
This question tests understanding of G protein-coupled receptor signal transduction, specifically distinguishing between Gs-mediated cAMP/PKA pathways and Gq-mediated PLC/IP3 pathways in hormone signaling. Signal transduction through GPCRs involves specific G protein subtypes: Gs stimulates adenylyl cyclase to produce cAMP and activate PKA, while Gq activates PLC to generate IP3 and DAG. The experimental evidence shows hormone E requires Gs (blocked by Gs inhibitor) but not PLC (still works with PLC inhibitor) for channel phosphorylation, indicating option A is correct - E signals through Gs to produce cAMP and activate PKA. The rapid timeframe (seconds) and phosphorylation outcome are consistent with PKA-mediated effects on ion channels. Common distractors like option C incorrectly emphasize the IP3 pathway despite the PLC inhibitor having no effect, while option D suggests RTK signaling inconsistent with GPCR/G protein involvement. When analyzing GPCR questions, use specific inhibitor effects to map the pathway: Gs inhibition blocking the response indicates cAMP/PKA involvement, while PLC inhibition having no effect rules out the IP3/Ca2+ branch.
A lab compared signaling from two hormones in the same cell type. Hormone X is lipid-soluble and circulates largely bound to a plasma carrier protein; hormone Y is water-soluble and circulates mostly free. In cells, X produced changes in mRNA abundance after 2 hours, while Y produced increased phosphorylation of a cytosolic protein within 1 minute. Which statement best distinguishes the receptor location and initial signaling mechanism for X versus Y?
X and Y both diffuse into the nucleus to activate phosphorylation cascades directly
X binds a cell-surface receptor coupled to G proteins; Y binds an intracellular receptor that directly binds DNA
X and Y both bind receptor tyrosine kinases but differ only in carrier protein binding
X binds an intracellular receptor that regulates transcription; Y binds a cell-surface receptor that initiates second messengers
Explanation
This question probes hormone transport, receptors, and signal transduction, contrasting lipid- and water-soluble hormone mechanisms. Signal transduction differs by receptor location: intracellular for lipid-soluble, surface for water-soluble, leading to transcriptional or rapid phosphorylation changes. Hormone X, lipid-soluble and carrier-bound, acts via intracellular receptors for slow mRNA effects, while Y uses surface receptors for quick phosphorylation. Choice B correctly distinguishes these, with X regulating transcription and Y initiating second messengers. Choice A reverses the mechanisms, a common mix-up flaw. For parallels, note solubility and timing to infer receptor type. Evaluate carrier protein roles in transport equilibrium.
Two hormones, A and B, each increase glycogen breakdown in hepatocytes but via different receptors. Hormone A binds a GPCR coupled to $G_s$ and increases cAMP, activating PKA. Hormone B binds a receptor tyrosine kinase (RTK), leading to autophosphorylation and recruitment of cytosolic adaptor proteins that activate a kinase cascade. Inhibiting adenylyl cyclase prevents A-induced glycogen breakdown but does not affect B-induced glycogen breakdown.
Which outcome would be expected if the RTK for hormone B is blocked while hormone A signaling remains intact?
Loss of A-induced glycogen breakdown because RTKs are required to activate $G_s$
Loss of B-induced glycogen breakdown with preserved A-induced glycogen breakdown
No change in either response because both pathways converge only at gene transcription
Enhanced B-induced glycogen breakdown due to increased cAMP from RTK blockade
Explanation
This question tests understanding of distinct signal transduction pathways - GPCR-cAMP versus RTK cascades - and their independence in mediating hormone responses like glycogen breakdown. Hormone A uses a GPCR-Gs-cAMP-PKA pathway while hormone B uses an RTK-kinase cascade pathway, representing two major but separate signaling mechanisms that can achieve similar cellular outcomes through different molecular intermediates. In this scenario, blocking the RTK for hormone B would specifically prevent B-induced glycogen breakdown by disrupting its unique signaling cascade, while leaving the A pathway intact since it operates through entirely different molecular components. The correct answer D accurately predicts loss of B-induced glycogen breakdown with preserved A-induced glycogen breakdown, reflecting the independence of these two pathways. A common distractor (B) incorrectly suggests that RTKs are required for GPCR signaling, when in fact these are completely separate receptor systems with distinct downstream effectors. When analyzing questions involving multiple hormone pathways, identify whether they use the same or different receptor types and signaling cascades to predict how selective inhibition will affect each response independently.
A steroid hormone (S) circulates mostly bound to a specific carrier protein. Target cells express an intracellular receptor (R) that, upon binding S, dimerizes and binds a DNA response element to alter transcription. In an experiment, carrier protein concentration in plasma-like medium is doubled while total S (free + bound) is held constant. Cells are exposed to this medium for 1 hour, and transcriptional activation of an S-responsive gene is measured.
Which outcome would be expected from doubling the carrier protein concentration at constant total S?
Decreased transcription due to reduced free S available to enter cells
No change in transcription because carrier proteins prevent S degradation inside cells
Increased transcription due to greater total S available for receptor binding
Increased transcription due to carrier-mediated endocytosis of S into the cytosol
Explanation
This question tests understanding of steroid hormone transport and the relationship between carrier protein binding and free hormone availability for intracellular receptor activation. Steroid hormones circulate bound to carrier proteins, but only the free (unbound) fraction can diffuse across cell membranes to bind intracellular receptors and activate transcription. In this scenario, doubling the carrier protein concentration while keeping total steroid S constant would shift the equilibrium toward more bound hormone, reducing the free fraction available to enter cells. The correct answer B accurately predicts decreased transcription due to reduced free S, as the increased carrier protein sequesters more hormone in the bound form. A common distractor (A) incorrectly assumes that total hormone concentration determines activity, ignoring the critical role of the free fraction in steroid hormone action. When analyzing steroid hormone signaling, always consider the equilibrium between bound and free hormone, remembering that only free hormone can cross membranes and activate intracellular receptors to alter gene transcription.
In a neuronal cell line, a peptide ligand (L) caused a transient increase in intracellular Ca$^{2+}$ measured by a fluorescent indicator. The response persisted when extracellular Ca$^{2+}$ was chelated but was eliminated by an inhibitor of phospholipase C (PLC). Which cellular response is most consistent with the pathway described?
Release of Ca$^{2+}$ from intracellular stores via IP$_3$-dependent channels
Ca$^{2+}$ influx through voltage-gated channels triggered by steroid receptor activation
Increased cGMP production by soluble guanylyl cyclase leading to Ca$^{2+}$ sequestration
Direct phosphorylation of PLC by L after L diffuses into the cytosol
Explanation
This question evaluates hormone transport, receptors, and signal transduction, focusing on Ca²⁺ mobilization via PLC pathways. Signal transduction involves GPCR activation of PLC, generating IP₃ to release ER Ca²⁺ stores. Ligand L activates PLC, increasing Ca²⁺ independently of extracellular sources. Choice D follows, with IP₃-dependent Ca²⁺ release matching persistence without external Ca²⁺. Choice B distracts with voltage-gated channels, flawed without depolarization cues. For similar cases, use chelators to distinguish sources. Link inhibitors to pathway enzymes.
A lab engineered a chimeric receptor with an extracellular domain that binds a peptide ligand and an intracellular domain derived from a receptor tyrosine kinase. Upon ligand addition, cells showed tyrosine phosphorylation and ERK activation, despite the ligand normally signaling through cAMP in wild-type cells. Which cellular response is most consistent with the signal transduction pathway in the chimeric receptor cells?
Activation of MAPK signaling downstream of receptor autophosphorylation on tyrosines
Activation of nuclear receptors leading to immediate transcription without phosphorylation
Increased cAMP because receptor tyrosine kinases directly synthesize cAMP
Decreased ERK activity because tyrosine phosphorylation inhibits all kinase cascades
Explanation
This question assesses hormone transport, receptors, and signal transduction, exploring chimeric receptor signaling. Signal transduction via RTKs includes autophosphorylation activating MAPK like ERK. The chimera uses RTK domain for tyrosine phosphorylation and ERK activation. Choice D aligns with MAPK downstream of autophosphorylation. Choice B distracts with nuclear receptors, ignoring kinase activity. For similar cases, identify domain functions in hybrids. Compare to wild-type pathways.
A hormone (H) produced opposite effects in two tissues: increased glycogen breakdown in liver and decreased contractility in cardiac tissue. In both tissues, H increased cAMP. Based on the scenario, which explanation best accounts for tissue-specific responses?
H must bind nuclear receptors in heart but GPCRs in liver to generate cAMP
cAMP can only activate one pathway, so one tissue’s observation must be experimental error
Carrier protein binding changes cAMP specificity between tissues
Different downstream effector proteins and gene expression programs interpret the same second messenger
Explanation
This question tests hormone transport, receptors, and signal transduction, addressing tissue-specific responses to shared messengers. Signal transduction allows cAMP to activate varied effectors for diverse outcomes across tissues. Hormone H increases cAMP but elicits opposite effects in liver and heart via different programs. Choice A explains this through downstream interpretation differences. Choice B dismisses observations as error, ignoring signaling complexity. In similar queries, recognize context-dependent messenger effects. Evaluate if pathways converge or diverge post-messenger.
A researcher applied insulin to skeletal muscle cells and observed increased phosphorylation of a membrane-proximal adaptor protein followed by increased translocation of GLUT4 to the plasma membrane. A pharmacologic inhibitor prevented receptor autophosphorylation on tyrosine residues. Which outcome would be expected if the receptor is blocked?
Increased GLUT4 translocation because blocking the receptor prevents its dephosphorylation
Unchanged GLUT4 translocation because insulin primarily signals through cGMP
Increased nuclear transcription within seconds due to direct insulin entry into the nucleus
Decreased GLUT4 translocation due to reduced downstream signaling from the receptor
Explanation
This question assesses hormone transport, receptors, and signal transduction, highlighting insulin's RTK pathway in muscle. Signal transduction starts with autophosphorylation, activating adaptors for GLUT4 translocation. Insulin binds RTK, phosphorylating adaptors to mobilize GLUT4. Receptor blockade reduces signaling and translocation, per choice D. Choice B suggests increased activity from dephosphorylation loss, but blockade prevents initiation. For related queries, identify RTK by tyrosine motifs. Examine inhibitor effects on proximal events.